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We measure homogeneous distances to M31 and 38 associated stellar systems ($-$16.8$\le M_V \le$ $-$6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From $>700$ orbits of new/archival ACS imaging, we identify $>4700$ RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 days and 0.04 mag. Based on Period-Wesenheit-Metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of $\sim20$ kpc (3%) and $\sim10$ kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that $\sim80$% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms $7-23$ kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess physical proximity of notable associations such as the NGC 147/185 pair and M33/AND XXII; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with $M_V > -9.5$, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system., 32 pages, 15 figures, accepted for publication on ApJ

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf (UFD) galaxies Pegasus III (Peg III) and Pisces II (Psc II), two of the most distant satellites in the halo of the Milky Way (MW). We measure the structure of both galaxies, derive mass-to-light ratios with newly determined absolute magnitudes, and compare our findings to expectations from UFD-mass simulations. For Peg III, we find an elliptical half-light radius of $a_h{{=}}1.88^{+0.42}_{-0.33}$ arcminutes ($118^{+31}_{-30}$ pc) and $M_V{=}{-4.17}^{+0.19}_{-0.22}$; for Psc II, we measure $a_h{=}1.31^{+0.10}_{-0.09}$ arcminutes ($69\pm8$ pc) and $M_V{=}{-4.28}^{+0.19}_{-0.16}$. We do not find any morphological features that indicate a significant interaction between the two has occurred, despite their close separation of only $\sim$40 kpc. Using proper motions (PMs) from Gaia early Data Release 3, we investigate the possibility of any past association by integrating orbits for the two UFDs in a MW-only and a combined MW and Large Magellanic Cloud (LMC) potential. We find that including the gravitational influence of the LMC is crucial, even for these outer-halo satellites, and that a possible orbital history exists where Peg III and Psc II experienced a close ($\sim$10-20 kpc) passage about each other just over $\sim$1 Gyr ago, followed by a collective passage around the LMC ($\sim$30-60 kpc) just under $\sim$1 Gyr ago. Considering the large uncertainties on the PMs and the restrictive priors imposed to derive them, improved PM measurements for Peg III and Psc II will be necessary to clarify their relationship. This would add to the rare findings of confirmed pairs of satellites within the Local Group., Comment: 18 pages, 11 figures; submitted to ApJ

We present resolved HI synthesis maps from the Very Large Array (VLA) of three interacting dwarf systems: the NGC 3664 dwarf pair, the NGC 3264 dwarf pair, and the UGC 4638 dwarf triplet. All three dwarf systems are captured at various stages of interaction and span a range of environments. We detect clear hallmarks of tidal interactions through the presence of HI bridges, and diffuse HI extensions that surround the dwarfs. We overlay the HI data on Pan-STARRS r-band images and find further evidence of tidal interactions through coincident distorted HI and tidal stellar features in NGC 3264 and UGC 4638, and an unwound spiral arm pointing towards its smaller companion in NGC 3264. In UGC 4638, both the gas and diffuse stars are extended to similar radii east of the primary, which could indicate that the smaller dwarf in the system has already completed one pass through the primary. We additionally find that our three systems, and those from the Local Volume TiNy Titans survey, are not HI deficient and thus the interaction has not resulted in a loss of gas from the systems. A comparison with non-interacting dwarf galaxies shows that the interactions have a significant impact on the kinematics of the systems. Our new resolved HI kinematics, combined with detailed stellar and HI morphologies, provide crucial constraints for future dynamical modelling of hierarchical mergers and the baryon cycle at the low-mass scale., Comment: 17 pages, 6 figures, accepted to AJ

