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With Gaia Data Release 3 (DR3), new and improved astrometric, photometric, and spectroscopic measurements for 1.8 billion stars have become available. Alongside this wealth of new data, however, there are challenges in finding efficient and accurate computational methods for their analysis. In this paper, we explore the feasibility of using machine learning regression as a method of extracting basic stellar parameters and line-of-sight extinctions from spectro-photometric data. To this end, we built a stable gradient-boosted random-forest regressor (xgboost), trained on spectroscopic data, capable of producing output parameters with reliable uncertainties from Gaia DR3 data (most notably the low-resolution XP spectra), without ground-based spectroscopic observations. Using Shapley additive explanations, we interpret how the predictions for each star are influenced by each data feature. For the training and testing of the network, we used high-quality parameters obtained from the StarHorse code for a sample of around eight million stars observed by major spectroscopic stellar surveys, complemented by curated samples of hot stars, very metal-poor stars, white dwarfs, and hot sub-dwarfs. The training data cover the whole sky, all Galactic components, and almost the full magnitude range of the Gaia DR3 XP sample of more than 217 million objects that also have reported parallaxes. We have achieved median uncertainties of 0.20 mag in V-band extinction, 0.01 dex in logarithmic effective temperature, 0.20 dex in surface gravity, 0.18 dex in metallicity, and 12% in mass (over the full Gaia DR3 XP sample, with considerable variations in precision as a function of magnitude and stellar type). We succeeded in predicting competitive results based on Gaia DR3 XP spectra compared to classical isochrone or spectral-energy distribution fitting methods we employed in earlier works, especially for parameters AV and Teff, along with the metallicity values. Finally, we showcase some potential applications of this new catalogue, including extinction maps, metallicity trends in the Milky Way, and extended maps of young massive stars, metal-poor stars, and metal-rich stars. [ABSTRACT FROM AUTHOR]

The most metal-poor stars (e.g., [Fe/H] ≤ –2.5) are the ancient fossils from the early assembly epoch of our Galaxy. They very likely formed before the the thick disk. Recent studies have shown that a non-negligible fraction of them have prograde planar orbits, which means that their origin is a puzzle. It has been suggested that a later-formed rotating bar could have driven these old stars from the inner Galaxy outward and transformed their orbits so that they became more dominated by rotation. However, it is unclear whether this mechanism can explain these stars as observed in the solar neighborhood. We explore whether this scenario is feasible by tracing these stars backward in an axisymmetric Milky Way potential with a bar as perturber. We integrated their orbits backward for 6 Gyr under two bar models: one model with a constant pattern speed, and the other with a decelerating speed. Our experiments show that for the constantly rotating bar model, the stars of interest are little affected by the bar and cannot have been driven from a spheroidal inner Milky Way to their current orbits. In the extreme case of a decelerating bar, some of the very metal-poor stars on planar and prograde orbits can be brought from the inner Milky Way, but ∼90% of them were nevertheless already dominated by rotation (Jϕ ≥ 1000 km s−1 kpc) 6 Gyr ago. The chance that these stars started with spheroid-like orbits with low rotation (Jϕ ≲ 600 km s−1 kpc) is very low (< 3%). We therefore conclude that within the solar neighborhood, the bar is unlikely to have shepherded a significant fraction of spheroid stars in the inner Galaxy to produce the overdensity of stars on prograde planar orbits that is observed today. [ABSTRACT FROM AUTHOR]

We present a detailed study of the chemical diversity of the metal-poor Milky Way using data from the GALAH DR3 survey. Considering 17 chemical abundances relative to iron ([X/Fe]) for 9923 stars, we employ principal component analysis (PCA) and extreme deconvolution (XD) to identify 10 distinct stellar groups. This approach, free from chemical or dynamical cuts, reveals known populations, including the accreted halo, thick disc, thin disc, and in situ halo. The thick disc is characterized by multiple substructures, suggesting it comprises stars formed in diverse environments. Our findings highlight the limited discriminatory power of magnesium in separating accreted and disc stars. Elements such as Ba, Al, Cu, and Sc are critical in distinguishing disc from accreted stars, while Ba, Y, Eu, and Zn differentiate disc and accreted stars from the in situ halo. This study demonstrates the potential power of combining a latent space representation of the data (PCA) with a clustering algorithm (XD) in Galactic archaeology, in providing new insights into the Galaxy's assembly and evolutionary history. [ABSTRACT FROM AUTHOR]

Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency's Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here, we present that the stellar kinematics traced by the classical Cepheids around the Perseus and Outer spiral arms in the Milky Way show strikingly different kinematical properties from each other: the radial and azimuthal velocities of Cepheids show positive and negative correlations in the Perseus and Outer arms, respectively. We also found that the dynamic spiral arms commonly seen in an N -body/hydrodynamic simulation of a Milky Way-like galaxy can naturally explain the observed kinematic trends. Furthermore, a comparison with such a simulation suggests that the Perseus arm is being disrupted, while the Outer arm is growing. Our findings suggest that two neighbouring spiral arms in distinct evolutionary phases – growing and disrupting phases – coexist in the Milky Way. [ABSTRACT FROM AUTHOR]

There is no doubt that the Milky Way is a barred galaxy; however, factors that establish its prominent morphology remain largely elusive. In this work, we attempt to constrain the history of the MW by tracing the present-day parameters and evolution of a set of MW and M31 analogues from the TNG50 simulations. We find that the strength of bars at |$z=0$| correlates well not only with the stellar mass build-up but also, more crucially, with the time of onset of stellar discs. Discs of strongly barred galaxies form early (⁠|$z \gtrsim 2-3$|⁠), compared to weakly and non-barred galaxies (⁠|$z \approx 1-1.5$|⁠). Although we are cautious to draw ultimate conclusions about the governing factor of discs formation due to the complexity and correlations between different phenomena, the observed morphological diversity can be tentatively explained by a substantial variation in the gas angular momentum around proto-galaxies already at |$z\approx 3-5$|⁠ ; in such a way, early discs formed from gas with larger angular momentum. By comparing the formation time-scales of discs of barred galaxies in the TNG50 sample, we infer that the MW has a strong bar (⁠|$0.35\lt A_2\lt 0.6$|⁠), and that its stellar disc started to dominate over the spheroidal component already at |$z \approx 2$|⁠ , with a mass of |$\approx 1 \pm 0.5 \times 10^{10} M_\odot$|⁠. We conclude that the presence of a strong bar in the MW is a natural manifestation of the early formation of the stellar disc, which made possible bursty but highly efficient star formation at high redshift. [ABSTRACT FROM AUTHOR]

We analyse the spatial anisotropy and the velocity anisotropy in a set of mock stellar haloes from the Aquarius simulations. The spatial anisotropy in each mock stellar halo rises progressively with the increasing distance from the halo centre, eventually reaching a maximum near the periphery. Excluding the bound satellites leads to a significant reduction of the spatial anisotropy in each halo. We compare the measured anisotropy in the mock stellar haloes with that from their sphericalized versions where all the shape- and substructure-induced anisotropies are erased. The growth of spatial anisotropy persists throughout the entire halo when the bound satellites are present but remains limited within the inner halo (⁠|${\lt} 60 \, {h^{-1}\, {\rm kpc}}$|⁠) after their exclusion. This indicates that the spatial anisotropy in the inner halo is induced by the diffuse substructures and the halo shape, whereas the outer halo anisotropy is dominated by the bound satellites. We find that the outer parts of the stellar halo are kinematically colder than the inner regions. The stellar orbits are predominantly radial, but they become rotationally dominated at certain radii that are marked by the prominent dips in the velocity anisotropy. Most of these dips disappear after the removal of the satellites. A few shallow dips arise occasionally due to the presence of diffuse streams and clouds. Our analysis suggests that a combined study of the spatial and velocity anisotropies can reveal the structure and the assembly history of the stellar haloes. [ABSTRACT FROM AUTHOR]

