We investigate the possibility that the dwarf galaxies Crater II and Hercules have previously been tidally stripped by the Milky Way. We present Magellan/IMACS spectra of candidate member stars in both objects. We identify 37 members of Crater II, 25 of which have velocity measurements in the literature, and we classify three stars within that subset as possible binaries. We find that including or removing these binary candidates does not change the derived velocity dispersion of Crater II. Excluding the binary candidates, we measure a velocity dispersion of sigma V-los = 2.7(-0.4)(+0.5) km s(-1), corresponding to M/L = 47(-13)(+17) M-circle dot/L-circle dot. We measure a mean metallicity of [Fe/H] = -1.95(-0.05)(+0.06),with a dispersion of sigma([Fe/H]) = 0.18(-0.08)(+0.06). Our velocity dispersion and metallicity measurements agree with previous measurements for Crater II, and confirm that the galaxy resides in a kinematically cold dark-matter halo. We also search for spectroscopic members stripped from Hercules in the possible extratidal stellar overdensities surrounding the dwarf. For both galaxies, we calculate proper motions using Gaia DR2 astrometry, and use their full 6D phase space information to evaluate the probability that their orbits approach sufficiently close to the Milky Way to experience tidal stripping. Given the available kinematic data, we find a probability of similar to 40% that Hercules has suffered tidal stripping. The proper motion of Crater II makes it almost certain to be stripped.

We present the first detailed elemental abundances in the ultra-faint Magellanic satellite galaxies Carina II (Car II) and Carina III (Car III). With high-resolution Magellan/MIKE spectroscopy, we determined the abundances of nine stars in Car II, including the first abundances of an RR Lyrae star in an ultra-faint dwarf galaxy (UFD), and two stars in Car III. The chemical abundances demonstrate that both systems are clearly galaxies and not globular clusters. The stars in these galaxies mostly display abundance trends matching those of other similarly faint dwarf galaxies: enhanced but declining [alpha/Fe] ratios, iron-peak elements matching the stellar halo, and unusually low neutron-capture element abundances. One star displays a low outlying [Sc/Fe] = -1.0. We detect a large Ba scatter in Car II, likely due to inhomogeneous enrichment by low-mass asymptotic giant branch star winds. The most striking abundance trend is for [Mg/Ca] in Car II, which decreases from +0.4 to -0.4 and indicates clear variation in the initial progenitor masses of enriching core-collapse supernovae. So far, the only UFDs displaying a similar [Mg/Ca] trend are likely satellites of the Large Magellanic Cloud. We find two stars with [Fe/H] <= -3.5 whose abundances likely trace the first generation of metal-free Population III stars and are well fit by Population III core-collapse supernova yields. An appendix describes our new abundance uncertainty analysis that propagates line-by-line stellar parameter uncertainties.

In this detailed geochemical, petrological, and microstructural study of felsite clast materials contained in Apollo breccia samples 12013, 14321, and 15405, little evidence was found for relatively enriched reservoirs of endogenic lunar volatiles. NanoSIMS measurements have revealed very low volatile abundances (<= 2-18 ppm hydrogen) in nominally anhydrous minerals (NAMS) plagioclase, potassic alkali feldspar, and SiO2 that make up a majority of these felsic lithologies. Yet these mineral assemblages and clast geochemistries on Earth would normally yield relatively high volatiles contents in their NAMS (similar to 20 to >= 80 ppm hydrogen). This difference is particularly notable in felsite 14321,1062 that exhibits extremely low volatile abundances (<= 2 ppm hydrogen) and a relatively low amount of microstructural evidence for shock metamorphism given that it is a clast of the most evolved (similar to 74 wt.% SiO2) rock-type returned from the Moon. If taken at face value, 'wet' felsic magmas (similar to 1 .2-1.7 wt.% water) are implied by the relatively high hydrogen contents of feldspar in felsite clasts in Apollo samples 12013 and 15405, but these results are likely misleading. These felsic clasts have microstructural features indicative of significantly higher shock stress than 14321,1062. These crustal lithologies likely obtained no more water from the lunar interior than the magma body producing 14321,1062. Rather, we suggest hydrogen was enriched in samples 12013 and 15405 by impact induced exchange, and/or partial assimilation of volatiles added to the surface of the Moon by a hydrated impactor (asteroid or comet) or the solar wind. Thus, the best estimate for magmatic water contents of felsic lunar magmas comes from 14321,1062 that leads to a calculated magmatic water content of <= 0.2 wt.%. This dry felsic magma has a slightly greater, but comparable water content to the ancient mafic magmas implied by the other lithologies that we have studied. Based on this and expanding evidence for a significantly dry ancient or early degassed Moon it is likely that some recent estimates (100's ppm) of the water abundances in the lunar parental magma ocean have been overestimated. Published by Elsevier Ltd.

