We report spectroscopic measurements of stars in the recently discovered young stellar association Price-Whelan+1 (PW+1), which was found in the vicinity of the Leading Arm (LA) of the Magellanic Stream (MS). We obtained Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW+1. and used The Cannon to determine their stellar parameters. We find that the mean metallicity of PW+1. is [Fe/H] = -1.23 with a small scatter of 0.06 dex and the mean RV is V-hel = 276.7 km s(-1) with a dispersion of 11.0 km s(-1). Our results are consistent in T-eff, log g, and [Fe/H] with the young and metal-poor characteristics (116 Myr and [Fe/H] = -1.1) determined for PW+1. from our discovery paper. We find a strong correlation between the spatial pattern of the PW.1. stars and the LA II. gas with an offset of -10 degrees.15 in LMS. and +1 degrees. 55 in B-MS. The similarity in metallicity, velocity, and spatial patterns indicates that PW.1. likely originated in LA II. We find that the spatial and kinematic separation between LA II. and PW+1. can be explained by ram pressure from Milky Way (MW) gas. Using orbit integrations that account for the LMC and MW halo and outer disk gas, we constrain the halo gas density at the orbital pericenter of PW+1 to be n(halo) (17 kpc) = 2.7(-2.0)(+3.4) x 10(-3) atoms and the disk gas density at the midplane at 20 kpc to be n(disk) (20 kpc, 0) = 6.0(-2.0)(+1.5) x 10(-2) atoms cm(-3). We, therefore, conclude that PW+1. formed from the LA II. of the MS, making it a powerful constraint on the MW-Magellanic interaction.

We present deep spectroscopy from Keck/DEIMOS of Andromeda I, III, V, VII, and X, all of which are dwarf spheroidal satellites of M31. The sample includes 256 spectroscopic members across all five dSphs. We confirm previous measurements of the velocity dispersions and dynamical masses, and we provide upper limits on bulk rotation. Our measurements confirm that M31 satellites obey the same relation between stellar mass and stellar metallicity as Milky Way (MW) satellites and other dwarf galaxies in the Local Group. The metallicity distributions show trends with stellar mass that are similar to those of MW satellites, including evidence in massive satellites for external influence, like pre-enrichment or gas accretion. We present the first measurements of individual element ratios, like [Si/Fe], in the M31 system, as well as measurements of the average [alpha/Fe] ratio. The trends of [alpha/Fe] with [Fe/H] also follow the same galaxy mass-dependent patterns as MW satellites. Less massive galaxies have more steeply declining slopes of [alpha/Fe] that begin at lower [Fe/H]. Finally, we compare the chemical evolution of M31 satellites to M31's Giant Stellar Stream and smooth halo. The properties of the M31 system support the theoretical prediction that the inner halo is composed primarily of massive galaxies that were accreted early. As a result, the inner halo exhibits higher [Fe/H] and [alpha/Fe] than surviving satellite galaxies.

We present the results of an extensive search for dwarf satellite galaxies around 10 primary host galaxies in the Local Volume (LV, D < 12 Mpc) using archival CFHT/MegaCam imaging data. The hosts span a wide range in properties, with stellar masses ranging from that of the Large Magellanic Cloud to similar to 3 times that of the Milky Way. The surveyed hosts are: NGC 1023, NGC 1156, NGC 2903, NGC 4258, NGC 4565, NGC 4631, NGC 5023, M51, M64, and M104. We detect satellite candidates using a consistent semi-automated detection algorithm that is optimized for the detection of low surface brightness objects. Depending on the host, our completeness limit is M-g similar to 8 to -10 (assuming the distance of the host). We detect objects with surface brightness down to mu(0,g) similar to 26 mag arcsec(-2) at greater than or similar to 90% completeness. The survey areas of the six best-surveyed hosts cover most of the inner projected R < 150 kpc area, which will roughly double the number of massive LV hosts surveyed at this level of area and luminosity completeness, once distances are measured for the candidates. The number of detected candidates range from 1 around M64 to 33 around NGC 4258. In total, 155 candidates are found, of which 93 are new. While we defer an analysis of the satellite luminosity functions of the hosts until distance information is available for the candidates, we do show that the candidates are primarily red, spheroidal systems with properties roughly consistent with known satellites in the Local Group.

