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We study gas inflows onto supermassive black holes using hydrodynamics simulations of isolated galaxies and idealized galaxy mergers with an explicit, multiphase interstellar medium (ISM). Our simulations use the recently developed ISM and stellar evolution model called Stars and MUltiphase Gas in GaLaxiEs (SMUGGLE). We implement a novel super-Lagrangian refinement scheme that increases the gas mass resolution in the immediate neighborhood of the black holes (BHs) to accurately resolve gas accretion. We do not include black hole feedback in our simulations. We find that the complex and turbulent nature of the SMUGGLE ISM leads to highly variable BH accretion. BH growth in SMUGGLE converges at gas mass resolutions $\lesssim3\times10^3{\rm M_\odot}$. We show that the low resolution simulations combined with the super-Lagrangian refinement scheme are able to produce central gas dynamics and BH accretion rates very similar to that of the uniform high resolution simulations. We further explore BH fueling by simulating galaxy mergers. The interaction between the galaxies causes an inflow of gas towards the galactic centres and results in elevated and bursty star formation. The peak gas densities near the BHs increase by orders of magnitude resulting in enhanced accretion. Our results support the idea that galaxy mergers can trigger AGN activity, although the instantaneous accretion rate depends strongly on the local ISM. We also show that the level of merger-induced enhancement of BH fueling predicted by the SMUGGLE model is much smaller compared to the predictions by simulations using an effective equation of state model of the ISM., 16 pages, 10 figures

Hydrogen emission lines can provide extensive information about star-forming galaxies in both the local and high-redshift Universe. We present a detailed Lyman continuum (LyC), Lyman-alpha (Ly{\alpha}), and Balmer line (H{\alpha} and H\b{eta}) radiative transfer study of a high-resolution isolated Milky-Way simulation using the Arepo-RT radiation hydrodynamics code with the SMUGGLE galaxy formation model. The realistic framework includes stellar feedback, non-equilibrium thermochemistry, and dust grain evolution in the interstellar medium (ISM). We extend our Cosmic Ly{\alpha} Transfer (COLT) code with photoionization equilibrium Monte Carlo radiative transfer for self-consistent end-to-end (non-)resonant line predictions. Accurate LyC reprocessing to recombination emission requires modelling pre-absorption by dust (27.5%), helium ionization (8.7%), and anisotropic escape fractions (7.9%), as these reduce the available budget for hydrogen line emission (55.9%). We investigate the role of the multiphase dusty ISM, disc geometry, gas kinematics, and star formation activity in governing the physics of emission and escape, focusing on the time variability, gas phase structure, and spatial, spectral, and viewing angle dependence of the emergent photons. Isolated disc simulations are well-suited for comprehensive observational comparisons with local H{\alpha} surveys, but would require a proper cosmological circumgalactic medium (CGM) environment as well as less dust absorption and rotational broadening to serve as analogs for high-redshift Ly{\alpha} emitting galaxies. Future applications of our framework to next-generation cosmological simulations of galaxy formation including radiation-hydrodynamics that resolve, Comment: 28 pages, 32 figures, MNRAS, in press

We investigate the formation of brightest cluster galaxies (BCGs) in the TNG300 cosmological simulation of the IllustrisTNG project. Our cluster sample consists of 700 haloes with $M_{200} \geq 5 \times 10^{13} \, \mathrm{M}_{\odot}$ at $z=0$, along with their progenitors at earlier epochs. This includes 280 systems with $M_{200} \geq 10^{14} \, \mathrm{M}_{\odot}$ at $z=0$, as well as three haloes with $M_{200} \geq 10^{15} \, \mathrm{M}_{\odot}$. We find that the stellar masses and star formation rates of our simulated BCGs are in good agreement with observations at $z \lesssim 0.4$, and that they have experienced, on average, $\sim$2 ($\sim$3) major mergers since $z=1$ ($z=2$). Separating the BCG from the intracluster light (ICL) by means of a fixed 30 kpc aperture, we find that the fraction of stellar mass contributed by ex situ (i.e. accreted) stars at $z=0$ is approximately 70, 80, and 90 per cent for the BCG, BCG+ICL, and ICL, respectively. Tracking our simulated BCGs back in time using the merger trees, we find that they became dominated by ex situ stars at $z \sim $1-2, and that half of the stars that are part of the BCG at $z=0$ formed early ($z \sim 3$) in other galaxies, but `assembled' onto the BCG until later times ($z \approx 0.8$ for the whole sample, $z \approx 0.5$ for BCGs in $M_{200} \geq 5 \times 10^{14} \, \mathrm{M}_{\odot}$ haloes). Finally, we show that the stellar mass profiles of BCGs are often dominated by ex situ stars at all radii, with stars from major mergers being found closer to the centre, while stars that were tidally stripped from other galaxies dominate the outer regions., 19 pages, 11 figures. Accepted for publication in MNRAS

The arrangement of dwarf galaxies in a thin plane surrounding the Milky Way has been thought to contradict the prevailing cosmological model of cold dark matter in the Universe. New work suggests that this arrangement may just be a temporary alignment, bringing our galaxy back into agreement with theoretical expectations once the radial distribution of satellites is taken into account., Comment: Invited 'News and Views' contribution for Nature Astronomy on paper by Sawala et al. 2022 "The Milky Way's plane of satellites is consistent with {\Lambda}CDM"