High precision proper motion (PM) measurements are available for approximately 20% of all known dwarf satellite galaxies of the Milky Way (MW). Here we extend the Bayesian framework of Patel et al. (2017b) to include all MW satellites with measured 6D phase space information and apply it with the Illustris-Dark simulation to constrain the MW's mass. Using the properties of each MW satellite individually, we find that the scatter among mass estimates is reduced when the magnitude of specific orbital angular momentum (j) is adopted rather than their combined instantaneous positions and velocities. We also find that high j satellites (i.e. Leo II) constrain the upper limits for the MW's mass and low j satellites rather than the highest speed satellites (i.e. Leo I and LMC), set the lower mass limits. When j of all classical satellites is used to simultaneously estimate the MW's mass, we conclude the halo mass is $0.85^{+0.23}_{-0.26}\times 10^{12} \rm \, M_{\odot}$ (including Sagittarius dSph) and $0.96^{+0.29}_{-0.28} \times 10^{12} \rm \, M_{\odot}$ (excluding Sagittarius dSph), cautioning that low j satellites on decaying orbits like Sagittarius dSph may bias the distribution. These estimates markedly reduce the current factor of two spread in the mass range of the MW. We also find a well-defined relationship between host halo mass and satellite j distribution, which yields the prediction that upcoming PMs for ultra-faint dwarfs should reveal j within $ 5\times10^3 - 10^4$ kpc km s$^{-1}$. This is a promising method to significantly constrain the cosmologically expected mass range for the MW and eventually M31 as more satellite PMs become available., 20 pages, 7 figures, submitted to ApJ

We report the discovery of 15 stars in the H3 survey that lie, in projection, near the tip of the trailing gaseous Magellanic Stream (MS). The stars have Galactocentric velocities $< -155$ km s$^{-1}$, Galactocentric distances of $\approx 40$ to 80 kpc (increasing along the MS), and [Fe/H] consistent with that of stars in the Small Magellanic Cloud. These 15 stars comprise 94% (15 of 16) of the H3 observed stars to date that have $R_{GAL} > 37.5$ kpc, $-$350 km s$^{-1} < V_{GSR} < -155$ km s$^{-1}$, and are not associated with the Sagittarius Stream. They represent a unique portion of the Milky Way's outer halo phase space distribution function and confirm that unrelaxed structure is detectable even at radii where H3 includes only a few hundred stars. Due to their statistical excess, their close association with the MS and H I compact clouds in the same region, both in position and velocity space, and their plausible correspondence with tidal debris in a published simulation, we identify these stars as debris of past Magellanic Cloud encounters. These stars are evidence for a stellar component of the tidal debris field far from the Clouds themselves and provide unique constraints on the interaction., Accepted for publication in ApJL

Satellite galaxies are predicted to generate gravitational density wakes as they orbit within the dark matter (DM) halos of their hosts, causing their orbits to decay over time. The recent infall of the Milky Way's (MW) most massive satellite galaxy, the Large Magellanic Cloud (LMC), affords us the unique opportunity to study this process in action. In this work, we present high-resolution ($m_{dm} = 4 \times 10^4 M_{\odot}$ ) N-body simulations of the MW-LMC interaction over the past 2 Gyr. We quantify the impact of the LMC's passage on the density and kinematics of the MW's DM halo and the observability of these structures in the MW's stellar halo. The LMC is found to generate pronounced Local and Global wakes in both the DM and stellar halos, leads to both local overdensities and distinct kinematic patterns that should be observable with ongoing and future surveys. Specifically, the Global Wake will result in redshifted radial velocities of stars in the North and blueshifts in the South, at distances larger than 45 kpc. The Local Wake traces the orbital path of the LMC through the halo (50-200 kpc), resulting in a stellar overdensity with a distinct, tangential kinematic pattern that persists to the present day. The detection of the MW's halo response will constrain: the infall mass of the LMC and its orbital trajectory, the mass of the MW, and it may inform us about the nature of the dark matter particle itself., 41 pages, 27 figures. Accepted to ApJ. Some terminology was changed. High-resolution images and figures can be found at https://bit.ly/2S25YzC

