A. B. Pace

The no-spin zone: rotation vs dispersion support in observed and simulated dwarf galaxies

We perform a systematic Bayesian analysis of rotation vs. dispersion support (v_(rot)/σ) in 40 dwarf galaxies throughout the Local Volume (LV) over a stellar mass range 10^(3.5) M_⊙ < M⋆ < 10^8 M_⊙. We find that the stars in ∼90% of the LV dwarf galaxies studied -- both satellites and isolated systems -- are dispersion-supported. In particular, we show that 7/10 *isolated* dwarfs in our sample have stellar populations with v_(rot)/σ<0.6. All have v_(rot)/σ≲2. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between v_(rot)/σ and distance to the closest L⋆ galaxy, nor between v_(rot)/σ and M⋆ within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies (10^9M⊙<M_(vir)<10^(10)M⊙) and show that the simulated *isolated* dIrr galaxies have stellar ellipticities and stellar v_(rot)/σ ratios that are consistent with the observed population of dIrrs *and* dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-supported systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.

Stellar Streams Discovered in the Dark Energy Survey

We perform a search for stellar streams around the Milky Way using the first 3 yr of multiband optical imaging data from the Dark Energy Survey (DES). We use DES data covering ∼5000 deg2 to a depth of guf0a0>uf0a023.5 with a relative photometric calibration uncertainty of <1%. This data set yields unprecedented sensitivity to the stellar density field in the southern celestial hemisphere, enabling the detection of faint stellar streams to a heliocentric distance of ∼50 kpc. We search for stellar streams using a matched filter in color–magnitude space derived from a synthetic isochrone of an old, metal-poor stellar population. Our detection technique recovers four previously known thin stellar streams: Phoenix, ATLAS, Tucana III, and a possible extension of Molonglo. In addition, we report the discovery of 11 new stellar streams. In general, the new streams detected by DES are fainter, more distant, and lower surface brightness than streams detected by similar techniques in previous photometric surveys. As a by-product of our stellar stream search, we find evidence for extratidal stellar structure associated with four globular clusters: NGC 288, NGC 1261, NGC 1851, and NGC 1904. The ever-growing sample of stellar streams will provide insight into the formation of the Galactic stellar halo, the Milky Way gravitational potential, and the large- and small-scale distribution of dark matter around the Milky Way.

The First Tidally Disrupted Ultra-faint Dwarf Galaxy?: A Spectroscopic Analysis of the Tucana III Stream

We present a spectroscopic study of the tidal tails and core of the Milky Way satellite Tucana III, collectively referred to as the Tucana III stream, using the 2dF+AAOmega spectrograph on the Anglo-Australian Telescope and the IMACS spectrograph on the Magellan Baade Telescope. In addition to recovering the brightest nine previously known member stars in the Tucana III core, we identify 22 members in the tidal tails. We observe strong evidence for a velocity gradient of 8.0 ± 0.4 km/s-1 deg-1 over at least 3° on the sky. Based on the continuity in velocity, we confirm that the Tucana III tails are real tidal extensions of Tucana III. The large velocity gradient of the stream implies that Tucana III is likely on a radial orbit. We successfully obtain metallicities for four members in the core and 12 members in the tails. We find that members close to the ends of the stream tend to be more metal-poor than members in the core, indicating a possible metallicity gradient between the center of the progenitor halo and its edge. The spread in metallicity suggests that the progenitor of the Tucana III stream is likely a dwarf galaxy rather than a star cluster. Furthermore, we find that with the precise photometry of the Dark Energy Survey data, there is a discernible color offset between metal-rich disk stars and metal-poor stream members. This metallicity-dependent color offers a more efficient method to recognize metal-poor targets and will increase the selection efficiency of stream members for future spectroscopic follow-up programs on stellar streams.

Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-faint Satellites in the Constellation Carina* ** ***

We present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the CarinaII (Car II) & Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including 2 binaries with variable radial velocities and 2 RR Lyrae stars. The other 14 members have a mean heliocentric velocity

Deep SOAR follow-up photometry of two Milky Way outer-halo companions discovered with Dark Energy Survey

v_{rm hel} = 477.2 pm 1.2

Modelling the Tucana III stream - a close passage with the LMC

km/s and a velocity dispersion of

Scaling Relations for Dark Matter Annihilation and Decay Profiles in Dwarf Spheroidal Galaxies

sigma_v = 3.4^{+1.2}_{-0.8}

Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I*

km/s. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of

Proper motions of Milky Way Ultra-Faint satellites with

1.0^{+0.8}_{-0.4} times 10^{6} M_odot

{\it Gaia}

, indicating a mass-to-light ratio of