Ultra-diffuse galaxies (UDGs) are the lowest-surface brightness galaxies known, with typical stellar masses of dwarf galaxies but sizes similar to larger galaxies like the Milky Way. The reason for their extended sizes is debated, with suggested internal processes like angular momentum, feedback or mergers versus external mechanisms or a combination of both. Observationally, we know that UDGs are red and quiescent in groups and clusters while their counterparts in the field are blue and star-forming. This dichotomy suggests environmental effects as main culprit. However, this scenario is challenged by recent observations of isolated quiescent UDGs in the field. Here we use $\Lambda$CDM cosmological hydrodynamical simulation to show that isolated quenched UDGs are formed as backsplash galaxies that were once satellites of another galactic, group or cluster halo but are today a few Mpc away from them. These interactions, albeit brief, remove the gas and tidally strip the outskirts of the dark matter haloes of the now quenched seemingly-isolated UDGs, which are born as star-forming field UDGs occupying dwarf-mass dark matter haloes. Quiescent UDGs may therefore be found in non-negligible numbers in filaments and voids, bearing the mark of past interactions as stripped outer haloes devoid of dark matter and gas compared to dwarfs with similar stellar content., Comment: Author's version for the main article and supplementary information (13 pages, 10 figures). Accepted for publication in Nature Astronomy

We study the formation of ultra-diffuse galaxies (UDGs) using the cosmological hydrodynamical simulation TNG50 of the Illustris-TNG suite. We define UDGs as dwarf galaxies in the stellar mass range $\rm{7.5 \leq log (M_{\star} / M_{\odot}) \leq 9 }$ that are in the $5\%$ most extended tail of the simulated mass-size relation. This results in a sample of UDGs with half-mass radii $\rm{r_{h \star } \gtrsim 2 \ kpc}$ and surface brightness between $\rm{24.5}$ and $\rm{28 \ mag \ arcsec^{-2}}$, similar to definitions of UDGs in observations. The large cosmological volume in TNG50 allows for a comparison of UDGs properties in different environments, from the field to galaxy clusters with virial mass $\rm{M_{200} \sim 2 \times 10^{14} ~ M_{\odot}}$. All UDGs in our sample have dwarf-mass haloes ($\rm{M_{200}\sim 10^{11} ~ M_{\odot} }$) and show the same environmental trends as normal dwarfs: field UDGs are star-forming and blue while satellite UDGs are typically quiescent and red. The TNG50 simulation predicts UDGs that populate preferentially higher spin haloes and more massive haloes at fixed $\rm{M_{\star}}$ compared to non-UDG dwarfs. This applies also to most satellite UDGs, which are actually ``born" UDGs in the field and infall into groups and clusters without significant changes to their size. We find, however, a small subset of satellite UDGs ($\lesssim 10 \%$) with present-day stellar size a factor $\geq 1.5$ larger than at infall, confirming that tidal effects, particularly in the lower mass dwarfs, are also a viable formation mechanism for some of these dwarfs, although subdominant in this simulation., 17 pages, 14 figures, accepted to MNRAS

We present a study of the stellar populations of globular clusters (GCs) in the Virgo Cluster core with a homogeneous spectroscopic catalog of 692 GCs within a major axis distance $R_{\rm maj} = $ 840 kpc from M87. We investigate radial and azimuthal variations in the mean age, total metallicity, [Fe/H], and $\alpha$-element abundance, of blue (metal-poor) and red (metal-rich) GCs using their co-added spectra. We find that the blue GCs have a steep radial gradient in [Z/H] within $R_{\rm maj} =$ 165 kpc, with roughly equal contributions from [Fe/H] and [$\alpha$/Fe], and flat gradients beyond. By contrast, the red GCs show a much shallower gradient in [Z/H], which is entirely driven by [Fe/H]. We use GC-tagged Illustris simulations to demonstrate an accretion scenario where more massive satellites (with more metal- and $\alpha$-rich GCs) sink further into the central galaxy than less massive ones, and where the gradient flattening occurs because of the low GC occupation fraction of low-mass dwarfs disrupted at larger distances. The dense environment around M87 may also cause the steep [$\alpha$/Fe] gradient of the blue GCs, mirroring what is seen in the dwarf galaxy population. The progenitors of red GCs have a narrower mass range than those of blue GCs, which makes their gradients shallower. We also explore spatial inhomogeneity in GC abundances, finding that the red GCs to the northwest of M87 are slightly more metal-rich. Future observations of GC stellar population gradients will be useful diagnostics of halo merger histories., Comment: 22 pages, 8 figures, 4 tables, accepted for publication in ApJ