We use low-amplitude long period variable (LA-LPV) candidates in Gaia DR3 to trace the kinematics and dynamics of the Milky Way bar. LA-LPVs, like other LPVs, are intrinsically bright and follow a tight period–luminosity relation, but unlike e.g. Mira variables, their radial velocity measurements are reliable due to their smaller pulsation amplitudes. We supplement the Gaia astrometric and radial velocity measurements with distance moduli assigned using a period–luminosity relation to acquire full 6D phase space information. The assigned distances are validated by comparing to geometric distances and StarHorse distances, which shows biases less than |$\sim 5~{{\rm per\ cent}}$|⁠. Our sample provides an unprecedented panoramic picture of the inner Galaxy with minimal selection effects. We map the kinematics of the inner Milky Way and find a significant kinematic signature corresponding to the Galactic bar. We measure the pattern speed of the Galactic bar using the continuity equation and find |$\Omega _{\rm b}=34.1\pm 2.4$|  km s |$^{-1}$|  kpc |$^{-1}$|⁠. We develop a simple robust and potential-independent method to measure the dynamical length of the bar using only kinematics and find |$R_{\rm b}\sim 4.0$|  kpc. We validate both measurements using N -body simulations. Assuming knowledge of the gravitational potential of the inner Milky Way, we analyse the orbital structure of the Galactic bar using orbital frequency ratios. The |$x_1$| orbits are the dominant bar-supporting orbital family in our sample. Amongst the selected bar stars, the |$x_1 v_1$| or 'banana' orbits constitute a larger fraction (⁠|$\sim 15~{{\rm per\ cent}}$|⁠) than other orbital families in the bar, implying that they are the dominant family contributing to the Galactic X-shape, although contributions from other orbital families are also present. [ABSTRACT FROM AUTHOR]

I report the discovery of a stellar stream (Sutlej) using Gaia DR3 (third data release) proper motions and XP metallicities located |$\sim$| 15° north of the Small Magellanic Cloud (SMC). The stream is composed of two parallel linear components ('branches') approximately |$\sim$| 8° × 0.6° in size and separated by 2.5°. The stars have a mean proper motion of (⁠|$\mu _{\rm RA},\mu _{\rm Dec.}$|⁠) = (+0.08 mas yr−1, −1.41 mas yr−1), which is quite similar to the proper motion of stars on the western side of the SMC. The colour–magnitude diagram of the stream stars has a clear red giant branch, horizontal branch, and main-sequence turn-off that are well matched by a parsec isochrone of 10 Gyr, [Fe/H] = −1.8 at 32 kpc, and a total stellar mass of |$\sim$| 33 000 M |$_{\odot }$|⁠. The stream is spread out over an area of 9.6 deg2 and has a surface brightness of 32.5 mag arcsec−2. The metallicity of the stream stars from Gaia XP spectra extends over |$-2.5$| |$\le$| [M/H] |$\le$| |$-1.0$| with a median of [M/H] = −1.8. The tangential velocity of the stream stars is 214 km s−1 compared to the values of 448 km s−1 for the Large Magellanic Cloud and 428 km s−1 for the SMC. While the radial velocity of the stream is not yet known, a comparison of the space velocities using a range of assumed radial velocities shows that the stream is unlikely to be associated with the Magellanic Clouds. The tangential velocity vector is misaligned with the stream by nearly 90°, which might indicate an important gravitational influence from the nearby Magellanic Clouds. [ABSTRACT FROM AUTHOR]

Aims. In this Letter we investigate the origin of the Oosterhoff dichotomy in light of recent discoveries related to several ancient merging events of external galaxies with the Milky Way (MW). In particular, we aim to clarify if the subdivision in terms of the Oosterhoff type between Galactic globular clusters (GGCs) and field RR Lyrae (RRLs) can be traced back to one or more ancient galaxies that merged with the MW in its past. Methods. We first explored the association of GGCs with the past merging events according to different literature studies. Subsequently, we compiled the positions, proper motions, and radial velocities of 10 138 field RRL variables from Gaia Data Release 3. To infer the distances, we adopted the MG–[Fe/H] relation, with [Fe/H] values estimated via empirical relationships involving individual periods and Fourier parameters. We then calculated the orbits and the integrals of motion using the Python library Galpy for the whole sample. By comparing the location of the field RRLs in the energy–angular momentum diagram with that of the GGCs, we determined their likely origin. Finally, using GaiaG-band light curves, we determined the Oosterhoff types of our RRL stars based on their location in the Bailey diagram. Results. The analysis of the Bailey diagrams for Galactic RRL stars and GGCs associated with an 'in situ' versus 'accreted' halo origin shows remarkable differences. The in situ sample shows a wide range of metallicities with a continuous distribution and no sign of the Oosterhoff dichotomy. Conversely, the accreted RRLs clearly show the Oosterhoff dichotomy and a significantly smaller dispersion in metallicity. Conclusions. Our results suggest that the Oosterhoff dichotomy was imported into the MW by the merging events that shaped the Galaxy. [ABSTRACT FROM AUTHOR]

Context. The Milky Way's history of recent disturbances is vividly demonstrated by a structure in the vertical phase-space distribution known as the Gaia phase spiral. A one-armed phase spiral has been seen widely across the Milky Way disc, while a two-armed one has only been observed in the solar neighbourhood. Aims. This study aims to determine the properties of the two-armed phase spiral and to put it in a Galactic context, with the ultimate goal of understanding the structure and history of the Milky Way disc. Methods. The Gaia DR3 data were used to trace and characterise the two-armed phase spiral. Special focus was put on the phase spiral's spatial distribution, rotational behaviour, and chemical characteristics. To quantify the properties of the phase spiral, we used a model that fits a spiral pattern to the phase space distribution of the stars. Results. We found that the two-armed phase spiral is detectable only within a narrow range of galactocentric distances and angular momenta in the solar neighbourhood, R = 8 ± 0.5 kpc, LZ = 1450 ± 50 kpc km s−1. Outside this region, the phase spiral is one-armed. The two-armed phase spiral rotates with the phase angle, in a similar way to the one-armed phase spiral, and changes axis ratio with phase angle. Additionally, stars within the phase-space overdensity caused by the two-armed phase spiral pattern have slightly higher mean metallicity than stars in the underdense regions of the pattern at equivalent galactocentric distances, angular momenta, and vertical orbit extents. Conclusions. The two-armed phase spiral rotates with phase angle and its effect can be seen in metallicity, in a similar way to the one-armed phase spiral. However, the limited range over which it can be found, and its variation in shape are quite different from the one-armed version, suggesting it is a much more localised phenomenon in the Galactic disc. [ABSTRACT FROM AUTHOR]

Context. The nuclear stellar disc (NSD) is a flat and dense stellar structure at the centre of the Milky Way. Previous work has identified the presence of metal-rich and metal-poor stars in the NSD, suggesting that they have different origins. The recent publication of photometric, metallicity, proper motion, and orbital catalogues allows the NSD stellar population to be characterised with unprecedented detail. Aims. We aim to explore the proper motions and orbits of NSD stars with different metallicities to assess whether they have different origins and to better understand the metallicity distribution in the NSD. Methods. We distinguished between metal-rich and metal-poor stars by applying a Gaussian mixture model, as done in previous work, and analysed the proper motions, orbits, and spatial distribution of stars with different metallicities. Results. We find that metal-rich stars exhibit a lower velocity dispersion, suggesting that they trace a kinematically cooler component compared to metal-poor ones. Furthermore, z-tube orbits are predominant among metal-rich stars, while chaotic/box orbits are more common among metal-poor ones. We also find that metal-rich and metal-poor stars show a similar extinction and are present throughout the analysed regions. As a secondary result, we detected a metallicity gradient in the metal-rich population with higher metallicity towards the centre of the NSD and a tentative gradient for the metal-poor stars, which is consistent with previous studies that did not distinguish between the two populations. Conclusions. Our results suggest that metal-rich stars trace the NSD, whereas metal-poor ones are related to the Galactic bar and probably constitute Galactic bar interlopers and/or are NSD stars that originated from accreted clusters. The detected metallicity gradients aligns with the currently accepted inside-out formation of the NSD. [ABSTRACT FROM AUTHOR]

