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Using the TNG100-1 cosmological simulations, we explore how galaxy properties, such as specific star formation rate (⁠|$\rm sSFR=SFR/\mathit{ M}_*$|⁠), gas fraction (⁠|$f_{\rm gas} \, = \, M_{\rm H}/M_{*}$|⁠), and star formation efficiency (⁠|$\rm SFE_{H} = SFR/\mathit{ M}_{H}$|⁠), change over the course of galaxy–galaxy interactions. We identify 18 534 distinct encounters from the reconstructed orbits of a sample of massive galaxies (⁠|$M_{*} > 10^{10} \,\, {\rm M}_{\odot }$|⁠) with companions within a stellar mass ratio of 0.1–10. Using these encounters, we study the variation of galaxy properties over time as they approach and move away from pericentric encounters over a redshift range of |$0 \le z \lt 1$|⁠. Following the closest pericentric encounters (⁠|$\le 50$| kpc) of a host galaxy with its companion, we find that sSFR is enhanced by a factor of |$1.6 \pm 0.1$| on average within the central stellar half-mass radius (R 1/2) compared to pre-encounter values. Our results show a time delay between pericentre and maximum sSFR enhancement of |$\sim$| 0.1 Gyr with a mean galaxy separation of 75 kpc. We similarly find that |$f_{\rm gas}$| is enhanced by a factor of |$1.2 \pm 0.1$|⁠ , and |$\rm SFE_{H}$| is enhanced by a factor of |$1.4 \pm 0.1$| following the pericentre of an encounter within the same time-scale. Additionally, we find evidence of inflowing gas towards the centre, measured by comparing the |$f_{\rm gas}$| and metallicity within the central R 1/2 to the galactic outskirts. We find that approximately 70 per cent of the peak sSFR enhancement can be attributed to the increase in |$\rm SFE_{H}$|⁠ , with the increase in |$f_{\rm gas}$| contributing the remaining 30 per cent. [ABSTRACT FROM AUTHOR]

Feedback from active galactic nuclei (AGNs) can strongly impact the host galaxies by driving high-velocity winds that impart substantial energy and momentum to the interstellar medium (ISM). In this work, we study the impact of these winds in isolated galaxies using high-resolution hydrodynamics simulations. Our simulations use the explicit ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs. Additionally, using a super-Lagrangian refinement scheme, we resolve AGN feedback coupling to the ISM at |$\sim$| 10–100 pc scales. We find that AGN feedback efficiently regulates the growth of supermassive black holes. However, its effect on star formation and outflows depends strongly on the relative strengths of AGN versus local stellar feedback and the geometrical structure of the gas disc. When the energy injected by AGN is subdominant to that of stellar feedback, there are no significant changes in the star formation rates or mass outflow rates of the host galaxy. Conversely, when the energy budget is dominated by the AGN, we see a significant decline in the star formation rates accompanied by an increase in outflows. Galaxies with thin gas discs like the Milky Way allow feedback to escape easily into the polar directions without doing much work on the ISM. In contrast, galaxies with thick and diffuse gas discs confine the initial expansion of the feedback bubble within the disc, resulting in more work done on the ISM. Phase space analysis indicates that outflows primarily comprise hot and diffuse gas, with a lack of cold and dense gas. [ABSTRACT FROM AUTHOR]

The Dark Energy Spectroscopic Instrument Legacy Imaging Surveys (DESI-LIS) comprise three distinct surveys: the Dark Energy Camera Legacy Survey (DECaLS), the Beijing–Arizona Sky Survey (BASS), and the Mayall z -band Legacy Survey (MzLS). The citizen science project Galaxy Zoo DECaLS 5 (GZD-5) has provided extensive and detailed morphology labels for a sample of 253 287 galaxies within the DECaLS survey. This data set has been foundational for numerous deep learning-based galaxy morphology classification studies. However, due to differences in signal-to-noise ratios and resolutions between the DECaLS images and those from BASS and MzLS (collectively referred to as BMz), a neural network trained on DECaLS images cannot be directly applied to BMz images due to distributional mismatch. In this study, we explore an unsupervised domain adaptation (UDA) method that fine-tunes a source domain model trained on DECaLS images with GZD-5 labels to BMz images, aiming to reduce bias in galaxy morphology classification within the BMz survey. Our source domain model, used as a starting point for UDA, achieves performance on the DECaLS galaxies' validation set comparable to the results of related works. For BMz galaxies, the fine-tuned target domain model significantly improves performance compared to the direct application of the source domain model, reaching a level comparable to that of the source domain. We also release a catalogue of detailed morphology classifications for 248 088 galaxies within the BMz survey, accompanied by usage recommendations. [ABSTRACT FROM AUTHOR]

We present a kinematic and spectroscopic analysis of 40 red giant branch stars, in 9 fields, exquisitely delineating the lower segment of the North West stream (NW-K2), which extends for |$\sim$| 80 kpc from the centre of the Andromeda galaxy. We measure the stream's systemic velocity as −439.3 |$^{+4.1}_{-3.8}$| km s |$^{-1}$| with a velocity dispersion =16.4 |$^{+5.6}_{-3.8}$| km s |$^{-1}$| that is in keeping with its progenitor being a dwarf galaxy. We find no detectable velocity gradient along the stream. We determine |$-1.3\pm0.1$| |$\le$| |$\langle[\mathrm{ Fe/H}]_{\rm spec}\rangle$| |$\le$| |$-1.2\pm0.8$| but find no metallicity gradient along the stream. We are able to plausibly associate NW-K2 with the globular clusters PAndAS-04, PAndAS-09, PAndAS-10, PAndAS-11, and PAndAS-12 but not with PAndAS-13 or PAndAS-15, which we find to be superimposed on the stream but not kinematically associated with it. [ABSTRACT FROM AUTHOR]

Context. The circumgalactic medium (CGM) plays a vital role in galaxy evolution, however, studying the emission from CGM is challenging due to its low surface brightness and the complexities involved in interpreting resonant lines such as Lyman-alpha (Lyα). Aims. The near-infrared coverage, unprecedented sensitivity, and high spatial resolution of the James Webb Space Telescope (JWST) enable us to study the optical strong lines associated with the extended Lyα "nebulae" at redshifts of 2−3. These lines serve as diagnostic tools to infer the physical conditions in the massive CGM gas reservoir of these systems. Methods. In deep medium-band images taken by the JWST, we serendipitously discovered the [O III] emission from the CGM surrounding a massive interacting galaxy system at a redshift of z ∼ 2.8, known to be embedded in a bright extended (100 kpc) Lyα "nebula". Results. This is the first time that the [O III] lines have been detected from a Lyα "nebula". The JWST images reveal that the CGM gas actually resides in narrow (∼2.5 kpc) filamentary structures with strong [O III] emission, tracing the same extent as the Lyα emission. An analysis of the [O III] suggests that the emitting CGM is fully ionized and is energetically dominated by mechanical heating. We also find that the inferred density and pressure are higher than those commonly predicted by simulations of the CGM. Conclusions. We conclude that the observed CGM emission originates from the gas expelled by the episodic feedback processes, cooling down and enriching the CGM, while traveling a distance of at least 60 kpc. These observations demonstrate how intensive feedback processes shape gas distribution and properties in the CGM around massive halos. While access to such deep, high-resolution imaging opens up a new discovery space for investigating the CGM, it also challenges numerical simulations with respect to explaining and reproducing the exquisitely complex structures revealed by the observations. [ABSTRACT FROM AUTHOR]

Context. Ultra-faint dwarfs (UFDs) are expected to be relics of the earliest galaxies to have formed in the Universe. Observations show the presence of a stellar halo around UFDs, which can give precious insights into UFD evolution. Indeed, stellar halos can form via tidal interaction, early supernova feedback, or merging events. Aims. This work investigates how merger properties impact the formation of stellar halos around UFDs, focusing on Tucana II, the most promising UFD assembled through mergers. Methods. We developed N-body simulations of dry isolated mergers between two UFDs, resolving their stellar component down to 1 M⊙. We built a suite of simulations by varying: the merger-specific i) angular momentum, l, and ii) kinetic energy k, iii) the merger mass ratio, M1/M2, iv) the dark-to-stellar mass ratio, MDM/M⋆, of the progenitors, and v) their stellar size, R1/2. To fully explore such a five-dimensional parameter space, we trained a neural network to emulate the properties of the resulting "post-merger" UFD, by quantifying the half-mass radius (R⋆) and the fraction of stars at radii > 5R⋆ (f5). Results. Our principal component analysis clearly shows that f5 (R⋆) is primarily determined by M1/M2 (R1/2), with R1/2 (M1/M2) playing a secondary role. Both f5 and R⋆ show almost no dependence on k, l, and MDM/M⋆ in the explored range. Using our emulator, we find that to form the stellar halo observed in Tucana II; that is, f5 = 10 ± 5% and R⋆ = 120 ± 30 pc, we need to merge progenitors with M1/M2 = 8−3+4 M 1 / M 2 = 8 − 3 + 4 $ M_1/M_2 = 8_{-3}^{+4} $ , the size of the more massive one being R1/2 = 97−18+25 R 1 / 2 = 97 − 18 + 25 $ R_{1/2} = 97^{+25}_{-18} $ pc. Such findings are corroborated by the consistency (χ2 ≃ 0.5 − 2) between stellar density profiles observed for Tucana II and those of simulations that have M1/M2 and R1/2 close to the values predicted by the emulator. Conclusions. The stellar halos of UFDs contain crucial information about the properties of their smaller progenitor galaxies. Ongoing and planned spectroscopic surveys will greatly increase the statistics of observed stars in UFDs, and thus of their associated stellar halos. By interpreting such observations with our simulations, we will provide new insights into the assembly history of UFDs, and thus the early galaxy formation process. [ABSTRACT FROM AUTHOR]