We study the role of group infall in the assembly and dynamics of galaxy clusters in $\Lambda$CDM. We select $10$ clusters with virial mass $M_{\rm 200} \sim 10^{14} \, M_{\odot}$ from the cosmological hydrodynamical simulation Illustris and follow their galaxies with stellar mass $M_{\star} \geq 1.5 \times 10^8 \, M_{\odot}$. A median of $\sim 38\%$ of surviving galaxies at $z=0$ are accreted as part of groups and did not infall directly from the field, albeit with significant cluster-to-cluster scatter. The evolution of these galaxy associations is quick, with observational signatures of their common origin eroding rapidly in $1$-$3$ Gyr after infall. Substructure plays a dominant role in fostering the conditions for galaxy mergers to happen, even within the cluster environment. Integrated over time, we identify (per cluster) an average of $17 \pm 6$ mergers that occur in infalling galaxy associations, of which $7 \pm 3$ occur well within the virial radius of their cluster hosts. The number of mergers show large dispersion from cluster to cluster, with our most massive system having $42$ mergers above our mass cut-off. These mergers, which are typically gas rich for dwarfs and a combination of gas rich and gas poor for $M_{\star} \sim 10^{11} \, M_{\odot}$, may contribute significantly within $\Lambda$CDM to the formation of specific morphologies, such as lenticulars (S0) and blue compact dwarfs in groups and clusters., Comment: 12 pages, 11 figures. Accepted for publication in MNRAS

Galaxies and their dark-matter halos have posed several challenges to the Dark Energy plus Cold Dark Matter (LCDM) cosmological model. These discrepancies between observations and theory intensify for the lowest-mass (`dwarf') galaxies. LCDM predictions for the number, spatial distribution, and internal structure of low-mass dark-matter halos have historically been at odds with observed dwarf galaxies, but this is partially expected, because many predictions modeled only the dark-matter component. Any robust LCDM prediction must include, hand-in-hand, a model for galaxy formation to understand how baryonic matter populates and affects dark-matter halos. In this article, we review the most notable challenges to LCDM regarding dwarf galaxies, and we discuss how recent cosmological numerical simulations have pinpointed baryonic solutions to these challenges. We identify remaining tensions, including the diversity of the inner dark-matter content, planes of satellites, stellar morphologies, and star-formation quenching. Their resolution, or validation as actual problems to LCDM, will likely require both refining galaxy formation models and improving numerical accuracy in simulations., Comment: Invited Review for Nature Astronomy, 25 pages (12 main article), 5 figures. Accepted version

Impulsive supernova feedback and non-standard dark matter models, such as self-interacting dark matter (SIDM), are the two main contenders for the role of the dominant core formation mechanism at the dwarf galaxy scale. Here we show that the impulsive supernova cycles that follow episodes of bursty star formation leave distinct features in the distribution function of stars: groups of stars with similar ages and metallicities develop overdense shells in phase space. If cores are formed through supernova feedback, we predict the presence of such features in star-forming dwarf galaxies with cored host halos. Their systematic absence would favor alternative dark matter models, such as SIDM, as the dominant core formation mechanism., Comment: 16 pages, 13 figures, arxiv version contains both the letter and the supplemental material \c{opyright} 2022 American Physical Society

We present a post-processing catalog of globular clusters (GCs) for the $39$ most massive groups and clusters in the TNG50 simulation of the IlllustrisTNG project (virial masses $M_{200} =[5\times 10^{12} \rm - 2 \times 10^{14}$] M$_{\odot}$). We tag GC particles to all galaxies with stellar mass $M_* \geq 5\times10^6$ M$_{\odot}$, and we calibrate their masses to reproduce the observed power-law relation between GC mass and halo mass for galaxies with $M_{200} \geq 10^{11}$ M$_{\odot}$ (corresponding to $M_* \sim 10^9$ $M_{\odot}$). Here we explore whether an extrapolation of this $M_{\rm GC}$-$M_{200}$ relation to lower-mass dwarfs is consistent with current observations. We find a good agreement between our predicted number and specific frequency of GCs in dwarfs with $\rm M_*=[5 \times 10^6 \rm - 10^9]$ M$_{\odot}$ and observations. Moreover, we predict a steep decline in the GC occupation fraction for dwarfs with $M_*, 16 pages, 8 figures (main body of paper). Resubmitted to MNRAS after positive referee report. The globular cluster catalogs for the 39 most massive groups and clusters in TNG50 are available for download from the IllustrisTNG public data release at www.tng-project.org/doppel22

Characterizing the predicted environments of dwarf galaxies like the Large Magellanic Cloud (LMC) is becoming increasingly important as next generation surveys push sensitivity limits into this low-mass regime at cosmological distances. We study the environmental effects of LMC-mass halos ($M_{200m} \sim 10^{11}$ M$_\odot$) on their populations of satellites ($M_\star \geq 10^4$ M$_\odot$) using a suite of zoom-in simulations from the Feedback In Realistic Environments (FIRE) project. Our simulations predict significant hot coronas with $T\sim10^5$ K and $M_\text{gas}\sim10^{9.5}$ M$_\odot$. We identify signatures of environmental quenching in dwarf satellite galaxies, particularly for satellites with intermediate mass ($M_\star = 10^{6-7}$ M$_\odot$). The gas content of such objects indicates ram-pressure as the likely quenching mechanism, sometimes aided by star formation feedback. Satellites of LMC-mass hosts replicate the stellar mass dependence of the quiescent fraction found in satellites of MW mass hosts (i.e. that the quiescent fraction increases as stellar mass decreases). Satellites of LMC-mass hosts have a wider variety of quenching times when compared to the strongly bi-modal distribution of quenching times of nearby centrals. Finally, we identify significant tidal stellar structures around four of our six LMC-analogs, suggesting that stellar streams may be common. These tidal features originated from satellites on close orbits, extend to $\sim$80 kpc from the central galaxy, and contain $\sim10^{6-7}$ M$_\odot$~of stars., 14 pages, 10 figures, submitted to MNRAS