We used Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify blue horizontal branch (BHB) stars. The measured average radial density profile follows a double power law in the range 5 < rgc/kpc < 120, with a density break at rgc ≈ 20 kpc. This description, however, falls short, depending on the chosen line of sight, with some regions showing no signature of a break in the profile and a wide range of density slopes, such as an outer slope −5.5 ≲ αout ≲ −4, pointing towards a highly anisotropic stellar halo. This explains, in part, the wide range of density profiles reported in the literature owing to different tracers and sky coverage. Using our detailed 3D stellar halo density map, we quantified the shape of the Pisces overdensity associated with the transient wake response of the Galaxy's (dark) halo to the Large Magellanic Cloud (LMC). Measured in the LMC's coordinate system, Pisces stands above the background, is 60° long and 25° wide, and is aligned with the LMC's orbit. This would correspond to a wake width of ∼32 kpc at ∼70 kpc. We do not find a statistically significant signature of the collective response in density as previously reported in the literature measured with K giant stars, despite our larger numbers. We release the catalogue constructed in this study with 95 446 possible BHB stars and their BHB probability. [ABSTRACT FROM AUTHOR]

Aims. The most metal-poor tail of the Milky Way ([Fe/H] ≤ −2.5) contains a population of stars on very prograde planar orbits, whose origins and evolution remain puzzling. One possible scenario is that they are shepherded by the bar from the inner Galaxy, where many of the old and low-metallicity stars in the Galaxy are located. Methods. To investigate this scenario, we used test-particle simulations with an axisymmetric background potential plus a central bar model. The test particles were generated by an extended distribution function (EDF) model based on the observational constraints of bulge stars. Results. According to the simulation results, a bar with a constant pattern speed is not efficient in terms of helping bring stars from the bulge to the solar vicinity. In contrast, when the model includes a decelerating bar, some bulge stars can gain rotation and move outwards as they are trapped in the bar's resonance regions. The resulting distribution of shepherded stars heavily depends on the present-day azimuthal angle between the bar and the Sun. The majority of the low-metallicity bulge stars driven outwards are distributed in the first and fourth quadrants of the Galaxy with respect to the Sun and about 10% of them are within 6 kpc from us. Conclusions. Our experiments indicate that the decelerating bar perturbation can be a contributing mechanism that may partially explain the presence of the most metal-poor stars with prograde planar orbits in the Solar neighbourhood, but it is unlikely to be the only one. [ABSTRACT FROM AUTHOR]

Aims. Our objective is to investigate the distribution of dust and associated large-scale structures of the Galaxy using optical linear polarization measurements of various open clusters located at different distances in the Galactic anticenter direction. Methods. We present R-band linear polarization observations of stars toward five open clusters: Kronberger 1, Berkeley 69, Berkeley 71, Berkeley 19, and King 8 in the anticenter direction. The polarization observations were carried out using the ARIES (Aryabhatta Research Institute of Observational Sciences) IMaging POLarimeter mounted on the 104 cm Sampurnanand telescope of ARIES, Nainital, making it the first study to target the polarization observations toward distant clusters (~6 kpc). We combined the observed polarization data with the distance information from the Gaia space telescope to infer the dust distribution along the line of sight. Results. The variation in the degree of polarization and extinction with distance reveals multiple dust layers in each cluster direction. In addition, common foreground-dust layers detected toward different cluster directions highlight global features such as spiral arms. Our results show that the dust clouds at 2 kpc toward Berkeley 69 and Berkeley 71 coincide with the Perseus arm, while the dust layer at ~4 kpc toward the distant clusters Berkeley 19 and King 8 indicates the presence of the Outer arm. The large-scale dust distribution obtained by combining our polarization results with previous polarization studies of nearby open clusters suggests that the anticenter direction is characterized by a low-extinction homogeneous dust distribution with a somewhat uniform orientation of the plane-of-sky component of the magnetic field along the line of sight. Conclusions. Our study demonstrates that polarization is useful as a tool for studying the large-scale dust distribution and structural features where kinematic distance methods are inadequate and cannot provide accurate distances to the dust clouds. The global dust distribution in the anticenter direction shows signatures of the intervening spiral arms. [ABSTRACT FROM AUTHOR]

We present the morphological parameters and global properties of dust-obscured star formation in typical star-forming galaxies at z = 4–6. Among 26 galaxies composed of 20 galaxies observed by the Cycle-8 ALMA Large Program, CRISTAL, and 6 galaxies from archival data, we individually detect rest-frame 158 μm dust continuum emission from 19 galaxies, 9 of which are reported for the first time. The derived far-infrared luminosities are in the range log10LIR [L⊙] = 10.9 − 12.4, an order of magnitude lower than previously detected massive dusty star-forming galaxies (DSFGs). We find the average relationship between the fraction of dust-obscured star formation (fobs) and the stellar mass to be consistent with previous results at z = 4–6 in a mass range of log10M* [M⊙]∼9.5 − 11.0 and to show potential evolution from z = 6 − 9. The individual fobs exhibits significant diversity, and we find a potential correlation with the spatial offset between the dust and UV continuum, suggesting that inhomogeneous dust reddening may cause the source-to-source scatter in fobs. The effective radii of the dust emission are on average ∼1.5 kpc and are about two times more extended than those seen in rest-frame UV. The infrared surface densities of these galaxies (ΣIR ∼ 2.0 × 1010 L⊙ kpc−2) are one order of magnitude lower than those of DSFGs that host compact central starbursts. On the basis of the comparable contribution of dust-obscured and dust-unobscured star formation along with their similar spatial extent, we suggest that typical star-forming galaxies at z = 4 − 6 form stars throughout the entirety of their disks. [ABSTRACT FROM AUTHOR]

We derived the metallicity profile of the Milky Way low-α disc population from 2 to 20 kpc from the Galactic centre in 1 Gyr age bins using the astroNN catalogue, and we show that it is highly structured, with a plateau between 4 and 7 kpc and a break at 10–12 kpc. We argue that these features result from the two main bar resonances, the corotation and the outer Lindblad resonance (OLR), respectively. We show that the break in the metallicity profile is most visible in stars having 7–8 Gyr, reaching an amplitude of about 0.4 dex, and it is the signpost of the position of the bar OLR. The bar formation was accompanied by an episode of radial migration triggered by it slowing down and it is responsible for spreading old metal-rich stars up to the OLR. The data show that the slowdown of the bar ended 6–7 Gyr ago. Based on numerical simulations that reproduce well the characteristic break observed in the metallicity profile, we argue that this implies that the bar formed in our Galaxy 8–10 Gyr ago. Analysis of the metallicity distribution as a function of radius shows no evidence of significant systematic outward radial migration after this first episode. We argue that the variation of the metallicity dispersion as a function of the guiding radius is dominated by the migration triggered by the bar, but also that the libration of orbits around the bar resonances induces a mixing that may have a significant impact on the observed metallicity dispersion. In contrast, the absence of a break in the metallicity profile of populations younger than ∼6 Gyr and the flattening of the gradient at younger ages is interpreted as evidence that the strength of the bar has decreased, loosening its barrier effect and allowing the gas and metals on both sides of the OLR to mix, erasing the break. Beyond the OLR, stars younger than 7 Gyr show very small metallicity dispersion, suggesting that no or limited mixing induced by the spiral arms has occurred in the outer disc. [ABSTRACT FROM AUTHOR]

The North–South asymmetry in the number density and bulk velocity of stars in the Solar neighbourhood provides valuable insights into the formation and evolution of the Milky Way disc. Our objective is to investigate the wave-like disc oscillations of mono-age stellar populations in the Solar neighbourhood using data from Gaia Data Release 3. We have selected a comprehensive sample of main-sequence turn-off stars. The ages of these stars can be accurately determined using isochrone fitting methods. Our findings indicate that the North–South density and mean vertical velocity asymmetries remain consistent across all age groups. The uniformity of perturbations across all subsamples suggests that all populations are responding to the same external influence, which likely affects them irrespective of their age. Moreover, the fact that these perturbations appear consistently implies they could be either ongoing or recent. Regarding vertical velocity dispersions, we observe that older stars exhibit larger dispersions. [ABSTRACT FROM AUTHOR]