Arp 220 is the prototypical ultraluminous infrared Galaxy (ULIRG), and one of the brightest objects in the extragalactic far-infrared sky. It is the result of a merger between two gas rich spiral galaxies which has triggered starbursting activity in the merger nuclear regions. Observations with the submillimeter array centred at a frequency of 345 GHz and with a synthesized beamsize of |$0.77\times 0.45$|  arcsec were used to search for polarized dust emission from the nuclear regions of Arp 220. Polarized dust emission was clearly detected at 6  |$\sigma$| significance associated with the brighter, western nucleus, with a peak polarization fraction of 2.7 |$\pm 0.35$|  per cent somewhat offset from the western nucleus. A suggestive 2.6  |$\sigma$| signal is seen from the fainter eastern nucleus. The dust emission polarization is oriented roughly perpendicular to the molecular disc in the western nucleus suggesting that the magnetic field responsible is orientated broadly in the plane of the disc, but may be being reordered by the interaction between the two nuclei. Unlike more evolved interacting systems, we see no indication that the magnetic field is being reordered by the outflow from the western nucleus. These observations are the first detection of dust polarization, and thus of magnetic fields, in the core of a ULIRG. [ABSTRACT FROM AUTHOR]

Estimating physical quantities such as the star formation rate, stellar mass, and active galactic nucleus (AGN) fraction of galaxies is a key step in understanding galaxy formation and evolution. In order to estimate the uncertainties in the predicted values for these quantities, in this paper we explore the impact of adopting four different AGN torus models in fitting the multiwavelength spectral energy distributions (SED) of galaxies. We also explore the impact of adopting two different geometries for the host, a spheroidal geometry, more appropriate for late-stage mergers, and a disc geometry, more appropriate for galaxies forming stars with secular processes. We use optical to submillimetre photometry from the Herschel Extragalactic Legacy Project (HELP) and utilize a Markov chain Monte Carlo SED-fitting code. We use exclusively radiative transfer models for the AGN torus as well as for the starburst and host galaxy. We concentrate on a sample of 200 galaxies at |$z\approx 2$|⁠ , selected in the ELAIS-N1 field. All galaxies have a detection at 250  |$\mu \mathrm{ m}$| which ensures the presence of a starburst. We find that the stellar mass and star formation rate of the galaxies can be robustly estimated by the SED fitting but the AGN fraction depends very much on the adopted torus model. We also find that the vast majority of the galaxies in our sample are better fitted by a spheroidal geometry and lie above the main sequence. Our method predicts systematically higher SFR and lower stellar mass than the popular energy balance method cigale. [ABSTRACT FROM AUTHOR]

We present results of our dynamical stream modelling for the North West Stream in the outer halo of the Andromeda galaxy (M31). Comprising two main segments, the North West Stream was thought to be a single structured arching around M31. However, recent evidence suggests that it is two separate, unrelated, streams. To test this hypothesis, we use observational data from six fields associated with the upper segment of the North West Stream together with eight fields and five globular clusters associated with the lower segment to constrain model orbits. We fit both segments of the stream using a fixed potential model for M31 and an orbit integrator to compare orbits with the observed streams. We measure the central tracks and predict proper motions for the upper segment (lower segment) finding |${\mu ^{*}_{\alpha }}$|  = 0.078 |$^{+0.015}_{-0.012}$| (0.085 |$^{+0.001}_{-0.002}$|⁠) mas yr−1 and |${\mu _{\delta }}$|  = |$-0.05^{+0.008}_{-0.009}$| (⁠|$-0.095^{+0.003}_{-0.005}$|⁠) mas yr−1. Our results support the hypothesis that the dwarf spheroidal galaxy Andromeda XXVII is the progenitor of the upper segment of the North West Stream and that the upper and lower segments do not comprise a single structure. We propose that the upper segment, which appears to be on an infall trajectory with M31, be renamed the 'Andromeda XXVII Stream' and the lower segment, also apparently infalling towards M31, retain the name 'North West Stream'. [ABSTRACT FROM AUTHOR]

The process of galaxy cluster formation likely leaves an imprint on the properties of its individual member galaxies. Understanding this process is essential for uncovering the evolutionary connections between galaxies and cosmic structures. Here, we study a sample of 10 protoclusters at |$z\sim 2$| –3 in different dynamical states that we estimate based on spectroscopic data of their members. We combine the dynamical information with Hubble Space Telescope imaging to measure galaxy sizes and pair fractions. Our analysis reveals a clear anticorrelation between the velocity dispersion of the protocluster and its galaxy pair fractions (indicative of merger rates). The velocity dispersion also anticorrelates with the dispersion in size among the member galaxies. These correlations may be explained by protoclusters in colder dynamical states maintaining a velocity dispersion and galaxy number density that boosts galaxy mergers, which in turn contributes to the structural expansion and compaction of galaxies. Our findings offer constraints for cosmological models regarding the evolution of galaxy morphology across different stages in the assembly of protoclusters. [ABSTRACT FROM AUTHOR]

To test the scenario that outflows accelerated by active galactic nuclei (AGN) have a major impact on galaxy-wide scales, we have analysed deep Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) data for the type-2 quasar/ultraluminous infrared galaxy F13451+1232 – an object that represents the major mergers considered in some models of galaxy evolution. After carefully accounting for the effects of atmospheric seeing that had smeared the emission from known compact nuclear outflows across the MUSE field of view, we find that the large-scale kinematics in F13451+1232 are consistent with gravitational motions that are expected in a galaxy merger. Therefore, the fast (⁠|$\mathrm{W_{80}}\gt 500$|  km s |$^{-1}$|⁠) warm-ionized AGN-driven outflows in this object are limited to the central |$\sim$| 100 pc of the galaxy, although we cannot rule out larger scale, lower velocity outflows. Moreover, we directly demonstrate that failure to account for the beam-smearing effects of atmospheric seeing would have led to the mass outflow rates and kinetic powers of spatially extended emission being overestimated by orders of magnitude. We also show that beam-smeared compact-outflow emission can be significant beyond radial distances of 3.5 arcsec (more than 8 times the radius of the seeing disc), and support the argument that some previous claims of large-scale outflows in active galaxies were likely the result of this effect rather than genuine galaxy-wide (⁠|$r\gt 5$|  kpc) outflows. Our study therefore provides further evidence that warm-ionized AGN-driven outflows are limited to the central kiloparsecs of galaxies and highlights the critical importance of accounting for atmospheric seeing in ground-based observational studies of active galaxies. [ABSTRACT FROM AUTHOR]

We combine X-ray data from the X-ray Multi-Mirror Mission 12th data release (4XMM DR12) and the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) Final Equatorial-Depth Survey (eFEDS) early data release catalogues with the Galaxy and Mass Assembly spectroscopic galaxy survey to form a sample of 526 local (⁠|$0.05\le z \le 0.3$|⁠), moderately luminous active galactic nuclei (AGNs; |$L_{\rm X} \ge 10^{42}$| erg s-1). We compare the distribution of AGN hosts with that of the general galaxy population, finding no evidence of AGNs being preferentially hosted within over/underdense environments at a 7-Mpc scale. Next, we compare the distances to nearest-neighbour galaxies and find that AGN host galaxies are separated from their neighbours by the same distance as galaxies in a matched non-AGN sample. Finally, we consider mid-IR colour, provided by the All-WISE survey, the |$L_{\rm X}/L_{6\, \mu \mathrm{m}}$| ratio, and X-ray hardness as tracers of AGN obscuration, to explore whether there is a change in the galaxy environment for obscured AGNs. We find no evidence of increasing AGN obscuration with decreasing separation in close galaxy pairs. We conclude that close galaxy–galaxy interactions may not be sufficiently disruptive to cause any observable increase in AGN activity. [ABSTRACT FROM AUTHOR]