Within $��$CDM, dwarf galaxies like the Large Magellanic Cloud (LMC) are expected to host numerous dark matter subhalos, several of which should host faint dwarf companions. Recent Gaia proper motions confirm new members of the LMC-system in addition to the previously known SMC, including two classical dwarf galaxies ($M_\ast$ > $10^5$ M$_{\odot}$; Carina and Fornax) as well as several ultra-faint dwarfs (Car2, Car3, Hor1, and Hyd1). We use the Feedback In Realistic Environments (FIRE) simulations to study the dark and luminous (down to ultrafaint masses, $M_\ast$ ~ $6\times10^3$ M$_{\odot}$) substructure population of isolated LMC-mass hosts ($M_\text{200}$ = $1-3\times10^{11}$ M$_{\odot}$) and place the Gaia + DES results in a cosmological context. By comparing number counts of subhalos in simulations with and without baryons, we find that, within 0.2 $r_\text{200}$, LMC-mass hosts deplete ~30% of their substructure, significantly lower than the ~70% of substructure depleted by Milky Way (MW) mass hosts. For our highest resolution runs ($m_\text{bary}$ = 880 M$_{\odot}$), ~5-10 subhalos form galaxies with $M_\ast$ > $10^4$ M$_{\odot}$, in agreement with the 7 observationally inferred pre-infall LMC companions. However, we find steeper simulated luminosity functions than observed, hinting at observation incompleteness at the faint end. The predicted DM content for classical satellites in FIRE agrees with observed estimates for Carina and Fornax, supporting the case for an LMC association. We predict that tidal stripping within the LMC potential lowers the inner dark matter density of ultra faint companions of the LMC. Thus, in addition to their orbital consistency, the low densities of dwarfs Car2, Hyd1, and Hyd2 reinforce their likelihood of Magellanic association., 19 pages, 9 figures, 4 tables. Accepted for publication in MNRAS

We use a dedicated 0.7-m telescope to image the halos of 119 galaxies in the Local Volume to $\mu_r \sim 28-30$ mag/arcsec$^2$. The sample is primarily from the 2MASS Large Galaxy Atlas and extended to include nearby dwarf galaxies and more distant giant ellipticals, and spans fully the galaxy colour-magnitude diagram including the blue cloud and red sequence. We present an initial overview, including deep images of our galaxies. Our observations reproduce previously reported low surface brightness structures, including extended plumes in M51, and a newly discovered tidally extended dwarf galaxy in NGC7331. Low surface brightness structures, or "envelopes", exceeding 50 kpc in diameter are found mostly in galaxies with $M_V, Comment: 34 pages, 27 figures, 4 tables, accepted for publication in MNRAS. The observations (FITS images) will be available by October 2019

Winds driven by stellar feedback are an essential part of the galactic ecosystem and are the main mechanism through which low-mass galaxies regulate their star formation. These winds are generally observed to be multi-phase with detections of entrained neutral and molecular gas. They are also thought to enrich the circum-galactic medium around galaxies with metals and dust. This ejected dust encodes information about the integrated star formation and outflow history of the galaxy. It is therefore, important to understand how much dust is entrained and driven out of the disc by galactic winds. Here we demonstrate that stellar feedback is efficient in driving dust-enriched winds and eject enough material to account for the amount of extraplanar dust observed in nearby galaxies. The amount of dust in the wind depends on the sites from where they are launched, with dustier galaxies launching more dust enriched outflows. Moreover, the outflowing cold-dense gas is significantly more dust-enriched than the volume filling hot tenuous material, naturally reproducing the complex multiphase structure of the outflowing wind observed in nearby galaxies. These results provide an important new insight into the dynamics, structure, and composition of galactic winds and their role in determining the dust content of the extragalactic gas in galaxies., 6 pages, 4 figures, submitted to ApJL, Comments welcome