Hot, |$\log (T/\mathrm{K})$| |$\sim$| 7.5, gas was recently discovered in the Milky Way in extragalactic sightlines. In order to determine its location, here we present sightlines to Galactic X-ray binaries (XRBs) passing through the Interstellar Medium (ISM). In this pilot study, we investigate absorption features of S |$_{\mathrm{XVI}}$|⁠ , Si |$_{\mathrm{XIV}}$|⁠ , and Ne |$_{\mathrm{X}}$| in the spectra of three XRBs, namely 4U 1735–44, 4U 1820–30, and Cyg X-2, using Chandra High Energy Transmission Grating archival observations. We do not detect any of these lines. We determine the 2 |$\sigma$| upper limit for the equivalent widths of the undetected absorption lines and the column densities of the corresponding ions. We note that the 2 |$\sigma$| upper limits for S |$_{\mathrm{XVI}}$| K |$\alpha$| and Si |$_{\mathrm{XIV}}$| K |$\alpha$| are an order of magnitude smaller than those previously detected in the extragalactic sightlines. Our finding suggests that if any gas at |$\log (T/\mathrm{K})$| |$\gt\ 7$| is present in the Galactic ISM, it is unlikely to be ubiquitous. This is an important result because it implies that S |$_{\mathrm{XVI}}$|⁠ , Si |$_{\mathrm{XIV}}$|⁠ , and Ne |$_{\mathrm{X}}$| absorption detected in extragalactic sightlines is not from the ISM, but is likely from a hot gas phase in the extraplanar region beyond the ISM or in the extended Circumgalactic Medium. [ABSTRACT FROM AUTHOR]

The question of whether the Milky Way's disc extends to low metallicity has been the subject of debate for many years. We aim to address the question by employing a large sample of giant stars with radial velocities and homogeneous metallicities based on the Gaia Data Release 3 BP/RP(XP) spectra. We study the 3D velocity distribution of stars in various metallicity ranges, including the very metal-poor (VMP) regime (⁠|$\mathrm{[M/H]} < -2.0$|⁠). We find that a clear, stand-alone disc population, i.e. that with a ratio of rotational velocity to velocity dispersion |$v/\sigma > 1$|⁠ , starts to emerge only around |$\mathrm{[M/H]} \sim -1.3$|⁠ , and is not visible for |$\mathrm{[M/H]} < -1.6$|⁠. Using Gaussian mixture modelling, we show that there are two halo populations in the VMP regime: one stationary and one with a net prograde rotation of |$\sim\!\! 80\ \mathrm{km\, s}^{-1}$|⁠. In this low-metallicity range, we are able to place constraints on the contribution of a rotation-supported thick disc sub-population to a maximum of |$\sim\!\! 3$| per cent in our sample. We compare our results to previous claims of discy VMP stars in both observations and simulations and find that having a prograde halo component could explain most of these. [ABSTRACT FROM AUTHOR]

We study solutions of the collisionless Boltzmann equation (CBE) in a functional Koopman representation. This facilitates the use of linear spectral techniques characteristic of the analysis of Schrödinger-type equations. For illustrative purposes, we consider the classical phase mixing of a non-interacting distribution function in a quartic potential. Solutions are determined perturbatively relative to a harmonic oscillator. We impose a form of coarse-graining by choosing a finite-dimensional basis to represent the distribution function and time evolution operators, which sets a minimum length-scale on phase space structure. We observe a relationship between the dimension of the representation and the multiplicity of the harmonic oscillator eigenvalues. System dynamics are understood in terms of degenerate subspaces of the linear operator spectra. Each subspace is associated with a mode of the harmonic oscillator, the first two being bending and breathing structures. The quartic potential splits the degenerate eigenvalues within each subspace. This facilitates the formation of spiral structure as deformations from the harmonic oscillator modes. We ultimately argue that this construction provides a promising avenue for study of self-interacting systems experiencing phase mixing, which is an outstanding problem in the context of the Gaia DR2 vertical phase space spirals. [ABSTRACT FROM AUTHOR]

Cosmological N -body simulations of galaxies operate at the level of 'star particles' with a mass resolution on the scale of thousands of solar masses. Turning these simulations into stellar mock catalogues requires 'upsampling' the star particles into individual stars following the same phase-space density. In this paper, we introduce two new upsampling methods. First, we describe GalaxyFlow , a sophisticated upsampling method that utilizes normalizing flows to both estimate the stellar phase-space density and sample from it. Secondly, we improve on existing upsamplers based on adaptive kernel density estimation (KDE), using maximum likelihood estimation to fine-tune the bandwidth for such algorithms in a way that improves both the density estimation accuracy and upsampling results. We demonstrate our upsampling techniques on a neighbourhood of the Solar location in two simulated galaxies: Auriga 6 and h277. Both yield smooth stellar distributions that closely resemble the stellar densities seen in the Gaia DR3 catalogue. Furthermore, we introduce a novel multimodel classifier test to compare the accuracy of different upsampling methods quantitatively. This test confirms that GalaxyFlow more accurately estimates the density of the underlying star particles than methods based on KDE, at the cost of being more computationally intensive. [ABSTRACT FROM AUTHOR]

Large-scale astrophysics data sets present an opportunity for new machine learning techniques to identify regions of interest that might otherwise be overlooked by traditional searches. To this end, we demonstrate how Classification Without Labels (CWoLa), a weakly supervised anomaly detection method, can help identify cold stellar streams within the more than one billion Milky Way stars observed by the Gaia satellite. CWoLa operates without the use of labelled streams or knowledge of astrophysical principles. Instead, it uses a classifier to distinguish between mixed samples for which the proportions of signal and background samples are unknown. As a proof of concept, we demonstrate that this computationally lightweight strategy is able to detect both simulated streams and the known stream GD-1 in data. Originally designed for high-energy collider physics, this technique may have broad applicability within astrophysics as well as other domains interested in identifying localized anomalies.

Overdensities in the radial phase space |$(r,v_{\rm r})$| of the Milky Way's halo have previously been associated with the phase-mixed debris of a highly radial merger event such as Gaia Sausage–Enceladus. We present and test an alternative theory in which the overdense 'chevrons' are instead composed of stars trapped in resonances with the Galactic bar. We develop an analytic model of resonant orbits in the isochrone potential, and complement this with a test particle simulation of a stellar halo in a realistic barred Milky Way potential. These models are used to predict the appearance of action space |$(J_\phi ,J_{\rm r})$| and radial phase space in the Solar neighbourhood. They are able to reproduce almost all salient features of the observed chevrons. In particular, both the analytic model and simulation predict that the chevrons are more prominent at |$v_{\rm r}\ \lt\ 0$| when viewed near the Sun, as is observed by Gaia. This is inconsistent with formation by an ancient merger event. We also associate individual chevrons with specific resonances. At a bar pattern speed of |$\Omega _\mathrm{b}=35$| km s |$^{-1}$|  kpc |$^{-1}$|⁠ , the two most prominent prograde chevrons align very closely with the corotation and outer Lindblad resonances. The former can be viewed as a highly eccentric extension of the Hercules stream. Finally, our model predicts that the |$v_{\rm r}$| asymmetry changes sign as a function of Galactic radius and azimuth, and we find evidence that this is indeed the case in the Milky Way. [ABSTRACT FROM AUTHOR]

We present results from simultaneous modelling of 2D (projected along the line of sight) position, proper motion, and line-of-sight velocity for Gaia - and Apache Point Observatory Galactic Evolution Experiment (APOGEE)-observed stars near the centre of the Sagittarius (Sgr) dwarf spheroidal galaxy. We use a mixture model that allows for independent sub-populations contributed by the Sgr galaxy, its nuclear star cluster Messier 54 (M54), and the Milky Way foreground. We find an offset of |$0.295\pm 0.029$| deg between the inferred centroids of Sgr and M54, corresponding to a (projected) physical separation of |$0.135\pm 0.013$| kpc. The detected offset might plausibly be driven by unmodelled asymmetry in Sgr's stellar configuration; however, standard criteria for model selection favour our symmetric model over an alternative that allows for bilateral asymmetry. We infer an offset between the proper motion centres of Sgr and M54 of |$[\Delta \mu _{\alpha }\cos \delta ,\Delta \mu _{\delta }]=[4.9, -19.7] \pm [6.8, 6.2] \ \mu \mathrm{ as}\,\mathrm{ yr}^{-1}$| (⁠|$[0.61, -2.46] \pm [0.85, 0.77] \ \mathrm{ km}\,\mathrm{ s}^{-1}$|⁠), with magnitude similar to the covariance expected due to spatially correlated systematic error. We infer an offset of |$4.1\pm 1.2 \ \mathrm{ km}\,\mathrm{ s}^{-1}$| in line-of-sight velocity. Using inferred values for the systemic positions and motions of Sgr and M54 as initial conditions, we calculate the recent orbital history of a simplified Sgr/M54 system, which we demonstrate to be sensitive to any line-of-sight distance offset between M54 and Sgr, and to the distribution of dark matter within Sgr. [ABSTRACT FROM AUTHOR]

