arXiv subjects

Anil C. Seth

Publications and source records attributed to Anil C. Seth.

At least 73 records · Page 4Linked to original sources

arXiv2015

Panchromatic Hubble Andromeda Treasury XII. Mapping Stellar Metallicity Distributions in M31

We present a study of spatial variations in the metallicity of old red giant branch stars in the Andromeda galaxy. Photometric metallicity estimates are derived by interpolating isochrones for over seven million stars in the Panchromatic Hubble Andromeda Treasury (PHAT) survey. This is the first systematic study of stellar metallicities over the inner 20 kpc of Andromeda's galactic disk. We see a clear metallicity gradient of $-0.020\pm0.004$ dex/kpc from $\sim4-20$ kpc assuming a constant RGB age. This metallicity gradient is derived after correcting for the effects of photometric bias and completeness and dust extinction and is quite insensitive to these effects. The unknown age gradient in M31's disk creates the dominant systematic uncertainty in our derived metallicity gradient. However, spectroscopic analyses of galaxies similar to M31 show that they typically have small age gradients that make this systematic error comparable to the 1$σ$ error on our metallicity gradient measurement. In addition to the metallicity gradient, we observe an asymmetric local enhancement in metallicity at radii of 3-6 kpc that appears to be associated with Andromeda's elongated bar. This same region also appears to have an enhanced stellar density and velocity dispersion.

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arXiv2015

The Panchromatic Hubble Andromeda Treasury VIII: A Wide-Area, High-Resolution Map of Dust Extinction in M31

We map the distribution of dust in M31 at 25pc resolution, using stellar photometry from the Panchromatic Hubble Andromeda Treasury. We develop a new mapping technique that models the NIR color-magnitude diagram (CMD) of red giant branch (RGB) stars. The model CMDs combine an unreddened foreground of RGB stars with a reddened background population viewed through a log-normal column density distribution of dust. Fits to the model constrain the median extinction, the width of the extinction distribution, and the fraction of reddened stars. The resulting extinction map has >4 times better resolution than maps of dust emission, while providing a more direct measurement of the dust column. There is superb morphological agreement between the new map and maps of the extinction inferred from dust emission by Draine et al. 2014. However, the widely-used Draine & Li (2007) dust models overpredict the observed extinction by a factor of ~2.5, suggesting that M31's true dust mass is lower and that dust grains are significantly more emissive than assumed in Draine et al. (2014). The discrepancy we identify is consistent with similar findings in the Milky Way by the Planck Collaboration (2015), but has a more complex dependence on parameters from the Draine & Li (2007) dust models. We also show that the discrepancy with the Draine et al. (2014) map is lowest where the interstellar radiation field has a harder spectrum than average. We discuss possible improvements to the CMD dust mapping technique, and explore further applications.

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arXiv2015

A Spectroscopic and Photometric Exploration of the C/M Ratio in the Disk of M31

We explore the ratio (C/M) of carbon-rich to oxygen-rich thermally pulsing asymptotic giant branch(TP-AGB) stars in the disk of M31 using a combination of moderate-resolution optical spectroscopy from the Spectroscopic Landscape of Andromeda's Stellar Halo (SPLASH) survey and six-filter Hubble Space Telescope photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey.Carbon stars were identified spectroscopically. Oxygen-rich M-stars were identifed using three different photometric definitions designed to mimic, and thus evaluate, selection techniques common in the literature. We calculate the C/M ratio as a function of galactocentric radius, present-day gas-phase oxygen abundance, stellar metallicity, age (via proxy defined as the ratio of TP-AGB stars to red giant branch, RGB, stars), and mean star formation rate over the last 400 Myr. We find statistically significant correlations between log(C/M) and all parameters. These trends are consistent across different M-star selection methods, though the fiducial values change. Of particular note is our observed relationship between log(C/M) and stellar metallicity, which is fully consistent with the trend seen across Local Group satellite galaxies. The fact that this trend persists in stellar populations with very different star formation histories indicates that the C/M ratio is governed by stellar properties alone.