Galaxy mergers are known to host abundant young massive cluster (YMC) populations, whose formation mechanism is still not well-understood. Here, we present a high-resolution galaxy merger simulation with explicit star formation and stellar feedback prescriptions to investigate how mergers affect the properties of the interstellar medium and YMCs. Compared with a controlled simulation of an isolated galaxy, the mass fraction of dense and high-pressure gas is much higher in mergers. Consequently, the mass function of both molecular clouds and YMCs becomes shallower and extends to higher masses. Moreover, cluster formation efficiency is significantly enhanced and correlates positively with the star formation rate surface density and gas pressure. We track the orbits of YMCs and investigate the time evolution of tidal fields during the course of the merger. At an early stage of the merger, the tidal field strength correlates positively with YMC mass, $\lambda_{\rm tid}\propto M^{0.71}$, which systematically affects the shape of the mass function and age distribution of the YMCs. At later times, most YMCs closely follow the orbits of their host galaxies, gradually sinking into the center of the merger remnant due to dynamical friction, and are quickly dissolved via efficient tidal disruption. Interestingly, YMCs formed during the first passage, mostly in tidal tails and bridges, are distributed over a wide range of galactocentric radii, greatly increasing their survivability because of the much weaker tidal field in the outskirts of the merger system. These YMCs are promising candidates for globular clusters that survive to the present day., Comment: 15 pages, 12 figures. MNRAS accepted on Mar. 9th

The tension between the diverging density profiles in Lambda Cold Dark Matter ($\Lambda$CDM) simulations and the constant-density inner regions of observed galaxies is a long-standing challenge known as the `core-cusp' problem. We demonstrate that the \texttt{SMUGGLE} galaxy formation model implemented in the \textsc{Arepo} moving mesh code forms constant-density cores in idealized dwarf galaxies of $M_\star \approx 8 \times 10^7$ M$_{\odot}$ with initially cuspy dark matter halos of $M_{200} \approx 10^{10}$ M$_{\odot}$. Identical initial conditions run with the Springel and Hernquist (2003; SH03) feedback model preserve cuspiness. Literature on the subject has pointed to the low density threshold for star formation, $\rho_\text{th}$, in SH03-like models as an obstacle to baryon-induced core formation. Using a \texttt{SMUGGLE} run with equal $\rho_\text{th}$ to SH03, we demonstrate that core formation can proceed at low density thresholds, indicating that $\rho_\text{th}$ is insufficient on its own to determine whether a galaxy develops a core. We suggest that the ability to resolve a multiphase interstellar medium at sufficiently high densities is a more reliable indicator of core formation than any individual model parameter. In \texttt{SMUGGLE}, core formation is accompanied by large degrees of non-circular motion, with gas rotational velocity profiles that consistently fall below the circular velocity $v_\text{circ} = \sqrt{GM/R}$ out to $\sim 2$ kpc. This may artificially mimic larger core sizes when derived from observable quantities compared to the size measured from the dark matter distribution ($\sim 0.5$ kpc), highlighting the need for careful modeling in the inner regions of dwarfs to infer the true distribution of dark matter., Comment: 18 pages, 15 figures, submitted to MNRAS

The abundance of the faintest galaxies provides insight into the nature of dark matter and the process of dwarf galaxy formation. In the LCDM scenario, low mass halos are so numerous that the efficiency of dwarf formation must decline sharply with decreasing halo mass in order to accommodate the relative scarcity of observed dwarfs and satellites in the Local Group. The nature of this decline contains important clues to the mechanisms regulating the onset of galaxy formation in the faintest systems. We explore here two possible models for the stellar mass ($M_*$)-halo mass ($M_{200}$) relation at the faint end, motivated by some of the latest LCDM cosmological hydrodynamical simulations. One model includes a sharp mass threshold below which no luminous galaxies form, as expected if galaxy formation proceeds only in systems above the Hydrogen-cooling limit. In the second model, $M_*$ scales as a steep power-law of $M_{200}$ with no explicit cutoff, as suggested by recent semianalytic work. Although both models predict satellite numbers around Milky Way-like galaxies consistent with current observations, they predict vastly different numbers of ultra-faint dwarfs and of satellites around isolated dwarf galaxies. Our results illustrate how the satellite mass function around dwarfs may be used to probe the $M_*$-$M_{200}$ relation at the faint end and to elucidate the mechanisms that determine which low-mass halos "light up" or remain dark in the LCDM scenario., Comment: 14 pages, 11 figures. Accepted version for publication in MNRAS

We present a suite of 16 high-resolution hydrodynamic simulations of an isolated dwarf galaxy (gaseous and stellar disk plus a stellar bulge) within an initially cuspy dark matter (DM) halo, including self-interactions between the DM particles (SIDM); as well as stochastic star formation and subsequent supernova feedback (SNF), implemented using the stellar feedback model SMUGGLE. The simulations start from identical initial conditions and we regulate the strength of SIDM and SNF by systematically varying the SIDM momentum transfer cross section and the gas density threshold for star formation. The DM halo forms a constant density core of similar size and shape for several combinations of those two parameters. Haloes with cores that are formed due to SIDM (adiabatic cusp-core transformation) have velocity dispersion profiles which are closer to isothermal than those of haloes with cores that are formed due to SNF in simulations with bursty star formation (impulsive cusp-core transformation). Impulsive SNF can generate positive stellar age gradients and increase random motion in the gas at the centre of the galaxy. Simulated galaxies in haloes with cores that were formed adiabatically are spatially more extended, with stellar metallicity gradients that are shallower (at late times) than those of galaxies in other simulations. Such observable properties of the gas and the stars, which indicate either an adiabatic or an impulsive evolution of the gravitational potential, may be used to determine whether observed cores in DM haloes are formed through self-interactions between the DM particles or in response to impulsive SNF., Comment: accepted for publication in MNRAS, contains 3 appendices; main paper: 18 pages, 11 figures; appendices: 8 pages, 7 figures; for questions/comments contact jburger@ucr.edu (Jan Burger)