Three-dimensional dust density maps are crucial for understanding the structure of the interstellar medium of the Milky Way and the processes that shape it. However, constructing these maps requires large data sets and the methods used to analyse them are computationally expensive and difficult to scale up. As a result, it has only recently become possible to map kiloparsec-scale regions of our Galaxy at parsec-scale grid sampling. We present all-sky three-dimensional dust density and extinction maps of the Milky Way out to 2.8 kpc in distance from the Sun using the fast and scalable Gaussian Process algorithm Dustribution. The sampling of the three-dimensional map is l, b, d  = 1° × 1° × 1.7 pc. The input extinction and distance catalogue contains 120 million stars with photometry and astrometry from Gaia DR2, 2MASS and AllWISE. This combines the strengths of optical and infrared data to probe deeper into the dusty regions of the Milky Way. We compare our maps with other published 3D dust maps. All maps quantitatively agree at the 0.001 mag pc−1 scale with many qualitatively similar features, although each map also has its own features. We recover Galactic features previously identified in the literature. Moreover, we also see a large under-density that may correspond to an inter-arm or -spur gap towards the Galactic Centre. [ABSTRACT FROM AUTHOR]

Based on a sample of K giant from Large sky Area Multi-Object fibre Spectroscopic Telescope Data Release 8 and a sample of RR Lyrae (RRL) from Gaia Data Release 3, we investigate the compositions of the Hercules–Aquila Cloud (HAC) and Virgo Overdensity (VOD) and their collective contribution to the tilt and triaxiality of the stellar halo (⁠|$r\, \lt \, 40\, {\rm kpc}$|⁠) as well as two breaks at |$\approx 15\, {\rm kpc}$| and 30 kpc. We apply the Gaussian mixture model to divide the stellar halo into the isotropic component and the radially biased anisotropic component, namely Gaia–Sausage–Enceladus (GSE), and find that both HAC and VOD are dominated by the GSE debris stars with weights of |$0.67^{+0.09}_{-0.07}$| and |$0.57^{+0.07}_{-0.06}$|⁠ , respectively. In addition, using the K giants with orbital parameters, we identify the member stars of known substructures, including GSE, Sagittarius (Sgr), Helmi Streams, Sequoia, Thamnos, Pontus, Wukong, and Metal-weak Thick Disc, to probe the compositions of low-eccentricity stars in the HAC and VOD regions. In density fittings of the RRL sample, we note that the absence of HAC and VOD has a weak effect on the shape of halo. Finally, we find that the radially biased anisotropic halo contributes majorly to the stellar halo that can be modelled with a tilted triaxial ellipsoid and a doubly broken power law with breaking radii at |$18.08^{+2.04}_{-3.22}\, {\rm kpc}$| and |$33.03^{+1.30}_{-1.21}\, {\rm kpc}$|⁠. This has important significance for understanding the status of large diffuse overdensities in the Milky Way. [ABSTRACT FROM AUTHOR]

We investigate the non-adiabatic effect of time-dependent deformations in the Milky Way (MW) halo potential on stellar streams. Specifically, we consider the MW's response to the infall of the Large Magellanic Cloud (LMC) and how this impacts our ability to recover the spherically averaged MW mass profile from observation using stream actions. Previously, action clustering methods have only been applied to static or adiabatic MW systems to constrain the properties of the host system. We use a time-evolving MW–LMC simulation described by basis function expansions. We find that for streams with realistic observational uncertainties on shorter orbital periods and without close encounters with the LMC, e.g. GD-1, the radial action distribution is sufficiently clustered to locally recover the spherical MW mass profile across the stream radial range within a |$2\sigma$| confidence interval determined using a Fisher information approach. For streams with longer orbital periods and close encounters with the LMC, e.g. Orphan–Chenab (OC), the radial action distribution disperses as the MW halo has deformed non-adiabatically. Hence, for OC streams generated in potentials that include an MW halo with any deformations, action clustering methods will fail to recover the spherical mass profile within a |$2\sigma$| uncertainty. Finally, we investigate whether the clustering of stream energies can provide similar constraints. Surprisingly, we find for OC-like streams, the recovered spherically averaged mass profiles demonstrate less sensitivity to the time-dependent deformations in the potential. [ABSTRACT FROM AUTHOR]

To advance our understanding of the evolution of the interstellar medium (ISM) of our Galaxy, numerical models of Milky Way (MW) type galaxies are widely used. However, most models only vaguely resemble the MW (e.g. in total mass), and often use imposed analytic potentials (which cannot evolve dynamically). This poses a problem in asserting their applicability for the interpretation of observations of our own Galaxy. The goal of this work is to identify a numerical model that is not only an MW-type galaxy, but one that can mimic some of the main observed structures of our Galaxy, using dynamically evolving potentials, so that it can be used as a base model to study the ISM cycle in a galaxy like our own. This paper introduces a suite of 15 MW-type galaxy models developed using the arepo numerical code, that are compared to Galactic observations of |$^{12}$| CO and H  i emission via longitude–velocity plots, from where we extract and compare the skeletons of major galactic features and the terminal gas velocities. We found that our best-fitting model to the overall structure, also reproduces some of the more specific observed features of the MW, including a bar with a pattern speed of |$30.0 \pm 0.2$| km s |$^{-1}$|  kpc |$^{-1}$|⁠ , and a bar half-length of |$3.2 \pm 0.8$|  kpc. Our model shows large streaming motions around spiral arms, and strong radial motions well beyond the inner bar. This model highlights the complex motions of a dynamic MW-type galaxy and has the potential to offer valuable insight into how our Galaxy regulates the ISM and star formation. [ABSTRACT FROM AUTHOR]

In order to constrain the evolutionary history of the Milky Way, we hunt for faint RR Lyrae stars (RRLs) using Dark Energy Camera data from the High cadence Transient Survey and the Halo Outskirts With Variable Stars survey. We report the detection of |$\sim$| 500 RRLs, including previously identified stars and |$\sim$| 90 RRLs not yet reported. We identify nine new RRLs beyond 100 kpc from the Sun, most of which are classified as fundamental-mode pulsators. The periods and amplitudes of the distant RRLs do not place them in either one of the two classical Oosterhoff groups, but in the Oosterhoff intermediate region. We detect two groups of clumped distant RRLs with similar distances and equatorial coordinates, which we interpret as an indication of their association with undiscovered bound or unbound satellites. We study the halo density profile using spheroidal and ellipsoidal (⁠|$q=0.7$|⁠) models, following a Markov chain Monte Carlo methodology. For a spheroidal halo, our derived radial profile is consistent with a broken power law with a break at |$18.1^{+2.1}_{-1.1}$| kpc separating the inner and the outer halo, and an outer slope of |$-4.47^{+0.11}_{-0.18}$|⁠. For an ellipsoidal halo, the break is located at |$24.3^{+2.6}_{-3.2}$| kpc and the outer slope is |$-4.57^{+0.17}_{-0.25}$|⁠. The break in the density profile is a feature visible in different directions of the halo. The similarity of these radial distributions with previous values reported in the literature seems to depend on the regions of the sky surveyed (direction and total area) and halo tracer used. Our findings are compatible with simulations and observations that predict that the outer regions of Milky Way-like galaxies are mainly composed of accreted material. [ABSTRACT FROM AUTHOR]