The intracluster medium (ICM) today is comprised largely of hot gas with clouds of cooler gas of unknown origin and lifespan. We analyse the evolution of cool gas (temperatures |$\lesssim 10^{4.5}$|  K) in the ICM of 352 galaxy clusters from the TNG-Cluster simulations, with present-day mass |$\sim 10^{14.3-15.4}\, {\rm M}_\odot$|⁠. We follow the main progenitors of these clusters over the past |$\sim 13$| billion years (since |$z\lesssim 7$|⁠) and find that, according to TNG-Cluster, the cool ICM mass increases with redshift at fixed cluster mass, implying that this cooler past of the ICM is due to more than just halo growth. The cool cluster gas at |$z\lesssim 0.5$| is mostly located in and around satellite galaxies, while at |$z\gtrsim 2$| cool gas can also accrete via filaments from the intergalactic medium. Lower-mass and higher-redshift clusters are more susceptible to cooling. The cool ICM mass correlates with the number of gaseous satellites and inversely with the central supermassive black hole (SMBH) mass. The average number of gaseous satellites decreases since |$z=2$|⁠ , correlating with the decline in the cool ICM mass over cosmic time, suggesting a link between the two. Concurrently, kinetic SMBH feedback shifts the ICM temperature distribution, decreasing the cool ICM mass inside–out. At |$z\approx 0.5$|⁠ , the predicted Mg  ii column densities are in the ballpark of recent observations, where satellites and other haloes contribute significantly to the total Mg  ii column density. Suggestively, a non-negligible amount of the ICM cool gas forms stars in situ at early times, reaching |$\sim 10^{2}\, {\rm M}_\odot \, {\rm ~yr^{-1}}$| and an H  |$\alpha$| surface brightness of |$\sim 10^{-17}\, {\rm ~erg\, s^{-1}\, cm^{-2}\, arcsec^{-2}}$| at |$z\approx 2$|⁠ , detectable with Euclid and JWST. [ABSTRACT FROM AUTHOR]

The stellar stripping of satellites in cluster haloes is understood to play an important role in the production of intracluster light. Increasingly, cosmological simulations have been utilized to investigate its origin and assembly. However, such simulations typically model individual galaxies at relatively coarse resolutions, raising concerns about their accuracy. Although there is a growing literature on the importance of numerical resolution for the accurate recovery of the mass-loss rates of dark matter (DM) haloes, there has been no comparable investigation into the numerical resolution required to accurately recover stellar mass-loss rates in galaxy clusters. Using N -body simulations of satellite galaxies orbiting in a cluster halo represented by a static external potential, we conduct a set of convergence tests in order to explore the role of numerical resolution and force softening length on stellar stripping efficiency. We consider a number of orbital configurations, satellite masses, and satellite morphologies. We find that stellar mass resolution is of minor importance relative to DM resolution. Resolving the central regions of satellite DM haloes is critical to accurately recover stellar mass-loss rates. Poorly resolved DM haloes develop cored inner profiles and, if this core is of comparable size to the stellar component of the satellite galaxy, this leads to significant overstripping. To prevent this, relatively high DM mass resolutions of around |$M_{\rm DM}\sim 10^{6}$|  M |$_{\odot }$|⁠ , better than those achieved by many contemporary cosmological simulations, are necessary. [ABSTRACT FROM AUTHOR]

We present a detailed study of the large-scale shock front in Stephan's Quintet, a by-product of past and ongoing interactions. Using integral-field spectroscopy from the new William Herschel Telescope Enhanced Area Velocity Explorer (WEAVE), recent 144 MHz observations from the LOFAR Two-metre Sky Survey, and archival data from the Very Large Array and JWST , we obtain new measurements of key shock properties and determine its impact on the system. Harnessing the WEAVE large integral field unit's field of view (90 |$\times$| 78 arcsec |$^{2}$|⁠), spectral resolution (⁠|$R\sim 2500$|⁠), and continuous wavelength coverage across the optical band, we perform robust emission-line modelling and dynamically locate the shock within the multiphase intergalactic medium with higher precision than previously possible. The shocking of the cold gas phase is hypersonic, and comparisons with shock models show that it can readily account for the observed emission-line ratios. In contrast, we demonstrate that the shock is relatively weak in the hot plasma visible in X-rays (with Mach number of |$\mathcal {M}\sim 2\!-\!4$|⁠), making it inefficient at producing the relativistic particles needed to explain the observed synchrotron emission. Instead, we propose that it has led to an adiabatic compression of the medium, which has increased the radio luminosity 10-fold. Comparison of the Balmer line-derived extinction map with the molecular gas and hot dust observed with JWST suggests that pre-existing dust may have survived the collision, allowing the condensation of H |$_2$| – a key channel for dissipating the shock energy. [ABSTRACT FROM AUTHOR]

We present the findings of a comprehensive and detailed analysis of merger tree data from ultra-high-resolution cosmological |$N$| -body simulations. The analysis, conducted with a particle mass resolution of |$5 \times 10^3\, h^{-1}\, M_{\odot }$| and a halo mass resolution of |$10^7\, h^{-1}\, M_{\odot }$|⁠ , provides sufficient accuracy to suppress numerical artefacts. This study elucidates the dynamical evolution of subhaloes associated with the Milky Way-like host haloes. Unlike more massive dark matter haloes, which have been extensively studied, these subhaloes follow a distinct mass evolution pattern: an initial accretion phase, followed by a tidal stripping phase where mass is lost due to the tidal forces of the host halo. The transition from accretion to stripping, where subhaloes reach their maximum mass, occurs around a redshift of |$z\simeq 1$|⁠. Smaller subhaloes reach this point earlier, while larger ones do so later. Our analysis reveals that over 80% of subhaloes have experienced mass loss, underscoring the universality of tidal stripping in subhalo evolution. Additionally, we derived the eccentricities and pericentre distances of subhalo orbits from the simulations and compare them with those of nearby satellite galaxies observed by the Gaia satellite. The results demonstrate a significant alignment between the orbital elements predicted by the cold dark matter model and the observed data, providing robust support for the model as a credible candidate for dark matter. [ABSTRACT FROM AUTHOR]

In a recent series of papers, supermassive black holes were used to discern pathways in galaxy evolution. By considering the black holes' coupling with their host galaxy's bulge/spheroid, the progression of mass within each component has shed light on the chronological sequence of galaxy speciation. Offsets between the galaxy-morphology-dependent |$M_{\rm bh}$| – |$M_{\rm \star ,sph}$| scaling relations trace a pattern of 'punctuated equilibrium' arising from merger-driven transitions between galaxy types, such as from spirals to dust-rich lenticulars and further to 'ellicular' and elliptical galaxies. This study delves deeper into the distinction between the ellicular galaxies – characterised by their intermediate-scale discs – and elliptical galaxies. Along the way, it is shown how some antitruncated large-scale discs in lenticular galaxies can arise from the coexistence of a steep intermediate-scale disc and a relatively shallow large-scale disc. This observation undermines application of the popular exponential-disc plus Sérsic-bulge model for lenticular galaxies and suggests some past bulge mass measurements have been overestimated. Furthermore, it is discussed how merger-driven disc-heating and blending likely leads to the spheroidalisation of discs and the conglomeration of multiple discs leads to the (high- n) Sérsicification of light profiles. The ellicular and elliptical galaxy distribution in the |$M_{\rm bh}$| – |$M_{\rm \star ,sph}$| diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The super-quadratic |$M_{\rm bh}$| – |$M_{\rm \star }$| relations, presented herein, for merger-built systems should aid studies of massive black hole collisions and the gravitational wave background. Finally, connections to dwarf compact elliptical and ultracompact dwarf galaxies, with their 100–1000 times higher |$M_{\rm bh}/M_{\rm \star ,sph}$| ratios, are presented. [ABSTRACT FROM AUTHOR]

The formation of dark matter-deficient galaxies (DMDGs) through tidal interactions has been a subject of growing interest, particularly with the discovery of galaxies such as NGC 1052-DF2. Previous studies suggested that strong tidal forces could strip dark matter (DM) from satellite galaxies, but the role of dynamical friction in this process has been largely overlooked. In this paper, we present self-consistent N -body simulations that incorporate the effects of dynamical friction on the tidal formation of NGC 1052-DF2, and compare them with the one without dynamical friction. We find that dynamical friction significantly accelerates the decay of the satellite galaxy's orbit, causing it to experience more frequent tidal stripping and leading to the earlier formation of a DM-deficient state, approximately |$7\!-\!8$|  Gyr after infall. This is a few Gyr earlier than simulations without dynamical friction. Our results suggest that DMDGs can form in a wider range of orbital configurations, particularly on more circular orbits, than previously thought. Furthermore, we find that globular clusters in the DM-deficient phase exhibit elevated velocity dispersion, providing an observational signature of this evolutionary stage. We also examine the evolution of satellite in the phase space of total energy versus angular momentum, and show that a vertically narrow feature in this phase space is a clear signature of pericentre passage. These findings broaden the understanding of how DMDGs form and highlight the critical role of dynamical friction in shaping the evolutionary history of satellite galaxies in massive haloes. [ABSTRACT FROM AUTHOR]

We used Five-hundred-meter Aperture Spherical radio Telescope (FAST) to conduct deep H  i imaging of the entire M 106 group region, and have discovered a few new H  i filaments and clouds. Three H  i clouds/filaments are found in a region connecting DDO 120 and NGC 4288, indicating an interaction between these two galaxies. The H  i features in this region suggest that DDO 120 is probably the origin of the H  i stream extending from the northern end of NGC 4288 to M 106. This structure is similar to the Small–Large Magellanic Cloud stream, but much longer, about 190 kpc. Furthermore, based on the distance measurements, we have determined the satellite galaxy members of M 106. With an absolute magnitude cutoff of |$M_{B}$|  = −10, we obtained a sample of 11 member satellite galaxies for M 106. Using the observed H  i mass with FAST, we studied the properties of satellite galaxies in M 106 and found that satellite galaxies with lower stellar masses exhibit more significant deviations from the star-forming main sequence in their specific star formation rates. Furthermore, the relationship between the H  i mass of satellite galaxies and optical diameter generally follows the field galaxies relation. We discuss the possible mechanisms leading to the quenching in the M 106 group based on the new data from FAST. [ABSTRACT FROM AUTHOR]