While it is generally believed that supermassive black holes (SMBH) lie in most galaxies with bulges, few SMBHs have been confirmed in bulgeless galaxies. Identifying such a population could provide important insights to the BH seed population and secular BH growth. To this end, we obtained near-infrared spectroscopic observations of a sample of low-redshift bulgeless galaxies with mid-infrared colors suggestive of AGN. We find additional evidence of AGN activity (such as coronal lines and broad permitted lines) in 69$\%$ (9/13) of the sample, demonstrating that mid-infrared selection is a powerful tool to detect AGN. More than half of the galaxies with confirmed AGN activity show fast outflows in [O III] in the optical and/or [Si VI] in the NIR, with the latter generally having much faster velocities that are also correlated to their spatial extent. We are also able to obtain virial BH masses for some targets and find they fall within the scatter of other late-type galaxies in the $M_{\rm{BH}}$-$M_{\rm{stellar}}$ relation. The fact that they lack a significant bulge component indicates that secular processes, likely independent of major mergers, grew these BHs to supermassive sizes. Finally, we analyze the rotational gas kinematics and find two notable exceptions: two AGN hosts with outflows that appear to be rotating faster than expected. There is an indication that these two galaxies have stellar masses significantly lower than expected from their dark matter halo masses. This, combined with the observed AGN activity and strong gas outflows may be evidence of the effects of AGN feedback., Accepted by ApJ. 25 pages, 8 figures, and 6 tables

We use the APOSTLE $\Lambda$CDM cosmological hydrodynamical simulations of the Local Group to study the recent accretion of massive satellites into the halo of Milky Way (MW)-sized galaxies. These systems are selected to be close analogues to the Large Magellanic Cloud (LMC), the most massive satellite of the MW. The simulations allow us to address, in a cosmological context, the impact of the Clouds on the MW, including the contribution of Magellanic satellites to the MW satellite population, and the constraints placed on the Galactic potential by the motion of the LMC. We show that LMC-like satellites are twice more common around Local Group-like primaries than around isolated halos of similar mass; these satellites come from large turnaround radii and are on highly eccentric orbits whose velocities at first pericentre are comparable with the primary's escape velocity. This implies $V_{\rm esc}^{\rm MW} (50 $ kpc$)\sim 365$ km/s, a strong constraint on Galactic potential models. LMC analogues contribute about 2 satellites with $M_*>10^5\, M_\odot$, having thus only a mild impact on the luminous satellite population of their hosts. At first pericentre, LMC-associated satellites are close to the LMC in position and velocity, and are distributed along the LMC's orbital plane. Their orbital angular momenta roughly align with the LMC's, but, interestingly, they may appear to "counter-rotate" the MW in some cases. These criteria refine earlier estimates of the LMC association of MW satellites: only the SMC, Hydrus1, Car3, Hor1, Tuc4, Ret2 and Phoenix2 are compatible with all criteria. Carina, Grus2, Hor2 and Fornax are less probable associates given their large LMC relative velocity., Comment: 17 pages, 14 figures, 1 table. Accepted for publication in MNRAS

We introduce the Stars and MUltiphase Gas in GaLaxiEs -- SMUGGLE model, an explicit and comprehensive stellar feedback model for the moving-mesh code arepo. This novel sub-resolution model resolves the multiphase gas structure of the interstellar medium and self-consistently generates gaseous outflows. The model implements crucial aspects of stellar feedback including photoionization, radiation pressure, energy and momentum injection from stellar winds and from supernovae. We explore this model in high-resolution isolated simulations of Milky Way-like disc galaxies. Stellar feedback regulates star formation to the observed level and naturally captures the establishment of a Kennicutt-Schmidt relation. This result is achieved independent of the numerical mass and spatial resolution of the simulations. Gaseous outflows are generated with average mass loading factors of the order of unity. Strong outflow activity is correlated with peaks in the star formation history of the galaxy with evidence that most of the ejected gas eventually rains down onto the disc in a galactic fountain flow that sustains late-time star formation. Finally, the interstellar gas in the galaxy shows a distinct multiphase distribution with a coexistence of cold, warm and hot phases., Comment: 29 pages, 15 figures, 3 tables, 1 appendix. Accepted for publication in MNRAS. Updated manuscript to match the published version