Our study focuses on characterizing the highly ionized gas within the Milky Way's (MW) Circumgalactic Medium (CGM) that gives rise to ionic transitions in the X-ray band 2–25 Å. Utilizing stacked Chandra /ACIS-S MEG and LETG spectra toward Quasar (QSO) sightlines, we employ the self-consistent hybrid ionization code phase to model our data. The stacked spectra are optimally described by three distinct gas phase components: warm [log (T /K) ∼ 5.5], warm-hot [log (T /K) ∼6], and hot [log (T /K) ∼ 7.5] components. These findings confirm the presence of the hot component in the MW's CGM indicating its coexistence with warm and warm-hot gas phases. We find this hot component to be homogeneous in temperature but inhomogeneous in column density. The gas in the hot component requires overabundances relative to solar to be consistent with the Dispersion Measure (DM) from the Galactic halo reported in the literature. For the hot phase we estimated a DM =  |$55.1^{+29.9}_{-23.7}$|  pc cm−3. We conclude that this phase is either enriched in Oxygen, Silicon, and Sulfur, or has metallicity over 6 times solar value, or a combination of both. We do not detect Fe L-shell absorption lines, implying O/Fe ≥ 4. The non-solar abundance ratios found in the supervirial gas component in the Galactic halo suggest that this phase arises from Galactic feedback. [ABSTRACT FROM AUTHOR]

In the conventional approach to decomposing a rotation curve into a set of contributions from mass model components, the measurements of the rotation curve at different radii are taken to be independent. It is clear, however, that radial correlations are present in such data, for instance (but not only) because the orbital speed depends on the mass distribution at all (or, minimally, inner) radii. We adopt a very simple parametric form for a covariance matrix and constrain its parameters using Gaussian process regression. Applied to the rotation curve of the Milky Way, this suggests the presence of correlations between neighbouring rotation curve points with amplitudes of |$\lt 10\, \mathrm{km}\, \mathrm{s}^{-1}$| over length scales of 1.5– |$2.5\, \mathrm{kpc}$| regardless of the assumed dark halo component. We show that accounting for such covariance can result in a ∼50 per cent lower total mass estimate for the Milky Way than when it is neglected, and that the uncertainty in model parameters increases such that it seems more representative of the uncertainty in the rotation curve measurement. The statistical uncertainty associated with the covariance is comparable to or exceeds the total systematic uncertainty budget. Our findings motivate including more detailed treatment of rotation curve covariance in future analyses. [ABSTRACT FROM AUTHOR]

Based on the integral field unit (IFU) data from Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, we develop a new method to select galaxies with biconical ionized structures, building a sample of 142 edge-on biconical ionized galaxies. We classify these 142 galaxies into 81 star-forming galaxies, 31 composite galaxies, and 30 AGNs (consisting of 23 Seyferts and 7 LI(N)ERs) according to the [N  ii ]-BPT diagram. The star-forming bicones have bar-like structures while AGN bicones display hourglass structures, and composite bicones exhibit transitional morphologies between them due to both black hole and star formation activities. Star-forming bicones have intense star formation activities in their central regions, and the primary driver of biconical structures is the central star formation rate surface density. The lack of difference in the strength of central black hole activities (traced by dust attenuation corrected [O  iii ]λ5007 luminosity and Eddington ratio) between Seyfert bicones and their control samples can be naturally explained as that the accretion disc and the galactic disc are not necessarily coplanar. Additionally, the biconical galaxies with central LI(N)ER-like line ratios are edge-on disc galaxies that show strong central dust attenuation. The radial gradients of H α surface brightness follow the r −2.35 relation, roughly consistent with r −2 profile, which is expected in the case of photoionization by a central point-like source. These observations indicate obscured AGNs or AGN echoes as the primary drivers of biconical structures in LI(N)ERs. [ABSTRACT FROM AUTHOR]

The Milky Way's (MW) inner stellar halo contains an [Fe/H]-rich component with highly eccentric orbits, often referred to as the 'last major merger.' Hypotheses for the origin of this component include Gaia -Sausage/Enceladus (GSE), where the progenitor collided with the MW proto-disc 8–11 Gyr ago, and the Virgo Radial Merger (VRM), where the progenitor collided with the MW disc within the last 3 Gyr. These two scenarios make different predictions about observable structure in local phase space, because the morphology of debris depends on how long it has had to phase mix. The recently identified phase-space folds in Gaia DR3 have positive caustic velocities, making them fundamentally different than the phase-mixed chevrons found in simulations at late times. Roughly 20 per cent of the stars in the prograde local stellar halo are associated with the observed caustics. Based on a simple phase-mixing model, the observed number of caustics are consistent with a merger that occurred 1–2 Gyr ago. We also compare the observed phase-space distribution to FIRE-2 Latte simulations of GSE-like mergers, using a quantitative measurement of phase mixing (2D causticality). The observed local phase-space distribution best matches the simulated data 1–2 Gyr after collision, and certainly not later than 3 Gyr. This is further evidence that the progenitor of the 'last major merger' did not collide with the MW proto-disc at early times, as is thought for the GSE, but instead collided with the MW disc within the last few Gyr, consistent with the body of work surrounding the VRM. [ABSTRACT FROM AUTHOR]

The Galactic disc is one of the main components of the Milky Way, which contributes most of the luminosity. Its structure is essential for understanding the formation and evolution of the Milky Way. Using 174 443 K-type dwarf stars observed by both Large Sky Area Multi-Object Fiber Spectroscopic Telescope and Gaia Data Release 3, we study the disc density profile in the local volume within 1200 pc. In the azimuthal dimension, we find strong asymmetric signal of the thin disc. The surface density and the scale height of the southern disc significantly change versus the azimuthal angle at the same Galactocentric distance R. Meanwhile, in the vertical dimension, the scale height of the northern disc has quite different trend than that of the southern one. The scale height of the southern disc shows a decreasing trend with ϕ ∼ −2.5°, and change to an increasing one with ϕ ∼ 5.0°. Meanwhile, the scale height of the northern disc has a consistently smaller increase. Finally, we divide the entire sample into three subsamples based on metallicity and all three subsamples show significant non-axisymmetric and north–south asymmetric signals in the Galactic disc. Furthermore, we find that the scale height of the metal-poor ([Fe/H] < −0.4 dex) subsample in the northern disc is greater than that of the metal-rich ([Fe/H] > −0.1 dex) subsample. However, in the southern disc, the scale height exhibits varying relationships across different metallicity slices. [ABSTRACT FROM AUTHOR]

We investigate a mechanism to form and keep a planar spatial distribution of satellite galaxies in the Milky Way (MW), which is called the satellite plane. It has been pointed out that the ΛCDM cosmological model hardly explains the existence of such a satellite plane, so it is regarded as one of the serious problems in the current cosmology. We here focus on a rotation of the gravitational potential of a host galaxy, i.e. a so-called figure rotation, following the previous suggestion that this effect can induce the tilt of a so-called tube orbit. Our calculation shows that a figure rotation of a triaxial potential forms a stable orbital plane perpendicular to the rotational axis of the potential. Thus, it is suggested that the MW's dark halo is rotating with its axis being around the normal line of the satellite plane. Additionally, we find that a small velocity dispersion of satellites is required to keep the flatness of the planar structure, namely the standard derivation of their velocities perpendicular to the satellite plane needs to be smaller than their mean rotational velocity on the plane. Although not all the MW's satellites satisfy this condition, a fraction of them, called member satellites, which are prominently on the plane, do satisfy it. We suggest that this picture explaining the observed satellite plane can be achieved by the filamentary accretion of dark matter associated with the formation of the MW and a group infall of member satellites along this cosmic filament. [ABSTRACT FROM AUTHOR]

The distribution of moving groups in the solar neighbourhood has been used to constrain dynamical properties of the Milky Way for decades. The kinematic bimodality between the main mode (Hyades, Pleiades, Coma Berenices, and Sirius) and Hercules can be explained by two different bar models – via the outer Lindblad resonance of a bar with a high pattern speed (∼55 km s−1 kpc−1), or via the corotation resonance of a bar with a low pattern speed (∼40 km s−1 kpc−1). Recent works directly studying the kinematics of bar stars and gas flows near the centre of the Galaxy have converged on the low pattern speed model. In this paper, we independently confirm this result by using Gaia DR3 to directly study the variation of Hercules across Galactic azimuth. We find that Hercules increases in V ϕ and becomes stronger as we move towards the minor axis of the bar, and decreases in V ϕ and becomes weaker as we move towards the major axis of the bar. This is in direct agreement with theoretical predictions of a low pattern speed bar model in which Hercules is formed by the corotation resonance with stars orbiting the bar's L4/L5 Lagrange points. [ABSTRACT FROM AUTHOR]