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arXiv2015

Measuring the mass of the central black hole in the bulgeless galaxy NGC 4395 from gas dynamical modeling

NGC 4395 is a bulgeless spiral galaxy, harboring one of the nearest known type 1 Seyfert nuclei. Although there is no consensus on the mass of its central engine, several estimates suggest it to be one of the lightest massive black holes (MBHs) known. We present the first direct dynamical measurement of the mass of this MBH from a combination of two-dimensional gas kinematic data, obtained with the adaptive optics assisted near infrared integral field spectrograph Gemini/NIFS, and high-resolution multiband photometric data from Hubble Space Telescope's Wide Field Camera 3 (HST/WFC3). We use the photometric data to model the shape and stellar mass-to-light ratio (M/L) of the nuclear star cluster. From the Gemini/NIFS observations, we derive the kinematics of warm molecular hydrogen gas as traced by emission through the H$_2$ 1--0 S(1) transition. These kinematics show a clear rotational signal, with a position angle orthogonal to NGC 4395's radio jet. Our best fitting tilted ring models of the kinematics of the molecular hydrogen gas contain a black hole with mass $M=4_{-3}^{+8}\times 10^5$ M$_\odot$ (3$σ$ uncertainties) embedded in a nuclear star cluster of mass $M=2 \times 10^6$ M$_\odot$. Our black hole mass measurement is in excellent agreement with the reverberation mapping mass estimate of Peterson et al. (2005), but shows some tension with other mass measurement methods based on accretion signals.

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arXiv2015

The Panchromatic Hubble Andromeda Treasury XI: The Spatially-Resolved Recent Star Formation History of M31

We measure the recent star formation history (SFH) across M31 using optical images taken with the \texit{Hubble Space Telescope} as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We fit the color-magnitude diagrams in ~9000 regions that are ~100 pc $\times$ 100 pc in projected size, covering a 0.5 square degree area (~380 kpc$^2$, deprojected) in the NE quadrant of M31. We show that the SFHs vary significantly on these small spatial scales but that there are also coherent galaxy-wide fluctuations in the SFH back to ~500 Myr, most notably in M31's 10-kpc star-forming ring. We find that the 10-kpc ring is at least 400 Myr old, showing ongoing star formation over the past ~500 Myr. This indicates the presence of molecular gas in the ring over at least 2 dynamical times at this radius. We also find that the ring's position is constant throughout this time, and is stationary at the level of 1 km/s, although there is evidence for broadening of the ring due to diffusion of stars into the disk. Based on existing models of M31's ring features, the lack of evolution in the ring's position makes a purely collisional ring origin highly unlikely. We find that the global SFR has been fairly constant over the last ~500 Myr, though it does show a small increase at 50 Myr that is 1.3 times the average SFR over the past 100 Myr. During the last ~500 Myr, ~60% of all SF occurs in the 10-kpc ring. Finally, we find that in the past 100 Myr, the average SFR over the PHAT survey area is $0.28\pm0.03$ M$_\odot$ yr$^{-1}$ with an average deprojected intensity of $7.3 \times 10^{-4}$ M$_\odot$ yr$^{-1}$ kpc$^{-2}$, which yields a total SFR of ~0.7 M$_\odot$ yr$^{-1}$ when extrapolated to the entire area of M31's disk. This SFR is consistent with measurements from broadband estimates. [abridged]