Aims. We present the discovery of an extended and faint tail observed in the isolated environment associated with galaxy NGC 3785. This study additionally provides observational evidence supporting the formation of ultra-diffuse galaxies (UDGs) at the end of the tail. Methods. We utilized the Gnuastro software to detect and analyze the low surface brightness structures in the optical g- and r-bands using data from the Dark Energy Camera Legacy Survey. We created a detection map to identify the faint tail and measured its length using cubic spline fitting. Additionally, we found 84 star-forming clumps along the tail and performed photometric analysis on the tail portion after applying a significance threshold on the signal-to-noise ratio. Results. We have measured the projected length of the tail, which is ∼390 kpc. We propose that this tail arises from the interaction of the NGC 3785 with a gas-rich galaxy, which ends up as a UDG at the end of the tail. [ABSTRACT FROM AUTHOR]

Context. Stellar tidal streams are a key tracer of galaxy evolution and have the potential to provide an indirect means for tracing dark matter. For the Local Group, many diffuse substructures have been identified and their link to galaxy evolution has been traced. However, the Local Group does not offer a statistically significant sample of stellar tidal streams. Thus, an analysis of a larger sample beyond the Local Group is required to better probe the frequency and characteristics of these streams to verify whether these properties are in agreement with the predictions of the ΛCDM model and its implementation in cosmological simulations, taking into account the impact of the baryonic physics modelling. Aims. The main scope of the Stellar Stream Legacy Survey is to obtain a statistically significant sample of stellar streams in the local Universe to be able to trace and study minor mergers and their contribution to galaxy evolution with respect to the ΛCDM theory. For that purpose, we are carrying out the first systematic survey of faint stellar debris from tidally disrupted dwarf satellites around nearby galaxies up to a distance of 100 Mpc. Methods. In this paper, we present a catalogue with the results of the first harvest of stellar tidal streams found by visual inspection in deep images of ∼700 galaxies from the Dark Energy Survey (DES). We also include, for the first time, a photometric characterisation of the streams obtained by measuring their surface brightnesses and colours. Results. We found a total of 63 streams in our sample at distances between 40 and 100 Mpc, including 58 that were not previously reported. We measured their average surface brightness for the g band, the r band and the z band, to be 28.35 ± 0.20, 27.81 ± 0.13, and 27.62 ± 0.09 mag arcsec−2, respectively. By applying a statistical analysis to our findings, we obtained a stream detection frequency of 9.1% ± 1.1% for the given surface brightness limit of the DES image sample, in agreement with previous studies. We identified stream progenitors in 5–14% of our stream sample, depending on the confidence level. Conclusions. The first catalogue of streams in the local Universe presented here will be complemented by future stream surveys within the Stellar Stream Legacy Survey and can be exploited in studies pertaining to galaxy evolution and cosmological models. In this work, we have learnt that the faintest measured stream surface brightness can be significantly brighter than the surface brightness limit of an image measured at the pixel level (in our case up to ∼1 mag arcsec−2 for the r band) mainly due to correlated noise present in the images. [ABSTRACT FROM AUTHOR]

Context. Near- to mid-infrared observations (from Spitzer and JWST) have revealed a hidden population of galaxies at redshift z = 3 − 6 called optically dark objects, which are believed to be massive and dusty star-formers. They contribute substantially to the cosmic star-formation rate (SFR) density at z ∼ 4 − 5 (up to 30 − 40%). Aims. While optically dark sources are widely recognized as a significant component of the stellar mass function, the history of their stellar mass assembly (and the evolution of their interstellar medium) remains unexplored. However, they are thought to be the progenitors of the more massive early-type galaxies found in present-day groups and clusters. It is thus important to examine the possible connection between dark sources and merging events in order to understand the environment in which they live. Methods. Here, we report our search for close companions in a sample of 19 optically-dark objects identified in the SMACS0723 JWST deep field. They were selected in the NIRCam F444W band and undetected below 2 μm. We restricted our analysis to the reddest (i.e., F277W–F444W > 1.3) and brightest (F444W < 26 mag) objects. Results. We identified KLAMA, an optically dark source showing a very close companion (angular distance < 0.5″). The spatially resolved SED fitting procedure indicates that all components lying within 1.5″ of who is it the dark source are indeed at z ∼ 5.7. Tidal features (leading to a whale-shaped morphology) corroborate the hypothesis that KLAMA is the most massive (log(M⋆/M⊙) > 10.3) and dusty (AV ∼ 3 at the core) system of an ongoing merger with a mass ratio of ∼10. Thus, around ten similar merger events would be required to double the stellar mass of KLAMA. Merging systems with properties similar to KLAMA are identified in the SERRA simulations, allowing us to reconstruct their stellar-mass assembly history and predict their molecular gas properties (in particular, the [CII] emission for the simulated system). Conclusions. The discovery of mergers within dark galaxies at the end of the Epoch of Reionization highlights the importance of conducting a statistical search for additional candidates in deep NIRCam fields. Such research will aid in our understanding of the significance of merging processes during the obscured phase of stellar-mass accumulation. [ABSTRACT FROM AUTHOR]

Context. It is now well known that certain massive galaxies undergo enormous enhancements in their star formation rate (SFR) when they undergo major mergers. These enhancements can be as high as 100 times the SFR of unperturbed galaxies of the same stellar mass. Previous works have found that the size of this boost in star formation (SF) is related to the morphology of and the proximity to the companion. The same trend has also been observed for the fraction of active galactic nuclei (AGN), where galaxies that are closer together tend to have higher AGN fractions. Aims. We aim to analyse the SF enhancement and AGN fraction evolution during the merger process by using a more timeline-like merger sequence. Additionally, we aim to determine the relation between the SF enhancement in mergers and the morphology of the galaxies involved. Methods. Taking advantage of the stellar masses (M*) and SFRs of the ∼600 nearby isolated mergers obtained in our previous study, we calculated the distance of each of our galaxies from the star-forming main sequence (MS; specific SFR (sSFR)/sSFRMS), which werefer to as the SF mode. We then analysed how the SF mode varies during the merger process as a function of morphology and M*. Additionally, we analysed the AGN content of our mergers, using multiple diagnostics based on emission line ratios and WISE colours. Results. We observed that, overall, merging galaxies show an SF mode that is governed by their morphology. Spirals typically show high SF mode values, while highly disturbed (HD) galaxies are generally even more enhanced (median values of +0.8 dex and +1.08 dex above the MS, respectively). In contrast, elliptical and lenticular galaxies show the lowest SF modes, as expected. However, even they show SF enhancement compared to their unperturbed counterparts. For example, their median SF mode is just within the 1-sigma scatter of the MS, and this can occur even before the galaxies have coalesced. We observed a trend for the SF mode to gradually increase with increasing merger stage. We did not find a clear dependency of the observed AGN fraction on the merger stage for the majority of our classification methods. Conclusions. We find mergers can significantly enhance SF in galaxies of all morphologies. For early-type galaxies, this could suggest that some gas was present prior to the merger, which may be triggered to form stars by the tidal interaction. As the SF enhancement continues throughout the merger process, this suggests that the enhancement may be a long-lived event, contrary to the short starbursts seen in some models. [ABSTRACT FROM AUTHOR]

We present new deep, wide-field imaging data from the Large Binocular Telescope (LBT) in g and r bands from the Smallest Scale of Hierarchy Survey (SSH) that reveal previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range 1.2 × 107 M⊙ to 1.4 × 108 M⊙. The six dwarfs are located 1-2 Mpc away from large galaxies, which implies that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the distances of ~3-4 Mpc at which the dwarfs lie, the identified tidal features are all resolved into individual stars in the LBT images and appear to consist of a population older than 1–2 Gyr. This excludes the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we witness the hierarchical merging assembly of these dwarfs with their satellite populations. This scenario is also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample, we estimate that a fraction of ~13% of the late-type dwarfs show signs of merging with satellites. This is in agreement with other recent independent studies and theoretical predictions within the ΛCDM cosmological framework. [ABSTRACT FROM AUTHOR]

Studies have shown that the morphologies of galaxies are substantially transformed following coalescence after a merger, but post-mergers are notoriously difficult to identify, especially in imaging that is shallow or low resolution. We train convolutional neural networks (CNNs) to identify simulated post-merger galaxies in a range of image qualities, modelled after five real surveys: the Sloan Digital Sky Survey (SDSS), the Dark Energy Camera Legacy Survey (DECaLS), the Canada–France Imaging Survey (CFIS), the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), and the Legacy Survey of Space and Time (LSST). Holding constant all variables other than imaging quality, we present the performance of the CNNs on reserved test set data for each image quality. The success of CNNs on a given data set is found to be sensitive to both imaging depth and resolution. We find that post-merger recovery generally increases with depth, but that limiting 5 |$\sigma$| point-source depths in excess of |$\sim 25$| mag, similar to what is achieved in CFIS, are only marginally beneficial. Finally, we present the results of a cross-survey inference experiment, and find that CNNs trained on a given image quality can sometimes be applied to different imaging data to good effect. The work presented here therefore represents a useful reference for the application of CNNs for merger searches in both current and future imaging surveys. [ABSTRACT FROM AUTHOR]

