The SPLASH Survey: Kinematics of Andromeda's Inner Spheroid

The combination of large size, high stellar density, high metallicity, and Sersic surface brightness profile of the spheroidal component of the Andromeda galaxy (M31) within R_proj ~ 20 kpc suggest that it is unlike any subcomponent of the Milky Way. In this work we capitalize on our proximity to and external view of M31 to probe the kinematical properties of this "inner spheroid." We employ a Markov chain Monte Carlo (MCMC) analysis of resolved stellar kinematics from Keck/DEIMOS spectra of 5651 red giant branch stars to disentangle M31's inner spheroid from its stellar disk. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins after accounting for a locally cold stellar disk as well as the Giant Southern Stream and associated tidal debris. For the first time, we detect significant spheroid rotation (v_rot ~ 50 km/s) beyond R_proj ~ 5 kpc. The velocity dispersion decreases from about 140 km/s at R_proj = 7 kpc to 120 km/s at R_proj = 14 kpc, consistent to 2 sigma with existing measurements and models. We calculate the probability that a given star is a member of the spheroid and find that the spheroid has a significant presence throughout the spatial extent of our sample. Lastly, we show that the flattening of the spheroid is due to velocity anisotropy in addition to rotation. Though this suggests that the inner spheroid of M31 more closely resembles an elliptical galaxy than a typical spiral galaxy bulge, it should be cautioned that our measurements are much farther out (2 - 14 r_eff) than for the comparison samples.Comment: Accepted for publication in Ap

The Panchromatic Hubble Andromeda Treasury I: Bright UV Stars in the Bulge of M31

As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle program, we observed a 12' \times 6.5' area of the bulge of M31 with the WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample of \sim4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and AGB-manqu\'e stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manqu\'e (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or {\alpha} abundances when the mass loss on the RGB is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch stars (EHB) and their progeny. We construct the first radial profiles of these stellar populations, and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UVX. We calculate that only a few percent of MS stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population.Comment: Accepted for publication in Ap

X-Ray Energy Deposition Model for Simulating Asteroid Response to a Nuclear Planetary Defense Mitigation Mission

In the event of a potentially catastrophic asteroid impact, with sufficient warning time, deploying a nuclear device remains a powerful option for planetary defense if a kinetic impactor or other means of deflection proves insufficient. Predicting the effectiveness of a potential nuclear deflection or disruption mission depends on accurate multiphysics simulations of the device's X-ray energy deposition into the asteroid and the resulting material ablation. The relevant physics in these simulations span many orders of magnitude, require a variety of different complex physics packages, and are computationally expensive. Having an efficient and accurate way of modeling this system is necessary for exploring a mission's sensitivity to the asteroid's range of physical properties. To expedite future simulations, we present a completed X-ray energy deposition model developed using the radiation-hydrodynamics code Kull that can be used to initiate a nuclear mitigation mission calculation. The model spans a wide variety of possible mission initial conditions: four different asteroid-like materials at a range of porosities, two different source spectra, and a broad range of radiation fluences, source durations, and angles of incidence. Using blowoff momentum as the primary metric, the model-initiated simulation results match the full radiation-hydrodynamics results to within 10%

Kinematics Of M31's Inner Spheroid Using SPLASH and PHAT Data

International audienceOur proximity to and external view of the Andromeda galaxy (M31) make it an excellent testbed for probing the relationship between structural subcomponents (e.g., disk and spheroid) of a large spiral galaxy. The combination of large size, high metallicity, and Sersic surface brightness profi le (with index n_Sersic 2-4) of M31's spheroid suggest that it is unlike any subcomponent of the Milky Way. We use resolved stellar kinematics from Keck/DEIMOS spectra of 5600 red giant branch stars to disentangle M31's spheroid from its stellar disk. Kinematical evidence is presented for the northern extension of the Giant Southern Stream. We show that accounting for the stream and associated tidal debris, in addition to a locally cold stellar disk, yields a systematic rotation pattern in the spheroid. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins. We detect significant rotation beyond r_proj 10 kpc. This is the first time that rotation has been detected in the spheroid past r_proj 1 kpc. The velocity dispersion decreases from 150 km/s at r_proj = 4 kpc to 115 km/s at r_proj = 14 kpc, consistent to 2sigma with previous measurements as well as with a bulge/halo model. We also calculate the probability that a given star is a member of the spheroid and that the spheroid has a signi ficant contribution at all radii in our sample along both the major and minor axes. Lastly, we show that the v_rot/sigma more closely resembles that of a so-called "fast rotating" elliptical galaxy than of a rotationally-flattened spiral galaxy bulge, but it should be cautioned that our M31 spheroid measurements are much further out (in units of effective radius) than for the comparison samples

Kinematics Of M31's Inner Spheroid Using SPLASH and PHAT Data

International audienceOur proximity to and external view of the Andromeda galaxy (M31) make it an excellent testbed for probing the relationship between structural subcomponents (e.g., disk and spheroid) of a large spiral galaxy. The combination of large size, high metallicity, and Sersic surface brightness profi le (with index n_Sersic 2-4) of M31's spheroid suggest that it is unlike any subcomponent of the Milky Way. We use resolved stellar kinematics from Keck/DEIMOS spectra of 5600 red giant branch stars to disentangle M31's spheroid from its stellar disk. Kinematical evidence is presented for the northern extension of the Giant Southern Stream. We show that accounting for the stream and associated tidal debris, in addition to a locally cold stellar disk, yields a systematic rotation pattern in the spheroid. We measure the mean velocity and dispersion of the spheroid in each of five spatial bins. We detect significant rotation beyond r_proj 10 kpc. This is the first time that rotation has been detected in the spheroid past r_proj 1 kpc. The velocity dispersion decreases from 150 km/s at r_proj = 4 kpc to 115 km/s at r_proj = 14 kpc, consistent to 2sigma with previous measurements as well as with a bulge/halo model. We also calculate the probability that a given star is a member of the spheroid and that the spheroid has a signi ficant contribution at all radii in our sample along both the major and minor axes. Lastly, we show that the v_rot/sigma more closely resembles that of a so-called "fast rotating" elliptical galaxy than of a rotationally-flattened spiral galaxy bulge, but it should be cautioned that our M31 spheroid measurements are much further out (in units of effective radius) than for the comparison samples

Keck Spectroscopy of M31's HST/MCT Region. II: Kinematical and Spectral Characteristics of Stellar Subpopulations

International audienceTrends in stellar kinematics in the inner regions of the Andromeda galaxy (M31) offer insight into the structure and formation of spiral galaxies. The synthesis of the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and PHAT (Panchromatic Hubble Andromeda Treasury) collaborations offers an unprecedented opportunity to understand the interplay between the assembly, star formation, and chemical enrichment histories of large disk galaxies

Keck Spectroscopy of M31's HST/MCT Region. I: Stellar Kinematics of the Disk and Bulge

International audienceTrends in stellar kinematics in the inner regions of the Andromeda galaxy (M31) offer insight into the structure and formation of spiral galaxies. The synthesis of the SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) and PHAT (Panchromatic Hubble Andromeda Treasury) collaborations offers an unprecedented opportunity to understand the interplay between the assembly, star formation, and chemical enrichment histories of large disk galaxies