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arXiv2015

The High-Mass Stellar Initial Mass Function in M31 Clusters

We have undertaken the largest systematic study of the high-mass stellar initial mass function (IMF) to date using the optical color-magnitude diagrams (CMDs) of 85 resolved, young (4 Myr < t < 25 Myr), intermediate mass star clusters (10^3-10^4 Msun), observed as part of the Panchromatic Hubble Andromeda Treasury (PHAT) program. We fit each cluster's CMD to measure its mass function (MF) slope for stars >2 Msun. For the ensemble of clusters, the distribution of stellar MF slopes is best described by $Γ=+1.45^{+0.03}_{-0.06}$ with a very small intrinsic scatter. The data also imply no significant dependencies of the MF slope on cluster age, mass, and size, providing direct observational evidence that the measured MF represents the IMF. This analysis implies that the high-mass IMF slope in M31 clusters is universal with a slope ($Γ=+1.45^{+0.03}_{-0.06}$) that is steeper than the canonical Kroupa (+1.30) and Salpeter (+1.35) values. Using our inference model on select Milky Way (MW) and LMC high-mass IMF studies from the literature, we find $Γ_{\rm MW} \sim+1.15\pm0.1$ and $Γ_{\rm LMC} \sim+1.3\pm0.1$, both with intrinsic scatter of ~0.3-0.4 dex. Thus, while the high-mass IMF in the Local Group may be universal, systematics in literature IMF studies preclude any definitive conclusions; homogenous investigations of the high-mass IMF in the local universe are needed to overcome this limitation. Consequently, the present study represents the most robust measurement of the high-mass IMF slope to date. We have grafted the M31 high-mass IMF slope onto widely used sub-solar mass Kroupa and Chabrier IMFs and show that commonly used UV- and Halpha-based star formation rates should be increased by a factor of ~1.3-1.5 and the number of stars with masses >8 Msun are ~25% fewer than expected for a Salpeter/Kroupa IMF. [abridged]

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arXiv2015

A clear age-velocity dispersion correlation in Andromeda's stellar disk

The stellar kinematics of galactic disks are key to constraining disk formation and evolution processes. In this paper, for the first time, we measure the stellar age-velocity dispersion correlation in the inner 20 kpc (3.5 disk scale lengths) of M31 and show that it is dramatically different from that in the Milky Way. We use optical Hubble Space Telescope/Advanced Camera for Surveys photometry of 5800 individual stars from the Panchromatic Hubble Andromeda Treasury (PHAT) survey and Keck/DEIMOS radial velocity measurements of the same stars from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. We show that the average line-of-sight velocity dispersion is a steadily increasing function of stellar age exterior to R=10 kpc, increasing from 30 km/s for the young upper main sequence stars to 90 km/s for the old red giant branch stars. This monotonic increase implies that a continuous or recurring process contributed to the evolution of the disk. Both the slope and normalization of the dispersion vs. age relation are significantly larger than in the Milky Way, allowing for the possibility that the disk of M31 has had a more violent history than the disk of the Milky Way, more in line with cosmological predictions. We also find evidence for an inhomogeneous distribution of stars from a second kinematical component in addition to the dominant disk component. One of the largest and hottest high-dispersion patches is present in all age bins, and may be the signature of the end of the long bar.

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arXiv2015

Tracing the Metal-Poor M31 Stellar Halo with Blue Horizontal Branch Stars

We have analyzed new HST/ACS and HST/WFC3 imaging in F475W and F814W of two previously-unobserved fields along the M31 minor axis to confirm our previous constraints on the shape of M31's inner stellar halo. Both of these new datasets reach a depth of at least F814W$<$27 and clearly detect the blue horizontal branch (BHB) of the field as a distinct feature of the color-magnitude diagram. We measure the density of BHB stars and the ratio of BHB to red giant branch stars in each field using identical techniques to our previous work. We find excellent agreement with our previous measurement of a power-law for the 2-D projected surface density with an index of 2.6$^{+0.3}_{-0.2}$ outside of 3 kpc, which flattens to $α<$1.2 inside of 3 kpc. Our findings confirm our previous suggestion that the field BHB stars in M31 are part of the halo population. However, the total halo profile is now known to differ from this BHB profile, which suggests that we have isolated the metal-poor component. This component appears to have an unbroken power-law profile from 3-150 kpc but accounts for only about half of the total halo stellar mass. Discrepancies between the BHB density profile and other measurements of the inner halo are therefore likely due to the different profile of the metal-rich halo component, which is not only steeper than the profile of the met al-poor component, but also has a larger core radius. These profile differences also help to explain the large ratio of BHB/RGB stars in our observations.