We present a study of ultra-diffuse galaxies (UDGs) in the Virgo Cluster based on deep imaging from the Next Generation Virgo Cluster Survey (NGVS). Applying a new definition for the UDG class based on galaxy scaling relations, we define samples of 44 and 26 UDGs using expansive and restrictive selection criteria, respectively. Our UDG sample includes objects that are significantly fainter than previously known UDGs: i.e., more than half are fainter than $\langle\mu\rangle_e \sim27.5$ mag arcsec$^{-2}$. The UDGs in Virgo's core region show some evidence for being structurally distinct from "normal" dwarf galaxies, but this separation disappears when considering the full sample of galaxies throughout the cluster. UDGs are more centrally concentrated in their spatial distribution than other Virgo galaxies of similar luminosity, while their morphologies demonstrate that at least some UDGs owe their diffuse nature to physical processes---such as tidal interactions or low-mass mergers---that are at play within the cluster environment. The globular cluster (GC) systems of Virgo UDGs have a wide range in specific frequency ($S_N$), with a higher mean $S_N$ than "normal" Virgo dwarfs, but a lower mean $S_N$ than Coma UDGs at fixed luminosity. Their GCs are predominantly blue, with a small contribution from red clusters in the more massive UDGs. The combined GC luminosity function is consistent with those observed in dwarf galaxies, showing no evidence of being anomalously luminous. The diversity in their morphologies and their GC properties suggests no single process has given rise to all objects within the UDG class. Based on the available evidence, we conclude that UDGs are simply those systems that occupy the extended tails of the galaxy size and surface brightness distributions., Comment: 31 pages, 16 figures, 4 tables, accepted for publication in ApJ

We study evolution of self-interacting dark matter (SIDM) subhalos in the Milky Way (MW) tidal field. The interaction between the subhalos and the MW's tides lead to more diverse dark matter distribution in the inner region, compared to their cold dark matter counterparts. We test this scenario with two MW satellite galaxies, Draco and Fornax, opposite extremes in the inner dark matter content, and find that they can be accommodated within the SIDM model proposed to explain the diverse rotation curves of spiral galaxies in the field., Comment: 6 pages, 3 figures. Updated figures and text. Accepted for publication in PRL

Globular clusters (GCs) are often used to estimate the dark matter content of galaxies, especially dwarfs, where other kinematic tracers are lacking. These estimates typically assume spherical symmetry and dynamical equilibrium, assumptions that may not hold for the sparse GC population of dwarfs in galaxy clusters. We use a catalog of GCs tagged onto the Illustris simulation to study the accuracy of GC-based mass estimates. We focus on galaxies in the stellar mass range 10$^{8} - 10^{11.8}$ M$_{\odot}$ identified in $9$ simulated Virgo-like clusters. Our results indicate that mass estimates are, on average, quite accurate in systems with GC numbers $N_{\rm GC} \geq 10$ and where the uncertainty of individual GC line-of-sight velocities is smaller than the inferred velocity dispersion, $\sigma_{\rm GC}$. In cases where $N_{\rm GC} \leq 10$, however, biases may result depending on how $\sigma_{\rm GC}$ is computed. We provide calibrations that may help alleviate these biases in methods widely used in the literature. As an application, we find a number of dwarfs with $M_{*} \sim 10^{8.5}\, M_{\odot}$ (comparable to the ultradiffuse galaxy DF2, notable for the low $\sigma_{GC}$ of its $10$ GCs) with $\sigma_{\rm GC} \sim 7$ - $15\; \rm km \rm s^{-1}$. These DF2 analogs correspond to relatively massive systems at their infall time ($M_{200} \sim 1$ - $3 \times 10^{11}$ $M_{\odot}$) which have retained only $3$-$17$ GCs and have been stripped of more than 95$\%$ of their dark matter. Our results suggest that extreme tidal mass loss in otherwise normal dwarf galaxies may be a possible formation channel for ultradiffuse objects like DF2., Comment: 19 pages, 15 figures. Accepted to MNRAS Dec. 11 2020

Recent studies using Gaia DR2 have identified a massive merger in the early history of the Milky Way (MW) whose debris is dominated by radial and counterrotating orbits. This event, dubbed the Gaia-Enceladus/Gaia-Sausage (GE/GS), is also hypothesized to have built the majority of the inner stellar halo. We use the cosmological hydrodynamic simulation Illustris to place this merger in the context of galaxy assembly within lambda cold dark matter. From ∼150 MW analogues, $\sim \!80 {{ \rm {per\ cent}}}$ have experienced at least one merger of similar mass and infall time as the GE/GS. Within this sample, 37 have debris as radial as the GE/GS, which we dub the ancient radial mergers (ARMs). Counterrotation is not rare among ARMs, with $43 {{ \rm {per\ cent}}}$ having $\gt 40 {{ \rm {per\ cent}}}$ of their debris in counterrotating orbits. However, the compactness inferred for the GE/GS debris given its large radial orbital anisotropy, β, and its substantial contribution to the stellar halo are difficult to reproduce. The median radius of ARM debris is r*,deb ≃ 45 kpc, while GE/GS is thought to be mostly contained within r ∼ 30 kpc. For most MW analogues, few mergers are required to build the inner stellar halo, and ARM debris only accounts for (median) $\sim \!12 {{ \rm {per\ cent}}}$ of inner accreted stars. Encouragingly, we find one ARM that is both compact and dominates the inner halo of its central, making it our best GE/GS analogue. Interestingly, this merger deposits a significant number of stars (M* ≃ 1.5 × 109 M⊙) in the outer halo, suggesting that an undiscovered section of GE/GS may await detection.