The formation and structure of the Milky Way have a fundamental role in our understanding of the universe and its evolution, and thanks to the Gaia mission and large spectroscopic surveys, we live an exceptional moment of data availability, allowing us to trace the building blocks of the Galactic disc and their relations. In this sense, we propose here the exploration of a large data set in a top-down fashion, elaborating a similarity map of the local Galactic volume in order to segregate and characterize its main components, searching for hints about their relations. We have used the t-SNE algorithm with chemical, orbital and kinematic properties of the stars to produce 2D manifolds and dissect their structure by isolating populations to further analyse their behaviour. The young thin disc could be clearly separated from the older thick disc, also showing a puzzling transition zone with hints about the aftermath of the Gaia–Sausage–Enceladus merger. Moving groups and resonant features also appear prominently in the maps, splitting the disc into inner and outer portions as consequence of the resonances produced by the Galactic bar. The dynamical halo appears as an extreme end related to the heated portion of the thick disc, showing sub-structures corresponding to known accreted populations. Open and globular clusters also appear in their chemical/evolutionary context. We present details of the developed strategy, an overview of the different populations and their relations, as well as a discussion and insights of our results in the scenario of Galactic evolution. [ABSTRACT FROM AUTHOR]

Context. The nuclear stellar disk has been the most prolific star-forming region in the Milky Way over the past ∼30 million years. Notably, the cumulative mass of the three clusters currently found in the nuclear stellar disk, the Quintuplet, the Arches, and the Nuclear clusters, amounts to just 10% of the total anticipated mass of young stars that formed in this period. This discrepancy, known as the missing cluster problem, is attributed to factors such as high stellar density and tidal forces. Traces of dissolving clusters may exist as comoving groups of stars, providing insights into the star formation history of the region. Recently, a new cluster candidate associated with an HII region was reported through the analysis of kinematic data Aims. Our aim is to determine whether the young and massive stellar objects in the region share proper motion, positions in the plane of the sky, and line-of-sight distances. We use reddening as a proxy for the distances. Methods. We reduced and analyzed integral field spectroscopy data from the KMOS instrument at the ESO VLT to locate possible massive young stellar objects in the field. Then, we identified young massive stars with astrophotometric data from the two different catalogs to analyze their extinction and kinematics. Results. We present a group of young stellar objects that share velocities, are close together in the plane of the sky, and are located at a similar depth in the nuclear stellar disk. Conclusions. The results presented here offer valuable insights into the missing clusters problem. They indicate that not all young massive stars in the Galactic center form in isolation; some of them seem to be the remnants of dissolved clusters or stellar associations. [ABSTRACT FROM AUTHOR]

Context. While orbital analysis studies were so far mainly focused on the Galactic halo, it is possible now to do these studies in the heavily obscured region close to the Galactic Centre. Aims. We aim to do a detailed orbital analysis of stars located in the nuclear stellar disc (NSD) of the Milky Way allowing us to trace the dynamical history of this structure. Methods. We integrated orbits of the observed stars in a non-axisymmetric potential. We used a Fourier transform to estimate the orbital frequencies. We compared two orbital classifications, one made by eye and the other with an algorithm, in order to identify the main orbital families. We also compared the Lyapunov and the frequency drift techniques to estimate the chaoticity of the orbits. Results. We identified several orbital families as chaotic, z-tube, x-tube, banana, fish, saucer, pretzel, 5:4, and 5:6 orbits. As expected for stars located in a NSD, the large majority of orbits are identified as z-tubes (or as a sub-family of z-tubes). Since the latter are parented by x2 orbits, this result supports the contribution of the bar (in which x2 orbits are dominant in the inner region) in the formation of the NSD. Moreover, most of the chaotic orbits are found to be contaminants from the bar or bulge which would confirm the predicted contamination from the most recent NSD models. Conclusions. Based on a detailed orbital analysis, we were able to classify orbits into various families, most of which are parented by x2-type orbits, which are dominant in the inner part of the bar. [ABSTRACT FROM AUTHOR]

Context. High-resolution 3D maps of interstellar dust are critical for probing the underlying physics shaping the structure of the interstellar medium, and for foreground correction of astrophysical observations affected by dust. Aims. We aim to construct a new 3D map of the spatial distribution of interstellar dust extinction out to a distance of 1.25 kpc from the Sun. Methods. We leveraged distance and extinction estimates to 54 million nearby stars derived from the Gaia BP/RP spectra. Using the stellar distance and extinction information, we inferred the spatial distribution of dust extinction. We modeled the logarithmic dust extinction with a Gaussian process in a spherical coordinate system via iterative charted refinement and a correlation kernel inferred in previous work. In total, our posterior has over 661 million degrees of freedom. We probed the posterior distribution using the variational inference method MGVI. Results. Our 3D dust map has an angular resolution of up to 14′ (Nside = 256), and we achieve parsec-scale distance resolution, sampling the dust in 516 logarithmically spaced distance bins spanning 69 pc to 1250 pc. We generated 12 samples from the variational posterior of the 3D dust distribution and release the samples alongside the mean 3D dust map and its corresponding uncertainty. Conclusions. Our map resolves the internal structure of hundreds of molecular clouds in the solar neighborhood and will be broadly useful for studies of star formation, Galactic structure, and young stellar populations. It is available for download in a variety of coordinate systems online and can also be queried via the publicly available dustmaps Python package. [ABSTRACT FROM AUTHOR]

At the time of this meeting, the latest Gaia data release is EDR3, published on 3 December 2020, but the next one, DR3, will appear soon, on 13 June 2022. This contribution describes, on the one hand, Gaia EDR3 results on massive stars and young stellar clusters, placing special emphasis on how a correct treatment of the astrometric and photometric calibration yields results that are simultaneously precise and accurate. On the other hand, it gives a brief description of the exciting results we can expect from Gaia DR3. [ABSTRACT FROM AUTHOR]

We present an update to Via Machinae , an automated stellar stream-finding algorithm based on the deep learning anomaly detector ANODE. Via Machinae identifies stellar streams within Gaia , using only angular positions, proper motions, and photometry, without reference to a model of the Milky Way potential for orbit integration or stellar distances. This new version, Via Machinae 2.0 , includes many improvements and refinements to nearly every step of the algorithm, that altogether result in more robust and visually distinct stream candidates than our original formulation. In this work, we also provide a quantitative estimate of the false positive rate of Via Machinae 2.0 by applying it to a simulated Gaia -mock catalogue based on galaxia , a smooth model of the Milky Way that does not contain substructure or stellar streams. Finally, we perform the first full-sky search for stellar streams with Via Machinae 2.0 , identifying 102 streams at high significance within the Gaia Data Release 2, of which only 10 have been previously identified. While follow-up observations for further confirmation are required, taking into account the false positive rate presented in this work, we expect approximately 90 of these stream candidates to correspond to real stellar structures. [ABSTRACT FROM AUTHOR]

Low-mass (sub)stellar objects represent the low end of the initial mass function, the transition to free-floating planets and a prominent interloper population in the search for high-redshift galaxies. To what accuracy can photometry only classify these? JWST/NIRCam has several advantages over Hubble Space Telescope (HST)/WFC3 near-infrared (NIR): more filters, a greater wavelength range, and greater spatial resolution. Here, we present a catalogue of (sub)stellar dwarfs identified in the cosmic evolution early release science survey (CEERS). We identify 518 stellar objects down to mF200W 28 using half-light radius, a full three magnitudes deeper than typical HST/WFC3 images. A k-means nearest neighbour (kNN) algorithm identifies and types these sources, using four HST/WFC3 and four NIRCam filters, trained on IRTF / spex spectra of nearby brown dwarfs. The kNN with four neighbours classifies well within two subtypes: e.g. M2±2, achieving ∼95 per cent precision and recall. In CEERS, we find 9 M8±2, 2 L6±2, 1 T4±2, and 15 T8±2. We compare the observed long wavelength NIRCam colours – not used in the kNN – to those expected for brown dwarf atmospheric models. The NIRCam F356W−F444W and F410M−F444W colours are redder by a magnitude for the type assigned by the kNN, hinting at a wider variety of atmospheres for these objects. We find a 300–350 pc scale height for M6±2 dwarfs plus a second structural component and a 150–200 pc scale height for T6±2 type dwarfs, consistent with literature values. A single M8±2 dwarf is spectroscopically confirmed at 4 kpc distance. [ABSTRACT FROM AUTHOR]