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arXiv2015

The Structure of Nuclear Star Clusters in Nearby Late-type Spiral Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging

We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of ten of the nearest and brightest nuclear clusters residing in late-type spiral galaxies, in seven bands that span the near-ultraviolet to the near-infrared. Structural properties of the clusters were measured by fitting two-dimensional surface brightness profiles to the images using GALFIT. The clusters exhibit a wide range of structural properties. For six of the ten clusters in our sample, we find changes in the effective radius with wavelength, suggesting radially varying stellar populations. In four of the objects, the effective radius increases with wavelength, indicating the presence of a younger population which is more concentrated than the bulk of the stars in the cluster. However, we find a general decrease in effective radius with wavelength in two of the objects in our sample, which may indicate extended, circumnuclear star formation. We also find a general trend of increasing roundness of the clusters at longer wavelengths, as well as a correlation between the axis ratios of the NCs and their host galaxies. These observations indicate that blue disks aligned with the host galaxy plane are a common feature of nuclear clusters in late-type galaxies, but are difficult to detect in galaxies that are close to face-on. In color-color diagrams spanning the near-UV through the near-IR, most of the clusters lie far from single-burst evolutionary tracks, showing evidence for multi-age populations. Most of the clusters have integrated colors consistent with a mix of an old population (> 1 Gyr) and a young population (~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity to populations with a mix of ages that would not be possible to achieve with imaging in optical bands only.

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arXiv2015

PHAT Stellar Cluster Survey. II. Andromeda Project Cluster Catalog

We construct a stellar cluster catalog for the Panchromatic Hubble Andromeda Treasury (PHAT) survey using image classifications collected from the Andromeda Project citizen science website. We identify 2,753 clusters and 2,270 background galaxies within ~0.5 deg$^2$ of PHAT imaging searched, or ~400 kpc$^2$ in deprojected area at the distance of the Andromeda galaxy (M31). These identifications result from 1.82 million classifications of ~20,000 individual images (totaling ~7 gigapixels) by tens of thousands of volunteers. We show that our crowd-sourced approach, which collects >80 classifications per image, provides a robust, repeatable method of cluster identification. The high spatial resolution Hubble Space Telescope images resolve individual stars in each cluster and are instrumental in the factor of ~6 increase in the number of clusters known within the survey footprint. We measure integrated photometry in six filter passbands, ranging from the near-UV to the near-IR. PHAT clusters span a range of ~8 magnitudes in F475W (g-band) luminosity, equivalent to ~4 decades in cluster mass. We perform catalog completeness analysis using >3000 synthetic cluster simulations to determine robust detection limits and demonstrate that the catalog is 50% complete down to ~500 solar masses for ages <100 Myr. We include catalogs of clusters, background galaxies, remaining unselected candidates, and synthetic cluster simulations, making all information publicly available to the community. The catalog published here serves as the definitive base data product for PHAT cluster science, providing a census of star clusters in an L$^*$ spiral galaxy with unmatched sensitivity and quality.

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arXiv2014

Extended Structure and Fate of the Nucleus in Henize 2-10

We investigate the structure and nuclear region of the black hole (BH) hosting galaxy Henize2-10. Surface brightness (SB) profiles are analyzed using Magellan/Megacam $g$- and $r$-band images. Excluding the central starburst, we find a best-fit two component Sérsic profile with $n_{\rm in} \sim 0.6$, $r_{\text{eff,in}} \sim$ 260~pc, and $n_{\rm out}\sim 1.8$, $r_{\text{eff, out}}\sim$ 1 kpc. Integrating out to our outer most data point (100 arc sec $\sim$ 4.3 kpc), we calculate $M_g=-19.2$ and $M_r=-19.8$. The corresponding enclosed stellar mass is $M_{\star}\sim(10\pm3)\times10^9$ M$_\odot$, $\sim3\times$ larger than previous estimates. Apart from the central $\lesssim$500 pc, with blue colors and an irregular morphology, the galaxy appears to be an early-type system. The outer color is quite red, $(g-r)_0=0.75$, suggesting a dominant old population. We study the nuclear region of the galaxy using archival Gemini/NIFS $K$-band adaptive optics spectroscopy and {\it Hubble Space Telescope} imaging. We place an upper limit on the BH mass of $\sim10^7M_{\odot}$ from the NIFS data, consistent with that from the $M_{\rm BH}$-radio-X-ray fundamental plane. No coronal lines are seen, but a Br$γ$ source is located at the position of the BH with a luminosity consistent with the X-ray emission. The starburst at the center of Henize 2-10 has led to the formation of several super star clusters, which are within $\sim$100 pc of the BH. We examine the fate of the nucleus by estimating the dynamical masses and dynamical friction timescales of the clusters. The most massive clusters ($\sim 10^6 M_\odot$) have $τ_{\rm dyn} \lesssim 200$ Myr, and thus Henize 2-10 may represent a rare snapshot of nuclear star cluster formation around a pre-existing massive BH.