We present the first detailed kinematic analysis of the proper motions (PMs) of stars in the Magellanic Bridge, from both the \textit{Gaia} Data Release 2 catalog and from \textit{Hubble Space Telescope} Advanced Camera for Surveys data. For the \textit{Gaia} data, we identify and select two populations of stars in the Bridge region, young main sequence (MS) and red giant stars. The spatial locations of the stars are compared against the known H {\small I} gas structure, finding a correlation between the MS stars and the H {\small I} gas. In the \textit{Hubble Space Telescope} fields our signal comes mainly from an older MS and turn-off population, and the proper motion baselines range between $\sim 4$ and 13 years. The PMs of these different populations are found to be consistent with each other, as well as across the two telescopes. When the absolute motion of the Small Magellanic Cloud is subtracted out, the residual Bridge motions display a general pattern of pointing away from the Small Magellanic Cloud towards the Large Magellanic Cloud. We compare in detail the kinematics of the stellar samples against numerical simulations of the interactions between the Small and Large Magellanic Clouds, and find general agreement between the kinematics of the observed populations and a simulation in which the Clouds have undergone a recent direct collision., 13 pages, 10 figures, 2 tables, submitted to ApJ, accepted February 8th, 2019

The 3D velocities of M31 and M33 are important for understanding the evolution and cosmological context of the Local Group. Their most massive stars are detected by Gaia, and we use Data Release 2 (DR2) to determine the galaxy proper motions (PMs). We select galaxy members based on, e.g., parallax, PM, color-magnitude-diagram location, and local stellar density. The PM rotation of both galaxies is confidently detected, consistent with the known line-of-sight rotation curves: $V_{\rm rot} = -206\pm86$ km s$^{-1}$ (counter-clockwise) for M31, and $V_{\rm rot} = 80\pm52$ km s$^{-1}$ (clockwise) for M33. We measure the center-of-mass PM of each galaxy relative to surrounding background quasars in DR2. This yields that $({\mu}_{\alpha*},{\mu}_{\delta})$ equals $(65 \pm 18 , -57 \pm 15)$ $\mu$as yr$^{-1}$ for M31, and $(31 \pm 19 , -29 \pm 16)$ $\mu$as yr$^{-1}$ for M33. In addition to the listed random errors, each component has an additional residual systematic error of 16 $\mu$as yr$^{-1}$. These results are consistent at 0.8$\sigma$ and 1.0$\sigma$ with the (2 and 3 times higher-accuracy) measurements already available from Hubble Space Telescope (HST) optical imaging and VLBA water maser observations, respectively. This lends confidence that all these measurements are robust. The new results imply that the M31 orbit towards the Milky Way is somewhat less radial than previously inferred, $V_{\rm tan, DR2+HST} = 57^{+35}_{-31}$ km s$^{-1}$, and strengthen arguments that M33 may be on its first infall into M31. The results highlight the future potential of Gaia for PM studies beyond the Milky Way satellite system., Comment: 15 pages, 7 figures, ApJ, in press

We use GAIA DR2 proper motions of the RIOTS4 field OB stars in the Small Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar of (v_RA, v_Dec) = (62 +/-7, -18+/-5) km/s and relative radial velocity +4.5 +/- 5.0 km/s. This unambiguously demonstrates that these two regions are kinematically distinct: the Wing is moving away from the Bar, and towards the Large Magellanic Cloud with a 3-D velocity of 64 +/- 10 km/s. This is consistent with models for a recent, direct collision between the Clouds. We present transverse velocity distributions for our field OB stars, confirming that unbound runaways comprise on the order of half our sample, possibly more. Using eclipsing binaries and double-lined spectroscopic binaries as tracers of dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers of runaways accelerated by supernova kicks, we find significant contributions from both populations. The data suggest that HMXBs have lower velocity dispersion relative to dynamically ejected binaries, consistent with the former corresponding to less energetic supernova kicks that failed to unbind the components. Evidence suggests that our fast runaways are dominated by dynamical, rather than supernova, ejections., Comment: Accepted to ApJ Letters. 10 pages, 4 figures