In this paper we explore the properties of the |$z=6.853$| obscured hyperluminous quasar COS-87259, discovered in the Cosmological Evolution Survey (COSMOS) field, with our recently developed Bayesian spectral energy distribution (SED) fitting code SMART (Spectral energy distributions Markov chain Analysis with Radiative Transfer models). SMART fits SEDs exclusively with multicomponent radiative transfer models that constitute four different types of pre-computed libraries for the active galactic nucleus (AGN) torus, the starburst, and the spheroidal or disc host. We explore two smooth radiative transfer models for the AGN torus and two two-phase models, in order to put constraints on the AGN fraction of the galaxy, the black hole mass, and its star formation rate (SFR). We find that either of the smooth tapered disc or the two-phase flared disc models provide a good fit to the SED of COS-87259. The best-fitting models predict an AGN fraction in the range |$86-92$| per cent, a bolometric AGN luminosity of |$5.8-10.3 \times 10^{13} L_\odot$|⁠ , a black hole mass of |$1.8-3.2 \times 10^{9} M_\odot$| (assuming the quasar is accreting at the Eddington limit), and an SFR in the range |$1985-2001~M_\odot \rm yr^{-1}$|⁠. The predicted space density of such objects in the redshift range |$4-7$| is |$\sim 20$| times more than that of co-eval unobscured quasars. [ABSTRACT FROM AUTHOR]

Interaction between galaxies plays a pivotal role in their evolution. Ongoing star formation in spiral galaxies can be affected by these processes. Interacting galaxy pairs provide an opportunity to study these effects. We select a sample of interacting galaxies in field environments at various interaction stages and are nearly face-on and chose galaxy pairs NGC 2207/IC 2163, NGC 4017/4016 (ARP 305), and NGC 7753/7752 (ARP 86). We use the UltraViolet Imaging Telescope (UVIT) onboard AstroSat to characterize the star-forming regions in the galaxy with a superior resolution of |$\mathrm{\sim 1.4\,\,\mathrm{ arcsec}}$|⁠. We identified and characterized star-forming regions in the UVIT images of the sample and correlated them with the neutral hydrogen (H  i) distribution. We detected localized regions of enhancement in star formation surface density (⁠|$\mathrm{\Sigma _{SFR}}$|⁠) and distortions in the sample of galaxies. We found this consistent with the distribution of H  i in the galaxy. These are possible evidence of past and ongoing interactions affecting the star formation properties in the galaxies. We then conducted a study to understand whether the observed enhancements hold true for a wider sample of interacting galaxies. We observe a moderate enhancement in the star formation rate (SFR) with the interaction class, with a maximum of 1.8 being in the merger class of galaxies. We studied the SFR enhancement for the main galaxies in our sample as a function of pair mass ratio and pair separation. We observe a strong anticorrelation between the SFR enhancement and pair mass ratio and no linear correlation between the enhancement and pair separation. This suggests that the enhancement in interaction-induced star formation may be more strongly influenced by the pair mass ratios, rather than the pair separation. We also infer that the pair separation can possibly act as a limiting parameter for the SFR enhancement. [ABSTRACT FROM AUTHOR]

Cloud–cloud collisions in splash bridges produced in gas-rich disc galaxy collisions offer a brief but interesting environment to study the effects of shocks and turbulence on star formation rates in the diffuse intergalactic medium, far from the significant feedback effects of massive star formation and active galactic nucleus. Expanding on our earlier work, we describe simulated collisions between counter-rotating disc galaxies of relatively similar mass, focusing on the thermal and kinematic effects of relative inclination and disc offset at the closest approach. This includes essential heating and cooling signatures, which go some way towards explaining the luminous power in H |$_2$| and [C  ii ] emission in the Taffy bridge, as well as providing a partial explanation of the turbulent nature of the recently observed compact CO-emitting clouds observed in Taffy by the Atacama Large Millimeter Array (ALMA). The models show counter-rotating disc collisions result in swirling, shearing kinematics for the gas in much of the post-collision bridge. Gas with little specific angular momentum due to collisions between counter-rotating streams accumulates near the centre of mass. The disturbances and mixing in the bridge drive continuing cloud collisions, differential shock heating, and cooling throughout. A wide range of relative gas phases and line-of-sight velocity distributions are found in the bridges, depending sensitively on initial disc orientations, and the resulting variety of cloud collision histories. Most cloud collisions can occur promptly or persist for quite a long duration. Cold and hot phases can largely overlap throughout the bridge or can be separated into different parts of the bridge. [ABSTRACT FROM AUTHOR]

Tidal features are a key observable prediction of the hierarchical model of galaxy formation and contain a wealth of information about the properties and history of a galaxy. Modern wide-field surveys such as LSST and Euclid will revolutionize the study of tidal features. However, the volume of data will prohibit visual inspection to identify features, thereby motivating a need to develop automated detection methods. This paper presents a visual classification of ∼2000 galaxies from the DECaLS survey into different tidal feature categories: arms, streams, shells , and diffuse. We trained a convolutional neural network (CNN) to reproduce the assigned visual classifications using these labels. Evaluated on a testing set where galaxies with tidal features were outnumbered |$\sim 1:10$|⁠ , our network performed very well and retrieved a median |$98.7\pm 0.3$|⁠ , |$99.1\pm 0.5$|⁠ , |$97.0\pm 0.8$|⁠ , and |$99.4^{+0.2}_{-0.6}$|  per  cent of the actual instances of arm, stream, shell , and diffuse features respectively for just 20 per cent contamination. A modified version that identified galaxies with any feature against those without achieved scores of |$0.981^{+0.001}_{-0.003}$|⁠ , |$0.834^{+0.014}_{-0.026}$|⁠ , |$0.974^{+0.008}_{-0.004}$|⁠ , and |$0.900^{+0.073}_{-0.015}$| for the accuracy, precision, recall, and F1 metrics, respectively. We used a gradient-weighted class activation mapping analysis to highlight important regions on images for a given classification to verify the network was classifying the galaxies correctly. This is the first demonstration of using CNNs to classify tidal features into sub-categories, and it will pave the way for the identification of different categories of tidal features in the vast samples of galaxies that forthcoming wide-field surveys will deliver. [ABSTRACT FROM AUTHOR]

We present a deep H  i survey at L band conducted with the Five-hundred-metre Aperture Spherical radio Telescope (FAST) over the COSMOS field. This survey is strategically designed to overlap with the MIGHTEE COSMOS field, aiming to combine the sensitivity of the FAST and high-resolution of the MeerKAT. We observed the field with FAST for approximately 11 hours covering |${\sim} 2$| square degrees, and reduced the raw data to H  i spectral cubes over the frequency range 1310–1420 MHz. The FAST-H  i data reach a median 3 |$\sigma$| column density of |$N_{\rm {H\,{{\small I}}}} \sim 2\times 10^{17}$|  cm |$^{-2}$| over a |${\sim} 5\, {\rm km}\, {\rm s}^{-1}$| channel width, allowing for studies of the distribution of H  i gas in various environments, such as in galaxies, the Circum-Galactic Medium (CGM) and Intergalactic Medium (IGM). We visually searched the spectral cubes for H  i sources, and found a total of 80 H  i detections, of which 56 have been cross-matched with the MIGHTEE-H  i catalogue. With the cross-matched sources, we compare their H  i masses and find that the total H  i mass fraction in the IGM and CGM surrounding the galaxy pairs is statistically higher than the H  i fraction surrounding the isolated galaxies by a difference of |${\sim} 13\pm 4$| per cent, indicating that the CGM and IGM associated with interacting systems are richer in neutral hydrogen compared to those around isolated galaxies in the local Universe. We also describe several FAST-MeerKAT synergy projects, highlighting the full potential of exploiting both single-dish and interferometric observations to study the distribution and evolution of the diffuse H  i gas. [ABSTRACT FROM AUTHOR]

We study the major merger fraction along the massive galaxy-quenching channel (traced with rest-frame |$\mathrm{NUV}-r$| color) at z = 0.2–0.7, aiming to examine the cosmic web detachment (CWD) scenario of galaxy quenching. In this scenario, the major merger fraction is expected to be high in green-valley galaxies as compared with those in star-forming and quiescent galaxies of similar stellar mass. We used photometry in the E-COSMOS field to select 1491 (2334) massive (⁠|$M_\ast \\gt 10^{9.5}\, M_{\odot }$|⁠) galaxies with |$m_i\\lt 22$|  mag (⁠|$m_z\\lt 22$|  mag) at z = 0.2–0.4 (z = 0.4–0.7) in the rest-frame color range of |$0.8\\lt r-K_{\rm s}\\lt 1.3$|⁠. We define a major galaxy–galaxy merger as a galaxy pair of comparable angular size and luminosity with tidal tails or bridges, and we identified such major mergers through visual inspection of Subaru HSC-SSP PDR 2 i - and z -band images. We classify 92 (123) galaxies as major merger galaxies at z = 0.2–0.4 (z = 0.4–0.7). The resulting major merger fraction is 5%–6% and this fraction does not change with galaxy color along the massive galaxy-quenching channel. The result is not consistent with the expectation based of the CWD scenario as the dominant mechanism of massive galaxy quenching. However, there are some caveats such as (i) the mergers that cause quenching may lose their visible merger signatures rapidly before they enter the green valley, (ii) our method may not trace the cosmic web sufficiently well, and (iii) because of our mass limit, most of the galaxies in our sample may have already experienced CWD events at higher redshifts than those studied here. Further studies with deeper data are desirable in the future. [ABSTRACT FROM AUTHOR]