We analyse 1602 microlensing events found in the VISTA Variables in the Via Lactea (VVV) near-infrared (NIR) survey data. We obtain spatially resolved, efficiency-corrected time-scale distributions across the Galactic bulge (|ℓ| < 10°, | b | < 5°), using a Bayesian hierarchical model. Spatially resolved peaks and means of the time-scale distributions, along with their marginal distributions in strips of longitude and latitude, are in agreement at a 1σ level with predictions based on the Besançon model of the Galaxy. We find that the event time-scales in the central bulge fields (|ℓ| < 5°) are on average shorter than the non-central (|ℓ| > 5°) fields, with the average peak of the lognormal time-scale distribution at 23.6 ± 1.9 d for the central fields and 29.0 ± 3.0 d for the non-central fields. Our ability to probe the structure of the bulge with this sample of NIR microlensing events is limited by the VVV survey's sparse cadence and relatively small number of detected microlensing events compared to dedicated optical surveys. Looking forward to future surveys, we investigate the capability of the Roman telescope to detect spatially resolved asymmetries in the time-scale distributions. We propose two pairs of Roman fields, centred on (ℓ = ±9, 5°, b  = −0.125°) and (ℓ = −5°, b  = ±1.375°) as good targets to measure the asymmetry in longitude and latitude, respectively. [ABSTRACT FROM AUTHOR]

We analyze the photometric data in the Wide layer of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) over ∼1200 deg2 to uncover new halo substructures beyond the distance, D ⊙ ∼ 30 kpc, from the Sun. For this purpose, we develop an isochrone filter for an old, metal-poor stellar system to extract the faint main-sequence stars at a range of distances. With this method, we detect not only the previously discovered substructures such as the Orphan Stream, but also a new overdensity toward Boötes at about D ⊙ ∼ 60 kpc and a new stream-like feature toward Pisces at around D ⊙ ∼ 60 kpc. It has been suggested that a small-scale overdensity exists in this direction of Pisces (the so-called Pisces Overdensity), but our results show that the overdensity is widely spread with a tidally elongated feature. Combining our results with the ongoing Hyper Suprime-Cam narrow-band survey and the near-future spectroscopic survey with Prime Focus Spectrograph (PFS) will allow us to place strong constraints on the origin of these halo substructures. [ABSTRACT FROM AUTHOR]

One of the major challenges we face is how to quickly and accurately create the three-dimensional (3D) density distributions of interstellar dust in the Milky Way using extinction and distance measurements of large samples of stars. In this study, we introduce a novel machine-learning approach that utilizes a convolution neural network, specifically a V-net, to infer the 3D distribution of dust density. Experiments are performed within two regions located towards the Galactic anticentre. The neural network is trained and tested using 10 000 simulations of dust density and line-of-sight extinction maps. Evaluation of the test sample confirms the successful generation of dust density maps from extinction maps by our model. Additionally, the performance of the trained network is evaluated using data from the literature. Our results demonstrate that our model is capable of capturing detailed dust density variations and can recover dust density maps while reducing the 'fingers of god' effect. Moving forward, we plan to apply this model to real observational data to obtain the fine distribution of dust at large and small scales in the Milky Way. [ABSTRACT FROM AUTHOR]

The warp is a well-known undulation of the Milky Way disc. Its structure has been widely studied, but only since Gaia DR2 has it been possible to reveal its kinematic signature beyond the solar neighbourhood. In this work, we present an analysis of the warp traced by Classical Cepheids by means of a Fourier decomposition of their height (Z) and, for the first time, of their vertical velocity (Vz). We find a clear but complex signal that in both variables reveals an asymmetrical warp. In Z , we find the warp to be almost symmetric in amplitude at the disc's outskirts, with the two extremes never being diametrically opposed at any radius and the line of nodes presenting a twist in the direction of stellar rotation for R > 11 kpc. For Vz , in addition to the usual m  = 1 mode, an m  = 2 mode is needed to represent the kinematic signal of the warp, reflecting its azimuthal asymmetry. The line of maximum vertical velocity is similarly twisted as the line of nodes and trails behind by ≈25°. We develop a new formalism to derive the pattern speed and change in amplitude with time |$\dot{A}$| of each Fourier mode at each radius, via a joint analysis of the Fourier decomposition in Z and Vz. By applying it to the Cepheids we find, for the m  = 1 mode, a constant pattern speed in the direction of stellar rotation of 9.2 ± 3.1 km s−1 kpc−1, a negligible |$\dot{A}$| up to R ≈ 14 kpc and a slight increase at larger radii, in agreement with previous works. [ABSTRACT FROM AUTHOR]

A massive molecular cloud complex represents local gravitational potential that can constrain the vertical distribution of surrounding stars and gas. This pinching effect results in the local corrugation of the scale height of stars and gas which is in addition to the global corrugation of the mid-plane of the disc. For the first time, we report observational evidence for this pinching on the H  i vertical structures in the Galactic region (20° < l < 40°), also called W41–W44 region. The H  i vertical distribution is modelled by a double Gaussian profile that physically represents a narrow dense gas distribution confined to the mid-plane embedded in a wider diffuse H  i. We find that the estimate of the H  i scale height distribution of wider components shows corrugated structures at the locations of molecular complexes, as theoretically predicted in literature. While the narrow component is less affected by the pinching, we found a hint of the disc being disrupted by the active dynamics in the local environment of the complex, for example, supernova explosions. Molecular complexes of mass of several |$10^6 \rm M_{\odot } $|⁠ , associated with the mini-starburst region W43 and the supernova remnant W41 show the strongest evidence for the pinching; here a broad trough, with an average width of ∼400 pc and height ∼300 pc, in the disc thickness of the wider component is prominently visible. Searching for similar effect on the stars as well as in the location of other complexes in the Milky Way and other galaxies will be useful to establish this phenomenon more firmly. [ABSTRACT FROM AUTHOR]

We have used data from the Outer Galaxy High-Resolution Survey (OGHReS) to refine the velocities, distances, and physical properties of a large sample of 3584 clumps detected in far-infrared/submillimetre emission in the Hi-GAL survey located in the ℓ = 250°–280° region of the Galactic plane. Using 12CO and 13CO spectra, we have determined reliable velocities to 3412 clumps (95 per cent of the sample). In comparison to the velocities from the Hi-GAL catalogue, we find good agreement for 80 per cent of the sample (within 5 km s−1). Using the higher resolution and sensitivity of OGHReS has allowed us to correct the velocity for 632 clumps and provide velocities for 687 clumps for which no velocity had been previously allocated. The velocities are used with a rotation curve to refine the distances to the clumps and to calculate the clumps' properties using a distance-dependent gas-to-dust ratio. We have determined reliable physical parameters for 3200 outer Galaxy dense clumps (∼90 per cent of the Hi-GAL sources in the region). We find a trend of decreasing luminosity-to-mass ratio with increasing Galactocentric distance, suggesting the star formation efficiency is lower in the outer Galaxy or that it is resulting in more lower mass stars than in the inner Galaxy. We also find a similar surface density for protostellar clumps located in the inner and outer Galaxy, revealing that the surface density requirements for star formation are the same across the Galactic disc. [ABSTRACT FROM AUTHOR]

Theoretical models of spiral arms suggest that the spiral arms provoke a vertical bulk motion in disc stars. By analysing the breathing motion, a coherent asymmetric vertical motion around the mid-plane of the Milky Way disc, with Gaia DR3, we found that a compressing breathing motion presents along the Local arm. On the other hand, with an N -body simulation of an isolated Milky Way-like disc galaxy, we found that the transient and dynamic spiral arms induce compressing breathing motions when the arms are in the growth phase, while the expanding breathing motion appears in the disruption phase. The observed clear alignment of the compressing breathing motion with the Local arm is similar to what is seen in the growth phase of the simulated spiral arms. Hence, we suggest that the Local arm's compressing breathing motion can be explained by the Local arm being in the growth phase of a transient and dynamic spiral arm. We also identified the tentative signatures of the expanding breathing motion associated with the Perseus arm and also the Outer arm coinciding with the compressing breathing motion. This may infer that the Perseus and Outer arms are in the disruption and growth phases, respectively. [ABSTRACT FROM AUTHOR]