The highest-speed local dark matter particles come from the Large Magellanic Cloud

Using N-body simulations of the Large Magellanic Cloud (LMC's) passage through the Milky Way (MW), tailored to reproduce observed kinematic properties of both galaxies, we show that the high-speed tail of the Solar Neighborhood dark matter distribution is overwhelmingly of LMC origin. Two populations contribute at high speeds: 1) Particles that were once bound to the LMC, and 2) MW halo particles that have been accelerated owing to the response of the halo to the recent passage of the LMC. These particles reach speeds of 700-900 km/s with respect to the Earth, near or somewhat higher that the local escape speed of the MW. The high-speed particles follow trajectories similar to the Solar reflex motion, with peak velocities reached in June. For low-mass dark matter, these high-speed particles can dominate the signal in direct-detection experiments, extending the reach of the experiments to lower mass and elastic scattering cross sections even with existing data sets. Our study shows that even non-disrupted MW satellite galaxies can leave a significant dark matter footprint in the Solar Neighborhood.Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

A Widespread, Clumpy Starburst in the Isolated Ongoing Dwarf Galaxy Merger dm1647+21

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

Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud

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 figure

Space Motions of the Dwarf Spheroidal Galaxies Draco and Sculptor based on HST Proper Motions with ~10-year Time Baseline

We present new proper motion (PM) measurements of the dwarf spheroidal galaxies (dSphs) Draco and Sculptor using multi-epoch images obtained with the Hubble Space Telescope ACS/WFC. Our PM results have uncertainties far lower than previous measurements, even made with the same instrument. The PM results for Draco and Sculptor are (mu_W,mu_N)_Dra = (-0.0562+/-0.0099,-0.1765+/-0.0100) mas/yr and (mu_W,mu_N)_Scl = (-0.0296+/-0.0209,-0.1358 +/-0.0214) mas/yr. The implied Galactocentric velocity vectors for Draco and Sculptor have radial and tangential components: (V_rad,V_tan)_Dra = (-88.6,161.4) +/- (4.4,5.6) km/s; and (V_rad,V_tan)_Scl = (72.6,200.2) +/- (1.3,10.8) km/s. We study the detailed orbital history of both Draco and Sculptor via numerical orbit integrations. Orbital periods of Draco and Sculptor are found to be 1-2 and 2-5 Gyrs, respectively, accounting for uncertainties in the MW mass. We also study the influence of the Large Magellanic Cloud (LMC) on the orbits of Draco and Sculptor. Overall, the inclusion of the LMC increases the scatter in the orbital results. Based on our calculations, Draco shows a rather wide range of orbital parameters depending on the MW mass and inclusion/exclusion of the LMC, but Sculptor's orbit is very well constrained with its most recent pericentric approach to the MW being 0.3-0.4 Gyr ago. Our new PMs imply that the orbital trajectories of both Draco and Sculptor are confined within the Disk of Satellites (DoS), better so than implied by earlier PM measurements, and likely rule out the possibility that these two galaxies were accreted together as part of a tightly bound group.Comment: 17 pages, 8 figures, 6 tables. Accepted for publication in Ap

Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall

A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological context requires knowledge of their 3D velocities and orbits. However, proper motion (PM) measurements have generally been of limited accuracy and available only for more massive dwarfs. We therefore present a new study of the kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that have been spectroscopically observed in the literature. We derive systemic PMs for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good consistency for the subset with measurements available from other studies. We derive the implied Galactocentric velocities, and calculate orbits in canonical MW halo potentials of "low" ($0.8 \times 10^{12} M_\odot$) and "high" mass ($1.6 \times 10^{12} M_\odot$). Comparison of the distributions of orbital apocenters and 3D velocities to the halo virial radius and escape velocity, respectively, suggests that the satellite kinematics are best explained in the high-mass halo. Tuc III, Crater II, and additional candidates have orbital pericenters small enough to imply significant tidal influences. Relevant to the missing satellite problem, the fact that fewer galaxies are observed to be near apocenter than near pericenter implies that there must be a population of distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital poles that do not align with the MW plane of satellites (given reasonable assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to establish whether they align; and 17 satellites align, of which 11 are co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are counter-orbiting. Group infall might have contributed to this, but no definitive association is found for the members of the Crater-Leo group.Comment: 18 pages, 7 figures, 4 tables, accepted for publication in A&