A comprehensive analysis of barred galaxies in triplet systems, drawn from the SDSS-based catalog of isolated galaxy triplets composed of 315 triplet systems (945 galaxy members) is presented. The primary objective for this study is to investigate the bar fraction ( f b a r ) within these systems and explore its correlation with various intrinsic and extrinsic properties of the triplets. Our final sample, after excluding highly inclined and merging galaxies, comprises 427 galaxy members embedded in 232 triplet systems. The final sample reveals a bar fraction of 42 % with a bar fraction that significantly increases in nearby systems (with smaller redshift) characterized by higher virial mass ( M v i r ), lower radial velocities (σ ) among their members, and larger angular separations ( r p ) and harmonic projected distances ( r h ) between the triplet members. Additionally, it has been noticed that the large-scale structure (LSS) influences the bar fraction, with f b a r decreasing as the distance from the primary galaxy (G1) to the first neighborhood increases ( d N N ). Furthermore, f b a r decreases as tidal strength generated by the galaxies in the L S S ( Q L S S )increases. These findings suggest that both the dynamical environment and the local interactions within triplet systems significantly impact the abundance of barred structures. This study provides valuable insights into the role of environmental factors in shaping galaxy morphology, particularly in systems with complex gravitational interactions. [ABSTRACT FROM AUTHOR]

We present an analysis of the galaxy merger rate in the redshift range |$4.0\lt z\lt 9.0$| (i.e. about 1.5 to 0.5 Gyr after the big bang) based on visually identified galaxy mergers from morphological parameter analysis. Our data set is based on high-resolution NIRCam JWST data (a combination of F 150 W and F 2000 W broad-band filters) in the low-to-moderate magnification (⁠|$\mu \lt 2$|⁠) regions of the Abell 2744 cluster field. From a parent set of 675 galaxies |$(M_{U}\in [-26.6,-17.9])$|⁠ , we identify 64 merger candidates from the Gini, |$M_{20}$| and asymmetry morphological parameters, leading to a merger fraction |$f_m=0.11\pm 0.04$|⁠. There is no evidence of redshift evolution of |$f_m$| even at the highest redshift considered, thus extending well into the epoch of reionization the constant trend seen previously at |$z\lesssim 6$|⁠. Furthermore, we investigate any potential redshift-dependent differences in the specific star formation rates between mergers and non-mergers. Our analysis reveals no significant correlation in this regard, with deviations in the studied redshift range typically falling within |$(1-1.5)\sigma$| from the null hypothesis that can be attributed to sample variance and measurement errors. Finally, we also demonstrate that the classification of a merging system is robust with respect to the observed (and equivalently rest frame) wavelength of the high-quality JWST broad-band images used. This preliminary study highlights the potential for progress in quantifying galaxy assembly through mergers during the epoch of reionization, with significant sample size growth expected from upcoming large JWST infrared imaging data sets. [ABSTRACT FROM AUTHOR]

Major mergers between galaxies are predicted to fuel their central supermassive black holes (SMBHs), particularly after coalescence. However, determining the prevalence of active galactic nuclei (AGNs) in mergers remains a challenge, because AGN diagnostics are sensitive to details of the central structure (e.g. nuclear gas clouds, geometry, and orientation of a dusty torus) that are partly decoupled from SMBH accretion. X-rays, expected to be ubiquitous among accreting systems, are detectable through non-Compton-thick screens of obscuring material, and thus offer the potential for a more complete assessment of AGNs in mergers. But extant statistical X-ray studies of AGNs in mergers have been limited by either sparse, heterogeneous, or shallow on-sky coverage. We use new X-ray observations from the first SRG/eROSITA all-sky data release to characterize the incidence, luminosity, and observability of AGNs in mergers. Combining machine learning and visual classification, we identify 923 post-mergers in Dark Energy Camera Legacy Survey (DECaLS) imaging and select 4565 interacting galaxy pairs (with separations <120 kpc and mass ratios within 1:10) from the Sloan Digital Sky Survey. We find that galaxies with X-ray AGNs are 2.0 |$\pm$| 0.24 times as likely to be identified as post-mergers compared to non-AGN controls, and that post-mergers are 1.8 |$\pm$| 0.1 times as likely to host an X-ray AGN as non-interacting controls. A multiwavelength census of X-ray, optical, and mid-IR-selected AGNs suggests a picture wherein the underlying AGN fraction increases during pair-phase interactions, that galaxy pairs within ~20 kpc become heavily obscured, and that the obscuration often clears post-coalescence. [ABSTRACT FROM AUTHOR]

Merging and interactions can radically transform galaxies. However, identifying these events based solely on structure is challenging as the status of observed mergers is not easily accessible. Fortunately, cosmological simulations are now able to produce more realistic galaxy morphologies, allowing us to directly trace galaxy transformation throughout the merger sequence. To advance the potential of observational analysis closer to what is possible in simulations, we introduce a supervised deep learning convolutional neural network and vision transformer hybrid framework, Mummi (MU lti M odel M erger I dentifier). Mummi is trained on realism-added synthetic data from IllustrisTNG100-1, and is comprised of a multistep ensemble of models to identify mergers and non-mergers, and to subsequently classify the mergers as interacting pairs or post-mergers. To train this ensemble of models, we generate a large imaging data set of 6.4 million images targeting UNIONS with RealSimCFIS. We show that Mummi offers a significant improvement over many previous machine learning classifiers, achieving 95 per cent pure classifications even at Gyr long time-scales when using a jury-based decision-making process, mitigating class imbalance issues that arise when identifying real galaxy mergers from |$z=0$| to 0.3. Additionally, we can divide the identified mergers into pairs and post-mergers at 96 per cent success rate. We drastically decrease the false positive rate in galaxy merger samples by 75 per cent. By applying Mummi to the UNIONS DR5-SDSS DR7 overlap, we report a catalogue of 13 448 high-confidence galaxy merger candidates. Finally, we demonstrate that Mummi produces powerful representations solely using supervised learning, which can be used to bridge galaxy morphologies in simulations and observations. [ABSTRACT FROM AUTHOR]

New, deep HST photometry allowed us to identify and study eight compact and bright (MV ≤ −5.8) star clusters in the outskirts of the star-forming isolated dwarf galaxy NGC 5238 (M* ≃ 108 M⊙). Five of these clusters are new discoveries, and six appear projected onto and/or aligned with the tidal tails recently discovered around this galaxy. The clusters are partially resolved into stars, and their colour magnitude diagrams reveal a well-developed red giant branch, implying ages older than 1–2 Gyr. Their integrated luminosity and structural parameters are typical of classical globular clusters, and one of them, with MV = −10.56 ± 0.07, is as bright as ω Cen, the brightest globular cluster in the Milky Way. Since the properties of this cluster are in the range spanned by those of nuclear star clusters we suggest that it may be the nuclear remnant of the disrupted satellite of NGC 5238 that produced the observed tidal tails. [ABSTRACT FROM AUTHOR]

Aims. Galaxy interactions can trigger drastic changes in the resolved star-forming properties of their constituents, but it remains unclear as to whether those changes are discernible from secular starburst triggers. In this Letter we investigate whether or not post-merger galaxies create unique star-forming trends on a kiloparsec scale. Methods. We present radial trends in star-formation-rate (SFR) surface density (ΣSFR) for 150 post-merger galaxies with moderate to extremely heightened global SFRs using observations from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. We juxtapose these profiles with those of noninteracting galaxies (excluding both galaxy pairs and post-merger galaxies) with similarly enhanced global SFRs. Results. Post-merger galaxies have a much stronger central starburst than isolated galaxies with similar global star-formation enhancements. Indeed, isolated starburst galaxies (SBs) lack a marked central enhancement and instead show a fairly uniform enhancement in ΣSFR with radius. Moreover, the difference in central star formation between post-merger galaxies and noninteracting galaxies is more radially extended and pronounced when the global enhancement in star formation is larger. We conclude that post-merger galaxies create a unique signature in their resolved star-forming properties that is distinct from secular processes that can trigger similar global SFR enhancements. [ABSTRACT FROM AUTHOR]