Like massive galaxies, dwarf galaxies are expected to undergo major mergers with other dwarfs. However, the end state of these mergers and the role that merging plays in regulating dwarf star formation are uncertain. Using imaging from the Hyper Suprime-Cam Subaru Strategic Program, we construct a sample of dwarf dwarf mergers and examine the star formation and host properties of the merging systems. These galaxies are selected via an automated detection algorithm from a sample of 6875 spectroscopically selected isolated dwarf galaxies at z < 0.12 and log(M, /Mb) < 9.6 from the Galaxy and Mass Assembly and Sloan Digital Sky Survey spectroscopic campaigns. We find a total tidal feature detection fraction of 3.29% (6.1% when considering only galaxies at z < 0.05). The tidal feature detection fraction rises strongly as a function of star formation activity; 15%-20% of galaxies with extremely high Ha equivalent width (Ha EW > 250 A) show signs of tidal debris. Galaxies that host tidal debris are also systematically bluer than the average galaxy at fixed stellar mass. These findings extend the observed dwarf dwarf merger sequence with a significant sample of dwarf galaxies, indicating that star formation triggered in mergers between dwarf galaxies continues after coalescence.

Many problems in contemporary astrophysics-from understanding the formation of black holes to untangling the chemical evolution of galaxies-rely on knowledge about binary stars. This, in turn, depends on the discovery and characterization of binary companions for large numbers of different kinds of stars in different chemical and dynamical environments. Current stellar spectroscopic surveys observe hundreds of thousands to millions of stars with (typically) few observational epochs, which allows for binary discovery but makes orbital characterization challenging. We use a custom Monte Carlo sampler (The Joker) to perform discovery and characterization of binary systems through radial velocities, in the regime of sparse, noisy, and poorly sampled multi-epoch data. We use it to generate posterior samplings in Keplerian parameters for 232,495 sources released in APOGEE Data Release 16. Our final catalog contains 19,635 high-confidence close-binary (P less than or similar to few years, a less than or similar to few

We present chemical abundances of red giant branch (RGB) stars in the dwarf spheroidal (dSph) satellite system of Andromeda (M31), using spectral synthesis of medium-resolution (R similar to 6000) spectra obtained with the Keck II telescope and Deep Imaging Multi-Object Spectrometer spectrograph via the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey. We coadd stars according to their similarity in photometric metallicity or effective temperature to obtain a signal-to-noise ratio (S/N) high enough to measure average [Fe/H] and [alpha/Fe] abundances. We validate our method using high S/N spectra of RGB stars in Milky Way globular clusters, as well as deep observations for a subset of the M31 dSphs in our sample. For this set of validation coadds, we compare the weighted average abundance of the individual stars with the abundance determined from the coadd. We present individual and coadded measurements of [Fe/H] and [alpha/Fe] for stars in 10 M31 dSphs, including the first [alpha/Fe] measurements for And IX, XIV, XV, and XVIII. These fainter, less massive dSphs show declining [alpha/Fe] relative to [Fe/H], implying an extended star formation history (SFH). In addition, these dSphs also follow the same mass-metallicity relation found in other Local Group satellites. The conclusions we infer from coadded spectra agree with those from previous measurements in brighter M31 dSphs with individual abundance measurements, as well as conclusions from photometric studies. These abundances greatly increase the number of spectroscopic measurements of the chemical composition of M31's less massive dwarf satellites, which are crucial to understanding their SFH and interaction with the M31 system.