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arXiv2014

The Panchromatic Hubble Andromeda Treasury V: Ages and Masses of the Year 1 Stellar Clusters

We present ages and masses for 601 star clusters in M31 from the analysis of the six filter integrated light measurements from near ultraviolet to near infrared wavelengths, made as part of the Panchromatic Hubble Andromeda Treasury (PHAT). We derive the ages and masses using a probabilistic technique, which accounts for the effects of stochastic sampling of the stellar initial mass function. Tests on synthetic data show that this method, in conjunction with the exquisite sensitivity of the PHAT observations and their broad wavelength baseline, provides robust age and mass recovery for clusters ranging from $\sim 10^2 - 2 \times 10^6 M_\odot$. We find that the cluster age distribution is consistent with being uniform over the past $100$ Myr, which suggests a weak effect of cluster disruption within M31. The age distribution of older ($>100$ Myr) clusters fall towards old ages, consistent with a power-law decline of index $-1$, likely from a combination of fading and disruption of the clusters. We find that the mass distribution of the whole sample can be well-described by a single power-law with a spectral index of $-1.9 \pm 0.1$ over the range of $10^3-3 \times 10^5 M_\odot$. However, if we subdivide the sample by galactocentric radius, we find that the age distributions remain unchanged. However, the mass spectral index varies significantly, showing best fit values between $-2.2$ and $-1.8$, with the shallower slope in the highest star formation intensity regions. We explore the robustness of our study to potential systematics and conclude that the cluster mass function may vary with respect to environment.

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arXiv2013

A New Approach to Detailed Structural Decomposition from the SPLASH and PHAT Surveys: Kicked-up Disk Stars in the Andromeda Galaxy?

We characterize the bulge, disk, and halo subcomponents in the Andromeda galaxy (M31) over the radial range 4 < R_proj < 225 kpc. The cospatial nature of these subcomponents renders them difficult to disentangle using surface brightness (SB) information alone, especially interior to ~20 kpc. Our new decomposition technique combines information from the luminosity function (LF) of over 1.5 million bright (20 < m_814W < 22) stars from the Panchromatic Hubble Andromeda Treasury (PHAT) survey, radial velocities of over 5000 red giant branch stars in the same magnitude range from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey, and integrated I-band SB profiles from various sources. We use an affine-invariant Markov chain Monte Carlo algorithm to fit an appropriate toy model to these three data sets. The bulge, disk, and halo SB profiles are modeled as a Sersic, exponential, and cored power-law, respectively, and the LFs are modeled as broken power-laws. We present probability distributions for each of 32 parameters describing the SB profiles and LFs of the three subcomponents. We find that the number of stars with a disk-like LF is ~5% larger than the the number with disk-like (dynamically cold) kinematics, suggesting that some stars born in the disk have been dynamically heated to the point that they are kinematically indistinguishable from halo members. This is the first kinematical evidence for a "kicked-up disk" halo population in M31. The fraction of kicked-up disk stars is consistent with that found in simulations. We also find evidence for a radially varying disk LF, consistent with a negative metallicity gradient in the stellar disk.