Aims. To unveil the influence of galaxy-galaxy interactions on the material transport driven by galactic bars toward the central regions of active galactic nucleus (AGN) galaxies, and to assess the efficiency of the combined mechanisms of interactions and bars in fueling massive black holes, we meticulously examine barred active galaxies in paired systems. Methods. Our study focuses on barred AGN galaxies in pairs with projected separations of rp < 100 kpc h−1 and relative radial velocities of ΔV < 500 km s−1 within z < 0.1, identified by the Sloan Digital Sky Survey (SDSS). To quantify the impact of interactions on material transport by galactic bars, we also constructed a suitable control sample of barred active galaxies without paired companions, matched in redshift, absolute r-band magnitude, stellar mass, color, and stellar age distributions. Additionally, we calculated the structural characteristics of galactic bars through two-dimensional image modeling, considering that bars exhibit a wide range of shapes and sizes, which may influence their ability to channel material. Results. From this study, we clearly found that nuclear activity (derived from the Lum[OIII]) increases as the projected separations between galaxy pair members decrease. Notably, barred AGN galaxies in close pairs (rp ⪅ 25 kpc h−1) exhibit significantly higher nuclear activity compared to galaxies in the control sample. Additionally, barred galaxies with a close pair companion show enhanced nuclear activity across all ranges of luminosity, stellar mass, and color. We also found that barred AGN galaxies with longer bar structures exhibit more efficient nuclear activity compared to those with shorter bars. This trend is especially pronounced in barred AGN galaxies within close pair systems, which show a significant excess of high Lum[OIII] values. Furthermore, we examined the central nuclear activity in barred AGNs undergoing major and minor interactions. Our findings show a clear escalation in nuclear activity as the pair projected separations decrease, particularly pronounced in major systems. Additionally, nuclear activity distributions in barred AGN samples within major and minor pairs exhibit similar trends. However, a significant deviation occurs among barred AGN galaxies in close pair systems within major interactions, showing a substantial excess of high Lum[OIII] values. This result is also reflected in the analysis of the accretion strength onto central black holes. These findings indicate that external perturbations from a nearby galaxy companion can influence gas flows induced by galactic bars, leading to increased nuclear activity in barred AGN galaxies within pair systems. Thus, the coexistence of both – bars and interactions – significantly amplifies central nuclear activity, thereby influencing the accretion processes onto massive black holes. [ABSTRACT FROM AUTHOR]

The observation of a massive galaxy with an extremely low dark matter content (i.e. NGC 1277) has posed questions about how such objects form and evolve in a hierarchical universe. We here report on the finding of several massive, dark matter-deficient galaxies in a set of 324 galaxy clusters theoretically modelled by means of full-physics hydrodynamical simulations. We first focus on two example galaxies selected amongst the most massive and dark matter-deficient ones. By tracing the evolution of these galaxies, we find that their lack of dark matter is a result of multiple pericentre passages. While orbiting their host halo, tidal interactions gradually strip away dark matter while preserving the stellar component. A statistical analysis of all massive satellite galaxies in the simulated clusters shows that the stellar-to-total mass ratio today is strongly influenced by the number of orbits and the distance at pericentres. Galaxies with more orbits and closer pericentres are more dark matter-deficient. Additionally, we find that massive, dark matter-deficient galaxies at the present day are either the remnants of very massive galaxies at infall or former central galaxies of infalling groups. We conclude that such massive yet dark matter-deficient galaxies exist and are natural by-products of typical cluster galaxy evolution, with no specific requirement for an exotic formation scenario. [ABSTRACT FROM AUTHOR]

Context. The Magellanic Clouds (MCs) are the Milky Way's most massive dwarf satellites. As they also represent the closest pair of galaxies in an ongoing tidal interaction while simultaneously infalling into the Milky Way halo, they provide a unique opportunity to study in detail an ongoing three-body encounter. Aims. We present the YMCA (Yes, Magellanic Clouds Again) survey: Probing the outer regions of the Magellanic system with VST, based on deep optical photometry carried out with the VLT Survey Telescope (VST). Methods. The YMCA survey targeted 110 square degrees, in the g and i filters, in the periphery of both MCs, including a long strip in between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The photometry of YMCA is sufficiently deep (50% complete down to g ≃ 23.5 − 24.0 mag) to allow for a detailed analysis of main-sequence stars in regions of the MCs that have remained relatively unexplored at these faint magnitudes. Results. The resulting colour–magnitude diagrams reveal that the outskirts of the MCs are predominantly characterised by intermediate-age and old stellar populations, with limited or negligible evidence of recent star formation. The analysis of the age distribution of star clusters (SCs) within the surveyed area, both already known and newly discovered candidates, hints at a close fly-by between the LMC and SMC that occurred ≃2.5 − 3.0 Gyr ago, in agreement with previous results. We also report the discovery of candidate SCs with ages within the so-called age-gap, thus questioning its real existence. [ABSTRACT FROM AUTHOR]

We present MeerKAT Fornax Survey H I observations of NGC 1427A, a blue irregular galaxy with a stellar mass of ∼2 × 109 M⊙ located near the centre of the Fornax galaxy cluster. Thanks to the excellent resolution (1–6 kpc spatially, 1.4 km s−1 in velocity) and H I column density sensitivity (∼4 × 1019 to ∼1018 cm−2 depending on resolution), our data deliver new insights on the long-debated interaction of this galaxy with the cluster environment. We confirm the presence of a broad, one-sided, starless H I tail stretching from the outer regions of the stellar body and pointing away from the cluster centre. We find the tail to have 50% more H I (4 × 108 M⊙) and to be 3 times longer (70 kpc) than in previous observations. In fact, we detect scattered H I clouds out to 300 kpc from the galaxy in the direction of the tail – possibly the most ancient remnant of the passage of NGC 1427A through the intracluster medium of Fornax. Both the velocity gradient along the H I tail and the peculiar kinematics of H I in the outer region of the stellar body are consistent with the effect of ram pressure given the line-of-sight motion of the galaxy within the cluster. However, several properties cannot be explained solely by ram pressure and suggest an ongoing tidal interaction. This includes: the close match between dense H I and stars within the disturbed stellar body; the abundant kinematically anomalous H I; and the inversion of the H I velocity gradient near the base of the H I tail. We rule out an interaction with the cluster tidal field, and conclude that NGC 1427A is the result of a high-speed galaxy encounter or of a merger started at least 300 Myr ago, where ram pressure shapes the distribution and kinematics of the H I in the perturbed outer stellar body and in the tidal tails. [ABSTRACT FROM AUTHOR]

Context. The standard cosmological scenario predicts a hierarchical formation for galaxies. Many substructures have been found in the Galactic halo, usually identified as clumps in kinematic spaces, like the energy-angular momentum space (E − Lz), under the hypothesis that these quantities should be conserved during the interaction. If these clumps also feature different chemical abundances, such as the metallicity distribution function (MDF), these two arguments together (different kinematic and chemical properties) are often used to motivate their association with distinct and independent merger debris. Aims. The aim of this study is to explore to what extent we can couple kinematic characteristics and metallicities of stars in the Galactic halo to reconstruct the accretion history of the Milky Way (MW). In particular, we want to understand whether different clumps in the E − Lz space with different MDFs should be associated with distinct merger debris. Methods. We analysed dissipationless, self-consistent, high-resolution N-body simulations of a MW-type galaxy accreting a satellite with a mass ratio of 1:10, with different orbital parameters and different metallicity gradients, which were assigned a posteriori. Results. We confirm that accreted stars from a ∼1:10 mass ratio merger event redistribute in a wide range of E and Lz, due to the dynamical friction process, and are thus not associated with a single region. Because satellite stars with different metallicities can be deposited in different regions of the E − Lz space (on average the more metal-rich ones end up more gravitationally bound to the MW), this implies that a single accretion of ∼1:10 can manifest with different MDFs, in different regions of the E − Lz space. Conclusions. Groups of stars with different E, Lz, and metallicities may be interpreted as originating from different satellite galaxies, but our analysis shows that these interpretations are not physically motivated. In fact, as we show, the coupling of kinematic information with MDFs to reconstruct the accretion history of the MW can bias the reconstructed merger tree towards increasing the number of past accretions and decreasing the masses of the progenitor galaxies. [ABSTRACT FROM AUTHOR]

Context. Galaxies evolve through a dynamic exchange of material with their immediate surrounding environment, the so-called circumgalactic medium (CGM). Understanding the physics of gas flows and the nature of the CGM is fundamental to studying galaxy evolution, especially at 4 ≤ z ≤ 6 (i.e., after the Epoch of Reionization) when galaxies rapidly assembled their masses and reached their chemical maturity. Galactic outflows are predicted to enrich the CGM with metals, although it has also been suggested that gas stripping in systems undergoing a major merger may play a role. Aims. In this work, we explore the metal enrichment of the medium around merging galaxies at z ∼ 4.5, observed by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE). To do so, we study the nature of the [CII] 158 μm emission in the CGM around these systems, using simulations to help disentangle the mechanisms contributing to the CGM metal pollution. Methods. By adopting an updated classification of major merger systems in the ALPINE survey, we selected and analyzed merging galaxies whose components can be spatially and/or spectrally resolved in a robust way. This makes it possible to distinguish between the [CII] emission coming from the single components of the system and that coming from the system as a whole. We also made use of the dustyGadget cosmological simulation to select synthetic analogs of observed galaxies and guide the interpretation of the observational results. Results. We find a large diffuse [CII] envelope (≳20 kpc) embedding all the merging systems, with at least 25% of the total [CII] emission coming from the medium between the galaxies. Using predictions from dustyGadget, we suggest that this emission has a multi-fold nature, with dynamical interactions between galaxies playing a major role in stripping the gas and enriching the medium with heavy elements. [ABSTRACT FROM AUTHOR]