We report on H-band spectra of chemically peculiar Mercury-Manganese (HgMn) stars obtained via the SDSS/APOGEE survey. As opposed to other varieties of chemically peculiar stars such as classical Ap/Bp stars, HgMn stars lack strong magnetic fields and are defined by extreme overabundances of Mn, Hg, and other heavy elements. A satisfactory explanation for the abundance patterns remains to be determined, but low rotational velocity is a requirement and involvement in binary/multiple systems may be as well. The APOGEE HgMn sample currently consists of 269 stars that were identified among the telluric standard stars as those whose metallic absorption content is limited to or dominated by the H-band Mn n lines. Due to the fainter magnitudes probed by the APOGEE survey as compared to past studies, only 9/269 stars in the sample were previously known as HgMn types. The 260 newly identified HgMn stars represents a more than doubling of the known sample. At least 32 per cent of the APOGEE sample are found to be binary or multiple systems, and from multi-epoch spectroscopy, we were able to determine orbital solutions for at least one component in 32 binaries. Many of the multilined systems include chemically peculiar companions, with noteworthy examples being the HgMn+Ap/Bp binary HD 5429, the HgMn+HgMn binary HD 298641, and the HgMn+Bp Mn + Am triple system HD 231263. As a further peculiarity, roughly half of the sample produces narrow emission in the C1 16895 A line, with widths and radial velocities that match those of the Mn II lines.

The stellar velocity distribution function in the solar vicinity is reexamined using data from the Sloan Digital Sky Survey Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey's DR16 and Gaia DR2. By exploiting APOGEE's ability to chemically discriminate with great reliability the thin-disk, thick-disk, and (accreted) halo populations, we can, for the first time, derive the three-dimensional velocity distribution functions (DFs) for these chemically separated populations. We employ this smaller but more data-rich APOGEE+Gaia sample to build a data-driven model of the local stellar population velocity DFs and use these as basis vectors for assessing the relative density proportions of these populations over the 5

We present [Fe/H] and [alpha/Fe] abundances, derived using spectral synthesis techniques, for stars in M31's outer stellar halo. The 21 [Fe/H] measurements and 7 [alpha/Fe] measurements are drawn from fields ranging from 43 to 165 kpc in projected distance from M31. We combine our measurements with existing literature measurements, and compare the resulting sample of 23 stars with [Fe/H] and 9 stars with [alpha/Fe] measurements in M31's outer halo with [alpha/Fe] and [Fe/H] measurements, also derived from spectral synthesis, in M31's inner stellar halo (r < 26 kpc) and dSph galaxies. The stars in M31's outer halo have [alpha/Fe] patterns that are consistent with the largest of M31's dSph satellites (And I and And VII). These abundances provide tentative evidence that the [alpha/Fe] abundances of stars in M31's outer halo are more similar to the abundances of Milky Way halo stars than to the abundances of stars in M31's inner halo. We also compare the spectral synthesis-based [Fe/H] measurements of stars in M31's halo with previous photometric [Fe/H] estimates, as a function of projected distance from M31. The spectral synthesis-based [Fe/H] measurements are consistent with a large-scale metallicity gradient previously observed in M31's stellar halo to projected distances as large as 100 kpc.

The spectral analysis and data products in Data Release 16 (DR16; 2019 December) from the high-resolution near-infrared Apache Point Observatory Galactic Evolution Experiment (APOGEE)-2/Sloan Digital Sky Survey (SDSS)-IV survey are described. Compared to the previous APOGEE data release (DR14; 2017 July), APOGEE DR16 includes about 200,000 new stellar spectra, of which 100,000 are from a new southern APOGEE instrument mounted on the 2.5 m du Pont telescope at Las Campanas Observatory in Chile. DR16 includes all data taken up to 2018 August, including data released in previous data releases. All of the data have been re-reduced and re-analyzed using the latest pipelines, resulting in a total of 473,307 spectra of 437,445 stars. Changes to the analysis methods for this release include, but are not limited to, the use of MARCS model atmospheres for calculation of the entire main grid of synthetic spectra used in the analysis, a new method for filling "holes" in the grids due to unconverged model atmospheres, and a new scheme for continuum normalization. Abundances of the neutron-capture element Ce are included for the first time. A new scheme for estimating uncertainties of the derived quantities using stars with multiple observations has been applied, and calibrated values of surface gravities for dwarf stars are now supplied. Compared to DR14, the radial velocities derived for this release more closely match those in the Gaia DR2 database, and a clear improvement in the spectral analysis of the coolest giants can be seen. The reduced spectra as well as the result of the analysis can be downloaded using links provided on the SDSS DR16 web page.