Aims. We present the first three-dimensional internal motions for individual stars in the Draco dwarf spheroidal galaxy. Methods. By combining first-epoch $Hubble$ $Space$ $Telescope$ observations and second-epoch $Gaia$ Data Release 2 positions, we measured the proper motions of $149$ sources in the direction of Draco. We determined the line-of-sight velocities for a sub-sample of $81$ red giant branch stars using medium resolution spectra acquired with the DEIMOS spectrograph at the Keck II telescope. Altogether, this resulted in a final sample of $45$ Draco members with high-precision and accurate 3D motions, which we present as a table in this paper. Results. Based on this high-quality dataset, we determined the velocity dispersions at a projected distance of $\sim120$ pc from the centre of Draco to be $\sigma_{R} =11.0^{+2.1}_{-1.5}$ km/s, $\sigma_{T}=9.9^{+2.3}_{-3.1}$ km/s and $\sigma_{LOS}=9.0^{+1.1}_{-1.1}$ km/s in the projected radial, tangential, and line-of-sight directions. This results in a velocity anisotropy $\beta=0.25^{+0.47}_{-1.38}$ at $r \gtrsim120$ pc. Tighter constraints may be obtained using the spherical Jeans equations and assuming constant anisotropy and Navarro-Frenk-White (NFW) mass profiles, also based on the assumption that the 3D velocity dispersion should be lower than $\approx 1/3$ of the escape velocity of the system. In this case, we constrain the maximum circular velocity $V_{max}$ of Draco to be in the range of $10.2-17.0$ km/s. The corresponding mass range is in good agreement with previous estimates based on line-of-sight velocities only. Conclusions. Our Jeans modelling supports the case for a cuspy dark matter profile in this galaxy. Firmer conclusions may be drawn by applying more sophisticated models to this dataset and with new datasets from upcoming $Gaia$ releases., Comment: 12 pages, 15 figures, 3 tables. Accepted for publication by A&A

We present a novel framework to self-consistently model the effects of radiation fields, dust physics and molecular chemistry (H$_2$) in the interstellar medium (ISM) of galaxies. The model combines a state-of-the-art radiation hydrodynamics module with a non-equilibrium thermochemistry module that accounts for H$_2$ coupled to a realistic dust formation and destruction model, all integrated into the new stellar feedback framework SMUGGLE. We test this model on high-resolution isolated Milky-Way (MW) simulations. We show that photoheating from young stars makes stellar feedback more efficient, but this effect is quite modest in low gas surface density galaxies like the MW. The multi-phase structure of the ISM, however, is highly dependent on the strength of the interstellar radiation field. We are also able to predict the distribution of H$_2$, that allow us to match the molecular Kennicutt-Schmidt (KS) relation, without calibrating for it. We show that the dust distribution is a complex function of density, temperature and ionization state of the gas which cannot be reproduced by simple scaling relations often used in the literature. Our model is only able to match the observed dust temperature distribution if radiation from the old stellar population is considered, implying that these stars have a non-negligible contribution to dust heating in the ISM. Our state-of-the-art model is well-suited for performing next generation cosmological galaxy formation simulations, which will be able to predict a wide range of resolved ($\sim 10$ pc) properties of galaxies., 17 Pages, 15 figures, Submitted to MNRAS, Comments welcome

We use deep Hubble Space Telescope imaging to derive a distance to the Virgo Cluster ultradiffuse galaxy (UDG) VCC 615 using the tip of the red giant branch (TRGB) distance estimator. We detect 5,023 stars within the galaxy, down to a 50% completeness limit of F814W = 28.0, using counts in the surrounding field to correct for contamination due to background sources and Virgo intracluster stars. We derive an extinction-corrected F814W tip magnitude of $m_{\rm tip,0} = 27.19^{+0.07}_{-0.05}$, yielding a distance of $d=17.7^{+0.6}_{-0.4}$ Mpc. This places VCC 615 on the far side of the Virgo Cluster ($d_{\rm Virgo} = 16.5 Mpc$), at a Virgocentric distance of 1.3 Mpc and near the virial radius of the main body of Virgo. Coupling this distance with the galaxy's observed radial velocity, we find that VCC 615 is on an outbound trajectory, having survived a recent passage through the inner parts of the cluster. Indeed, our orbit modeling gives a 50% chance the galaxy passed inside the Virgo core (r, 18 pages, 10 figures, accepted for publication in The Astrophysical Journal

We present a summary of the predictions from numerical simulations to our understanding of dwarf galaxies. It centers the discussion around the Λ Cold Dark Matter scenario (ΛCDM) but discusses also implications for alternative dark matter models. Four key predictions are identified: the abundance of dwarf galaxies, their dark matter content, their relation with environment and the existence of dwarf satellites orbiting dwarf field galaxies. We discuss tensions with observations and identify the most exciting predictions expected from simulations in the future, including i) the existence of “dark galaxies” (dark matter halos without stars), ii) the ability to resolve the structure (size, morphology, dark matter distribution) in dwarfs and iii) the number of ultra-faint satellites around dwarf galaxies. All of these predictions shall inform future observations, not only the faintest galaxies to be discovered within the Local Volume but also distant dwarfs driving galaxy formation in the early universe.