We present a new measurement of the systemic proper motion of the Small Magellanic Cloud (SMC), based on an expanded set of 30 fields containing background quasars and spanning a $\sim$3 year baseline, using the \textit{Hubble Space Telescope} (\textit{HST}) Wide Field Camera 3. Combining this data with our previous 5 \textit{HST} fields, and an additional 8 measurements from the \textit{Gaia}-Tycho Astrometric Solution Catalog, brings us to a total of 43 SMC fields. We measure a systemic motion of $\mu_{W}$ = $-0.82$ $\pm$ 0.02 (random) $\pm$ 0.10 (systematic) mas yr$^{-1}$ and $\mu_{N}$ = $-1.21$ $\pm$ 0.01 (random) $\pm$ 0.03 (systematic) mas yr$^{-1}$. After subtraction of the systemic motion, we find little evidence for rotation, but find an ordered mean motion radially away from the SMC in the outer regions of the galaxy, indicating that the SMC is in the process of tidal disruption. We model the past interactions of the Clouds with each other based on the measured present-day relative velocity between them of $103 \pm 26$ km s$^{-1}$. We find that in 97\% of our considered cases, the Clouds experienced a direct collision $147 \pm 33$ Myr ago, with a mean impact parameter of $7.5 \pm 2.5$ kpc., Comment: 13 pages, 12 figures, 3 tables, accepted to ApJ

We present Hubble Space Telescope (HST) absolute proper motion (PM) measurements for 20 globular clusters (GCs) in the Milky Way (MW) halo at Galactocentric distances $R_{\rm GC} \approx 10-100$ kpc, with median per-coordinate PM uncertainty 0.06 mas yr$^{-1}$. Young and old halo GCs do not show systematic differences in their 3D Galactocentric velocities, derived from combination with existing line-of-sight velocities. We confirm the association of Arp 2, Pal 12, Terzan 7, and Terzan 8 with the Sagittarius (Sgr) stream. These clusters and NGC 6101 have tangential velocity $V_{\rm tan} > 290$ km s$^{-1}$, whereas all other clusters have $V_{\rm tan} < 200$ km s$^{-1}$. NGC 2419, the most distant GC in our sample, is also likely associated with the Sgr stream, whereas NGC 4147, NGC 5024, and NGC 5053 definitely are not. We use the distribution of orbital parameters derived using the 3D velocities to separate halo GCs that either formed within the MW or were accreted. We also assess the specific formation history of e.g. Pyxis and Terzan 8. We constrain the MW mass via an estimator that considers the full 6D phase-space information for 16 of the GCs from $R_{\rm GC} = 10$ to 40 kpc. The velocity dispersion anisotropy parameter $\beta = 0.609^{+0.130}_{-0.229}$. The enclosed mass $M (

The outer regions of galactic disks have received increased attention since ultraviolet observations with GALEX demonstrated that nearly 30% of galaxies have UV emission beyond their optical extents, indicating star formation activity. These galaxies have been termed extended UV (XUV) disks. Here, we address whether these observations contradict the gas surface density threshold for star formation inferred from Halpha radial profiles of galaxies. We run smoothed particle hydrodynamics simulations of isolated disk galaxies with fiducial star formation prescriptions and show that over-densities owing to the presence of spiral structure can induce star formation in extended gas disks. For direct comparison with observations, we use the 3-D radiative transfer code Sunrise to create simulated FUV and K_s band images. We find that galaxies classified as Type I XUV disks are a natural consequence of spiral patterns, but we are unable to reproduce Type II XUV disks. We also compare our results to studies of the Kennicutt-Schmidt relation in outer disks., Comment: Published in ApJ

Previous models of galactic disk heating in interactions invoke restrictive assumptions not necessarily valid in modern LCDM contexts: that satellites and orbits are rigid and circular, with slow decay over many orbital times from dynamical friction. This leads to a linear scaling of disk heating with satellite mass: disk heights and velocity dispersions scale ~M_sat/M_disk. In turn, observed disk thicknesses present strong constraints on merger histories: the implication for the Milky Way is that, 14 pages, 3 figures, accepted to ApJ (minor revisions to match accepted version)

Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances >200 kpc (i.e., Mvir > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends roughly 100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.

Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the buildup of stellar mass in low-metallicity systems. We present the first VLT/MUSE optical IFU observations of the interacting dwarf pair dm1647+21, selected from the TiNy Titans survey. The H$\alpha$ emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 M$_{\odot}$ yr$^{-1}$, 2.7 times higher than the SFR inferred from SDSS data. The implied specific SFR (sSFR) for the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of $>$ 50. Examining the spatially-resolved maps of classic optical line diagnostics, we find the ISM excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies: rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus be more distributed., Comment: Accepted for publication in ApJ. 11 pages, 5 figures, 1 table. Figures slightly degraded to meet arXiv size restrictions. For more information about TiNy Titans see https://lavinia.as.arizona.edu/~tinytitans/

We introduce TiNy Titans (TNT), the first systematic study of star formation and the subsequent processing of the interstellar medium in interacting dwarf galaxies. Here we present the first results from a multiwavelength observational program based on a sample of 104 dwarf galaxy pairs selected from a range of environments within the spectroscopic portion of the Sloan Digital Sky Survey and caught in various stages of interaction. The TNT dwarf pairs span mass ratios of M/M 100 Å, occur in 20% of the TNT dwarf pairs, regardless of environment, compared to only 6%–8% of the matched unpaired dwarfs. Starbursts can be triggered throughout the merger (i.e., out to large pair separations) and not just approaching coalescence. Despite their enhanced star formation and triggered starbursts, most TNT dwarf pairs have similar gas fractions relative to unpaired dwarfs of the same stellar mass. Thus, there may be significant reservoirs of diffuse, non-star-forming neutral gas surrounding the dwarf pairs, or the gas consumption timescales may be long in the starburst phase. The only TNT dwarf pairs with low gas fractions (f) and the only dwarfs, either paired or unpaired, with Hα EQW < 2 Å are found near massive galaxy hosts. We conclude that dwarf–dwarf interactions are significant drivers of galaxy evolution at the low-mass end, but ultimately environment is responsible for the quenching of star formation. This preliminary study is a precursor to an ongoing high-resolution H i and optical imaging program to constrain the spatial distribution of star formation and gas throughout the course of the dwarf–dwarf merger sequence.

We present proper motions for the Large and Small Magellanic Clouds (LMC and SMC) based on three epochs of Hubble Space Telescope data, spanning a ~7 yr baseline, and centered on fields with background QSOs. The first two epochs, the subject of past analyses, were obtained with ACS/HRC, and have been reanalyzed here. The new third epoch with WFC3/UVIS increases the time baseline and provides better control of systematics. The three-epoch data yield proper-motion random errors of only 1%-2% per field. For the LMC this is sufficient to constrain the internal proper-motion dynamics, as will be discussed in a separate paper. Here we focus on the implied center-of-mass proper motions: μ W, LMC = –1.910 ± 0.020 mas yr–1, μ N, LMC = 0.229 ± 0.047 mas yr–1, and μ W, SMC = –0.772 ± 0.063 mas yr–1, μ N, SMC = –1.117 ± 0.061 mas yr–1. We combine the results with a revised understanding of the solar motion in the Milky Way to derive Galactocentric velocities: v tot, LMC = 321 ± 24 km s–1 and v tot, SMC = 217 ± 26 km s–1. Our proper-motion uncertainties are now dominated by limitations in our understanding of the internal kinematics and geometry of the Clouds, and our velocity uncertainties are dominated by distance errors. Orbit calculations for the Clouds around the Milky Way allow a range of orbital periods, depending on the uncertain masses of the Milky Way and LMC. Periods 4 Gyr are ruled out, which poses a challenge for traditional Magellanic Stream models. First-infall orbits are preferred (as supported by other arguments as well) if one imposes the requirement that the LMC and SMC must have been a bound pair for at least several Gyr.