Physical Properties of Complex C Halo Clouds

Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey of the tail of Complex C are presented and the halo clouds associated with this complex cataloged. The properties of the Complex C clouds are compared to clouds cataloged at the tail of the Magellanic Stream to provide insight into the origin and destruction mechanism of Complex C. Magellanic Stream and Complex C clouds show similarities in their mass distributions (slope = -0.7 and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative of a warm component), which may indicate a common origin and/or physical process breaking down the clouds. The clouds cataloged at the tail of Complex C extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to 10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase structure in Complex C, possibly due to its low metallicity and inefficient cooling compared to other halo clouds. From assuming the Complex C clouds are in pressure equilibrium with a hot halo medium, we find a median halo density of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a z-height of ~3 kpc. Using the same argument for the Stream results in a median halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are consistent with previous observational constraints and cosmological simulations. We also assess the derived cloud and halo properties with three dimensional grid simulations of halo HI clouds and find the temperature is generally consistent within a factor of 1.5 and the volume densities, pressures and halo densities are consistent within a factor of 3.Comment: Accepted for publication in AJ. 54 pages, including 6 tables and 16 figure

Gas and Star Formation in Satellites of Milky Way Analogs

We have imaged the entirety of eight (plus one partial) Milky Way (MW)–like satellite systems, a total of 42 (45) satellites, from the Satellites Around Galactic Analogs II catalog in both Hα and H i with the Canada–France–Hawaii Telescope and the Jansky Very Large Array. In these eight systems we have identified four cases where a satellite appears to be currently undergoing ram pressure stripping (RPS) as its H i gas collides with the circumgalactic medium (CGM) of its host. We also see a clear suppression of gas fraction (M HI/M *) with decreasing (projected) satellite–host separation—to our knowledge, the first time this has been observed in a sample of MW-like systems. Comparisons to the Auriga, A Project Of Simulating The Local Environment, and TNG50 cosmological zoom-in simulations show consistent global behavior, but they systematically underpredict gas fractions across all satellites by roughly 0.5 dex. Using a simplistic RPS model, we estimate the average peak CGM density that satellites in these systems have encountered to be logρcgm/gcm−3≈−27.3 . Furthermore, we see tentative evidence that these satellites are following a specific star formation rate to gas fraction relation that is distinct from field galaxies. Finally, we detect one new gas-rich satellite in the UGC 903 system with an optical size and surface brightness meeting the standard criteria to be considered an ultra-diffuse galaxy

A Population of Accreted SMC Stars in the LMC

We present an analysis of the stellar kinematics of the Large Magellanic Cloud based on ~5900 new and existing velocities of massive red supergiants, oxygen-rich and carbon-rich AGB stars, and other giants. After correcting the line-of-sight velocities for the LMC's space motion and accounting for asymmetric drift in the AGB population, we derive a rotation curve that is consistent with all of the tracers used, as well as that of published HI data. The amplitude of the rotation curve is v_0=87+/-5 km s^-1 beyond a radius R_0=2.4+/-0.1 kpc, and has a position angle of the kinematic line of nodes of theta=142 degrees +/-5 degrees. By examining the outliers from our fits, we identify a population of 376 stars, or >~5% of our sample, that have line-of-sight velocities that apparently oppose the sense of rotation of the LMC disk. We find that these kinematically distinct stars are either counter-rotating in a plane closely aligned with the LMC disk, or rotating in the same sense as the LMC disk, but in a plane that is inclined by 54 degrees +/- 2 degrees to the LMC. Their kinematics clearly link them to two known HI arms, which have previously been interpreted as being pulled out from the LMC. We measure metallicities from the Ca triplet lines of ~1000 LMC field stars and 30 stars in the kinematically distinct population. For the LMC field, we find a median [Fe/H]=-0.56 +/- 0.02 with dispersion of 0.5 dex, while for the kinematically distinct stars the median [Fe/H] is -1.25 +/- 0.13 with a dispersion of 0.7 dex. The metallicity differences provide strong evidence that the kinematically distinct population originated in the SMC. This interpretation has the consequence that the HI arms kinematically associated with the stars are likely falling into the LMC, instead of being pulled out.Comment: 12 pages, 8 color figures, accepted for publication in the Astrophysical Journa

AN ULTRA-FAINT GALAXY CANDIDATE DISCOVERED in EARLY DATA from the MAGELLANIC SATELLITES SURVEY

We report a new ultra-faint stellar system found in Dark Energy Camera data from the first observing run of the Magellanic Satellites Survey (MagLiteS). MagLiteS J0644-5953 (Pictor II or Pic II) is a low surface brightness (μ = 28.5+1 -1 mag arcsec-2 within its half-light radius) resolved overdensity of old and metal-poor stars located at a heliocentric distance of 45+5 -4 kpc. The physical size (r1/2 = 46+15 -11) and low luminosity (Mv = -3.2+0.4 -0.5 mag) of this satellite are consistent with the locus of spectroscopically confirmed ultra-faint galaxies. MagLiteS J0644-5953 (Pic II) is located 11.3+3.1 -0.9 kpc from the Large Magellanic Cloud (LMC), and comparisons with simulation results in the literature suggest that this satellite was likely accreted with the LMC. The close proximity of MagLiteS J0644-5953 (Pic II) to the LMC also makes it the most likely ultra-faint galaxy candidate to still be gravitationally bound to the LMC.Peer reviewe