We present Magellan/IMACS spectroscopy of three recently discovered ultra-faint Milky Way satellites, Grus.II, Tucana.IV, and Tucana.V. We measure systemic velocities of vhel = -110.0. 0.5 km s-1, = v 15.9+ hel 1.71.8 km s-1, and = - v 36.2+ hel 2.22.5 km s-1 for the three objects, respectively. Their large relative velocities demonstrate that the satellites are unrelated despite their close physical proximity. We determine a velocity dispersion for Tuc.IV of s = 4.3+ 1.01.7 km s-1, but we cannot resolve the velocity dispersions of the other two systems. For Gru.II, we place an upper limit (90%.confidence) on the dispersion of s.<.1.9 km s-1, and for Tuc.V, we do not obtain any useful limits. All three satellites have metallicities below[Fe H] = -2.1, but none has a detectable metallicity spread. We determine proper motions for each satellite based on Gaia astrometry and compute their orbits around the Milky Way. Gru.II is on a tightly bound orbit with a pericenter of 25+ 76 kpc and orbital eccentricity of 0.45+ 0.050.08. Tuc.V likely has an apocenter beyond 100.kpc and could be approaching the Milky Way for the first time. The current orbit of Tuc.IV is similar to that of Gru.II, with a pericenter of 25+ 811 kpc and an eccentricity of 0.36+ 0.060.13. However, a backward integration of the position of Tuc.IV demonstrates that it collided with the Large Magellanic Cloud at an impact parameter of 4. kpc 120.Myr ago, deflecting its trajectory and possibly altering its internal kinematics. Based on their sizes, masses, and metallicities, we classify Gru.II and Tuc.IV as likely dwarf galaxies, but the nature of Tuc.V remains uncertain.

We present a Bayesian method to identify multiple (chemodynamic) stellar populations in dwarf spheroidal galaxies (dSphs) using velocity, metallicity, and positional stellar data without the assumption of spherical symmetry. We apply this method to a new Keck/Deep Imaging Multi-Object Spectrograph (DEIMOS) spectroscopic survey of the Ursa Minor (UMi) dSph. We identify 892 likely members, making this the largest UMi sample with line-of-sight velocity and metallicity measurements. Our Bayesian method detects two distinct chemodynamic populations with high significance (in logarithmic Bayes factor, ln B similar to 33). The metal-rich ([Fe/H] = -2.05 +/- 0.03) population is kinematically colder (radial velocity dispersion of sigma(v) =4.9(-1.0)(+0.8)km s(-1)) and more centrally concentrated than the metal-poor ([Fe/H]=-2.29(-0.06)(+0.05)) and kinematically hotter population (sigma(v)=11.5(-0.8)(+0.9)km s(-1)). Furthermore, we apply the same analysis to an independent Multiple Mirror Telescope (MMT)/Hectochelle data set and confirm the existence of two chemodynamic populations in UMi. In both data sets, the metal-rich population is significantly flattened (epsilon = 0.75 +/- 0.03) and the metal-poor population is closer to spherical (epsilon=0.33(-0.09)(+0.12)). Despite the presence of two populations, we are able to robustly estimate the slope of the dynamical mass profile. We found hints for prolate rotation of order similar to 2 km s(-1) in the MMT data set, but further observations are required to verify this. The flattened metal-rich population invalidates assumptions built into simple dynamical mass estimators, so we computed new astrophysical dark matter annihilation (J) and decay profiles based on the rounder, hotter metal-poor population and inferred log(10)(J(0 degrees.5)/GeV(2)cm(-5))approximate to 19.1 for the Keck data set. Our results paint a more complex picture of the evolution of UMi than previously discussed.

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

The lowest luminosity (L < 10(5) L-circle dot) Milky Way satellite galaxies represent the extreme lower limit of the galaxy luminosity function. These ultra-faint dwarfs are the oldest, most dark matter-dominated, most metal-poor, and least chemically evolved stellar systems known. They therefore provide unique windows into the formation of the first galaxies and the behavior of dark matter on small scales. In this review, we summarize the discovery of ultra-faint dwarfs in the Sloan Digital Sky Survey in 2005 and the subsequent observational and theoretical progress in understanding their nature and origin. We describe their stellar kinematics, chemical abundance patterns, structural properties, stellar populations, orbits, and luminosity function, as well as what can be learned from each type of measurement. We conclude the following:

Draco C1 is a known symbiotic binary star system composed of a carbon red giant and a hot, compact companion-likely a white dwarf-belonging to the Draco dwarf spheroidal galaxy. From near-infrared spectroscopic observations taken by the Apache Point Observatory Galactic Evolution Experiment (APOGEE-2), part of Sloan Digital Sky Survey IV, we provide updated stellar parameters for the cool, giant component, and constrain the temperature and mass of the hot, compact companion. Prior measurements of the periodicity of the system, based on only a few epochs of radial velocity data or relatively short baseline photometric observations, were sufficient only to place lower limits on the orbital period (P > 300 days). For the first time, we report precise orbital parameters for the binary system: with 43 radial velocity measurements from APOGEE spanning an observational baseline of more than 3 yr, we definitively derive the period of the system to be 1220.0(-3.5)(+3.7) days. Based on the newly derived orbital period and separation of the system, together with estimates of the radius of the red giant star, we find that the hot companion must be accreting matter from the dense wind of its evolved companion.

We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M-V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z similar to 6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II's cluster is offset from the center of the galaxy by a projected distance of 23 +/- 3 pc. This offset could be an indication of a small (similar to 50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of 0.31(-0.06)(+0.05) and is aligned with the orientation of Eri II within 3 degrees 6 degrees, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.