Aims. Galaxy mergers represent a fundamental physical process under hierarchical structure formation, but their role in triggering active galactic nuclei (AGNs) is still unclear. We aim to investigate the merger-AGN connection using state-of-the-art observations and novel methods for detecting mergers and AGNs. Methods. We selected stellar mass-limited samples at redshift z < 1 from the Kilo-Degree Survey (KiDS), focussing on the KiDS-N-W2 field with a wide range of multi-wavelength data. We analysed three AGN types, selected in the mid-infrared (MIR), X-ray, and via spectral energy distribution (SED) modelling. To identify mergers, we used convolutional neural networks (CNNs) trained on two cosmological simulations. We created mass- and redshift-matched control samples of non-mergers and non-AGNs. Results. We first investigated the merger-AGN connection using a binary AGN/non-AGN classification. We observed a clear AGN excess (of a factor of 2–3) in mergers with respect to non-mergers for the MIR AGNs, along with a mild excess for the X-ray and SED AGNs. This result indicates that mergers could trigger all three types, but are more connected to the MIR AGNs. About half of the MIR AGNs are in mergers but it is unclear whether mergers are the main trigger. For the X-ray and SED AGNs, mergers are unlikely to be the dominant triggering mechanism. We also explored the connection using the continuous AGN fraction fAGN parameter. Mergers exhibit a clear excess of high fAGN values relative to non-mergers, for all AGN types. We unveil the first merger fraction fmerger − fAGN relation with two distinct regimes. When the AGN is not very dominant, the relation is only mildly increasing or even flat, with the MIR AGNs showing the highest fmerger. In the regime of very dominant AGNs (fAGN ≥ 0.8), fmerger shows the same steeply rising trend with increasing fAGN for all AGN types. These trends are also seen when plotted against AGN bolometric luminosity. We conclude that mergers are most closely connected to dust-obscured AGNs, generally linked to a fast-growing phase of the supermassive black hole. Such mergers therefore stand as the main (or even the sole) fuelling mechanism of the most powerful AGNs. [ABSTRACT FROM AUTHOR]

Markarian 231 (Mrk 231) is one of the closest and brightest ultra-luminous infrared galaxies that is considered to be in a late galaxy merger stage. The optical spectrum of Mrk 231 presents peculiar features and has been the subject of detailed studies. In this paper we study the optical long-slit spectroscopy of the central arcsecond of Mrk 231 as well as six off-nuclear spectral positions observed using the multi-object double spectrographs at the Large Binocular Telescope. Additionally, we carefully examine acquisition images taken before the spectroscopic observations. We find that both the nuclear and off-nuclear spectra of Mrk 231 exhibit broad Hα emission lines, which could be attributed to high-velocity outflows from the nucleus of Mrk 231, an extreme circumnuclear starburst, or a disturbed circumnuclear disc and broad line region associated with the merger of galaxies. The narrow Hα emission line has three components in every spectrum, which could be indicative of low-velocity outflows or a disturbed structure of the narrow-line region. The region to the south of the nucleus exhibits weak Hβ emission, and the regions to the north show no or extremely faint Hβ emission. The continuum has a stronger blue component in the south compared to the north. The central spectrum has prominent broad absorption lines at a redshift of ∼0.0262, which is distinct from the redshift of the emission line system, ∼0.0422. Broad absorption lines belonging to at least three systems are present in all of the off-nuclear spectra, but the regions to the south have their most prominent absorption lines at a redshift of ∼0.0262, and all three systems have absorption lines of similar strengths in the regions to the north. A careful examination of the acquisition images of Mrk 231 shows that there is diffuse emission in the region adjacent to the northern tail, extending in the north-east direction almost orthogonally, which might be an indication of a more complex interaction history. [ABSTRACT FROM AUTHOR]

Understanding the relationship between quenching mechanisms that transform star-forming galaxies into quiescent ones and galaxy properties remains a challenge. We investigate the gas and stellar properties of 69 double-peaked galaxies selected from the Sloan Digital Sky Survey (SDSS) and observed as part of the Mapping Nearby Galaxies at APO (MaNGA) survey to elucidate the mechanisms driving star formation quenching within these systems. We study their star formation activity along with their physical properties: colour, morphology, environment, stellar age, and gas ionisation diagnostics. We find that these 69 double-peaked MaNGA galaxies encompass a higher fraction of galaxies in the green valley, based on the corrected NUV − r colour, compared to our defined control samples of single-peaked emission line galaxies. This green colour traces intermediate stellar populations compared to blue and red galaxies, as indicated by the Dn(4000) index. These green galaxies show no recent star formation bursts within the last 2 Gyr. They exhibit emission line ratios that are suggestive of both ongoing star formation and nuclear activity. They are predominantly located in isolated or small group environments, indicating that internal processes rather than external factors drive their quenching. Moreover, optical emission lines detected throughout a significant area of these systems suggest that gas depletion is unlikely to be the primary quenching mechanism. These findings support a scenario where quenching is instead caused by reduced star formation efficiency. [ABSTRACT FROM AUTHOR]

This observational study investigates the effects of interaction on the kinematics and chemical abundance of the peculiar galaxy ESO 287-IG50, which may be a polar ring galaxy in an ongoing formation process. The study utilized BVRI broad band imagery and longslit spectroscopy in the wavelength range of 4240-8700 Å. The STARLIGHT stellar population synthesis code was used to analyze the data, and standard diagnostic diagrams were employed to classify the main ionizing source of selected emission-line regions. Image analysis using filtering techniques revealed an inner ring with perpendicular structures at both ends, which could be the inner part of a bisymmetric spiral structure. Photometric analysis showed dusty filaments crossing the central structure, which was identified as the redder region of the galaxy, dominated by a non-negligible amount of dust. Shell-like structures, which could be remnants of a galaxy merging process, were also found. Image analysis through filtering revealed what appears to be an inner ring, with perpendicular structures at both ends, which could be the inner part of a bisymmetric spiral structure. Photometric analysis shows dusty filaments crossing the central structure. This region would be dominated by a non negligible amount of dust, identified as the redder region of the galaxy. A heliocentric radial velocity of 17 689±45 km s−1 was measured, and the velocity profile exhibited a clear rotational behavior, with peak velocities of 110 km s−1 to the SW and 80 km s−1 to the other side. The analysis of the nuclear region using the STARLIGHT code revealed a stellar population consisting of approximately one-third young stars and two-thirds old stars. The predominance of an aged stellar population, a distinctive feature in galaxies undergoing interaction processes, can be attributed to the prolonged evolutionary period of this galaxy, as evidenced by the shell structures we identified as indicators of this interaction process. The [NII] λ 6584Å/H α ratio suggests that ESO 287-IG50 may be an AGN due to the excess of Nitrogen relative to Hydrogen in the residual spectrum, a feature not yet reported in the literature. Studying the line ratios and EW(H α ), we notice that this galaxy exhibits a peculiar AGN, with a non-stellar origin ionization mechanism. [ABSTRACT FROM AUTHOR]

In this study, we explore the impact of the galactic interaction/mergers on the central oxygen abundance. We analyse 234 star-forming galaxies included in the Calar Alto Legacy Integral Field Area survey with integrated molecular gas observations from the Atacama Pathfinder EXperiment millimeter telescope and the CARMA interferometer. This database has the most optical integral field spectroscopy data with CO data for yet, with integrated measurements within |$\sim 1~{R_{\rm{eff}}}$|⁠. Our sample includes 125 isolated galaxies (control sample) and 109 galaxies in different merging stages. We find that despite whether the merging galaxies show an increase or decrease in their molecular gas fraction, the oxygen abundance does not vary significantly, in comparison to our control sample. Therefore, the enhancement and suppression of oxygen abundance are similar in both isolated galaxies and interacting/merging galaxies. On the contrary, regardless of the merger stage (including isolated sample), galaxies that present an increase in their specific star formation rate present a metallicity dilution. We suggest that both internal and external events affect the chemical composition of merging galaxies. [ABSTRACT FROM AUTHOR]

We present novel insights into the interplay between tidal forces and star formation in interacting galaxies before their first pericentre passage. We investigate seven close pair galaxies devoid of visible tidal disturbances, such as tails, bridges, and shells. Using integral field spectroscopy data of extended Calar Alto Legacy Integral Field Area, we unveil a previously unreported phenomenon: H  |$\alpha$| emission, a proxy for recent star formation, exhibits a significant enhancement in regions facing the companion galaxy, reaching up to 1.9 times higher flux compared to opposite directions. Notably, fainter companions within pairs display a more pronounced one-sided H  |$\alpha$| excess, exceeding the typical range observed in isolated galaxies with 2 |$\sigma$| confidence level. Furthermore, the observed H  |$\alpha$| excess in fainter companion galaxies exhibits a heightened prominence at the outer galactic regions. These findings suggest that tidal forces generated before the first pericentre passage exert a stronger influence on fainter galaxies due to their shallower potential wells by their brighter companions. This unveils a more intricate interplay between gravitational interactions and star formation history within interacting galaxies than previously understood, highlighting the need further to explore the early stages of interaction in galaxy evolution. [ABSTRACT FROM AUTHOR]