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 sub(proj) ~ 20 kpc ...suggests 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 (upsilon sub(rot) ~ 50 km s super(-1)) beyond R sub(proj) ~ 5 kpc. The velocity dispersion decreases from about 140 km s sub(-1) at R sub(proj) = 7 kpc to 120 km s sub(-1) at R sub(proj) = 14 kpc, consistent to 2sigma 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 (2-14r sub(eff)) than for the comparison samples.
Abstract
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%.
As part of the Panchromatic Hubble Andromeda Treasury multi-cycle program, we observed a 12' x 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 ~4000 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-manque 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-manque (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 red giant branch is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch (EHB) stars 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 UV excess. We calculate that only a few percent of main-sequence 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.
NGC 205 and M32, close satellites of the M31 galaxy, are our nearest examples of a dwarf elliptical (dE) galaxy and a compact elliptical (cE) galaxy, respectively. Careful surface photometry of the ...two satellites has revealed distortions in their outer isophotes, long presumed to be the result of their tidal interaction with M31. In NGC 205, this tidal distortion hypothesis was supported by the finding of a sharp downturn/reversal in the rotation curve spatially coincident with the isophotal distortions. I present a study of NGC 205's orbit and progenitor properties using restricted N-body simulations and a genetic algorithm. Using the photometric and kinematical data as constraints, this allows for an effective exploration of the parameter space defining NGC 205's orbit. NGC 205 is represented as a static Hernquist potential with embedded mass-less test particles serving as tracers of surface brightness. I explore three distinct, initially stable test particle configurations: cold rotating disk, warm rotating disk, and hot, pressure-supported spheroid. Each model reproduces some, but not all, of NGC 205's observed features, leading me to speculate that a rotating progenitor with substantial pressure support could match all of the observables. Furthermore, plausible combinations of mass and scale length for the spheroid progenitor model reproduce the observed velocity dispersion profile. For all three models, orbits best matching the observables place the satellite 11 ± 9 kpc behind M31 moving at very large velocities: 300–500 km s−1 on primarily radial orbits. Given that the observed radial component is only 54 km s−1, this implies a large tangential motion for NGC 205, moving from northwest to southeast. This suggests NGC 205 is not associated with the observed stellar arc northeast of NGC 205. Furthermore, NGC 205's velocity appears near escape velocity, signifying NGC 205 is likely on its first M31 passage. Studies of M32 are made challenging by the fact that the galaxy is projected against the bright/complicated inner region of M31. Crowding/blending are severe in the inner parts while M31 contamination is severe in the outer parts. To deal with this, I combine Keck/DEIMOS integrated-light spectroscopy in the inner parts with resolved stellar spectroscopy in the outer parts, using spatial and kinematical information to statistically account for M31 contamination. This is the first resolved-star kinematical study of any cE galaxy. In contrast to previous studies that extended out to r special characters omitted 30" ∼ 1 special characters omitted ∼ 100 pc, this study measures the rotation curve and velocity dispersion profile out to r ∼ 250" and higher order Gauss-Hermite moments out to r ∼ 70". The rotation and dispersion profiles extend well beyond the radius (r ∼ 150") where the isophotes are distorted. Unlike NGC 205, M32's kinematics are regular and symmetric and do not show obvious sharp gradients across the region of isophotal elongation and twists. The kinematics are interpreted using three-integral axisymmetric dynamical equilibrium models constructed using Schwarzschild's orbit superposition technique. Models with a constant mass-to-light ratio can fit the data remarkably well. But since this requires an increasing tangential anisotropy with radius, invoking the presence of a dark halo may be more plausible. Such a dark halo is definitely required to bind a half-dozen fast-moving stars observed at the largest radii, but these stars may not be an equilibrium component of M32. The observed regularity of the stellar kinematics, as well as the possible extended dark halo, are unexpected if M31 tides are significant at large radii. This suggests that tidal stripping may not be as significant for shaping cE galaxies as has often been argued.
CARBON STARS IN THE SATELLITES AND HALO OF M31 Hamren, Katherine; Beaton, Rachael L.; Guhathakurta, Puragra ...
Astrophysical journal/The Astrophysical journal,
09/2016, Volume:
828, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of ...Andromeda's Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color-color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample's eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.
CARBON STARS IN THE SATELLITES AND HALO OF M31 Hamren, Katherine; Guhathakurta, Puragra; Rockosi, Constance M. ...
The Astrophysical journal,
09/2016, Volume:
828, Issue:
1
Journal Article
Peer reviewed
We spectroscopically identify a sample of carbon stars in the satellites and halo of M31 using moderate-resolution optical spectroscopy from the Spectroscopic and Photometric Landscape of Andromeda’s ...Stellar Halo survey. We present the photometric properties of our sample of 41 stars, including their brightness with respect to the tip of the red giant branch (TRGB) and their distributions in various color–color spaces. This analysis reveals a bluer population of carbon stars fainter than the TRGB and a redder population of carbon stars brighter than the TRGB. We then apply principal component analysis to determine the sample’s eigenspectra and eigencoefficients. Correlating the eigencoefficients with various observable properties reveals the spectral features that trace effective temperature and metallicity. Putting the spectroscopic and photometric information together, we find the carbon stars in the satellites and halo of M31 to be minimally impacted by dust and internal dynamics. We also find that while there is evidence to suggest that the sub-TRGB stars are extrinsic in origin, it is also possible that they are are particularly faint members of the asymptotic giant branch.
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.
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