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arXiv2013

A Radio-Selected Black Hole X-ray Binary Candidate in the Milky Way Globular Cluster M62

We report the discovery of a candidate stellar-mass black hole in the Milky Way globular cluster M62. We detected the black hole candidate, which we term M62-VLA1, in the core of the cluster using deep radio continuum imaging from the Karl G. Jansky Very Large Array. M62-VLA1 is a faint source, with a flux density of 18.7 +/- 1.9 microJy at 6.2 GHz and a flat radio spectrum (alpha=-0.24 +/- 0.42, for S_nu = nu^alpha). M62 is the second Milky Way cluster with a candidate stellar-mass black hole; unlike the two candidate black holes previously found in the cluster M22, M62-VLA1 is associated with a Chandra X-ray source, supporting its identification as a black hole X-ray binary. Measurements of its radio and X-ray luminosity, while not simultaneous, place M62-VLA1 squarely on the well-established radio--X-ray correlation for stellar-mass black holes. In archival Hubble Space Telescope imaging, M62-VLA1 is coincident with a star near the lower red giant branch. This possible optical counterpart shows a blue excess, H alpha emission, and optical variability. The radio, X-ray, and optical properties of M62-VLA1 are very similar to those for V404 Cyg, one of the best-studied quiescent stellar-mass black holes. We cannot yet rule out alternative scenarios for the radio source, such as a flaring neutron star or background galaxy; future observations are necessary to determine whether M62-VLA1 is indeed an accreting stellar-mass black hole.

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arXiv2012

The Panchromatic Hubble Andromeda Treasury III. Measuring Ages and Masses of Partially Resolved Stellar Clusters

The apparent age and mass of a stellar cluster can be strongly affected by stochastic sampling of the stellar initial mass function, when inferred from the integrated color of low mass clusters (less than ~10^4 solar masses). We use simulated star clusters to show that these effects are minimized when the brightest, rapidly evolving stars in a cluster can be resolved, and the light of the fainter, more numerous unresolved stars can be analyzed separately. When comparing the light from the less luminous cluster members to models of unresolved light, more accurate age estimates can be obtained than when analyzing the integrated light from the entire cluster under the assumption that the initial mass function is fully populated. We show the success of this technique first using simulated clusters, and then with a stellar cluster in M31. This method represents one way of accounting for the discrete, stochastic sampling of the stellar initial mass function in less massive clusters and can be leveraged in studies of clusters throughout the Local Group and other nearby galaxies.

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arXiv2012

The Panchromatic Hubble Andromeda Treasury II. Tracing the Inner M31 Halo with Blue Horizontal Branch Stars

We attempt to constrain the shape of M31's inner stellar halo by tracing the surface density of blue horizontal branch (BHB) stars at galactocentric distances ranging from 2 kpc to 35 kpc. Our measurements make use of resolved stellar photometry from a section of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, supplemented by several archival Hubble Space Telescope observations. We find that the ratio of BHB to red giant stars is relatively constant outside of 10 kpc, suggesting that the BHB is as reliable a tracer of the halo population as the red giant branch. In the inner halo, we do not expect BHB stars to be produced by the high metallicity bulge and disk, making BHB stars a good candidate to be a reliable tracer of the stellar halo to much smaller galactocentric distances. If we assume a power-law profile r^(-α) for the 2-D projected surface density BHB distribution, we obtain a high-quality fit with a 2-D power-law index of α=2.6^{+0.3}_{-0.2} outside of 3 kpc, which flattens to α<1.2 inside of 3 kpc. This slope is consistent with previous measurements but is anchored to a radial baseline that extends much farther inward. Finally, assuming azimuthal symmetry and a constant mass-to-light ratio, the best-fitting profile yields a total halo stellar mass of 2.1^{+1.7}_{-0.4} x 10^9 M_sun. These properties are comparable with both simulations of stellar halo formation formed by satellite disruption alone, and with simulations that include some in situ formation of halo stars.

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arXiv2012

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-manqué 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-manqué (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 α 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.

astro-ph.CO↗

arXiv2012

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.

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