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Register a new userThe Panchromatic Hubble Andromeda Treasury V: Ages and Masses of the Year 1 Stellar Clusters
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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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The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31s star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31s disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged)
We present the Bayesian Extinction And Stellar Tool (BEAST), a probabilistic approach to modeling the dust extinguished photometric spectral energy distribution of an individual star while accounting for observational uncertainties common to large resolved star surveys. Given a set of photometric measurements and an observational uncertainty model, the BEAST infers the physical properties of the stellar source using stellar evolution and atmosphere models and constrains the line of sight extinction using a newly developed mixture model that encompasses the full range of dust extinction curves seen in the Local Group. The BEAST is specifically formulated for use with large multi-band surveys of resolved stellar populations. Our approach accounts for measurement uncertainties and any covariance between them due to stellar crowding (both systematic biases and uncertainties in the bias) and absolute flux calibration, thereby incorporating the full information content of the measurement. We illustrate the accuracy and precision possible with the BEAST using data from the Panchromatic Hubble Andromeda Treasury. While the BEAST has been developed for this survey, it can be easily applied to similar existing and planned resolved star surveys.
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 Andromedas galactic disk. We see a clear metallicity gradient of $-0.020pm0.004$ 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 M31s 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$sigma$ 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 Andromedas elongated bar. This same region also appears to have an enhanced stellar density and velocity dispersion.
We map the star formation history across M31 by fitting stellar evolution models to color-magnitude diagrams of each 83${times}$83$$ (0.3$times$1.4 kpc, deprojected) region of the PHAT survey outside of the innermost 6${times}$12$$ portion. We find that most of the star formation occurred prior to $sim$8 Gyr ago, followed by a relatively quiescent period until $sim$4 Gyr ago, a subsequent star formation episode about 2 Gyr ago and a return to relative quiescence. There appears to be little, if any, structure visible for populations with ages older than 2 Gyr, suggesting significant mixing since that epoch. Finally, assuming a Kroupa IMF from 0.1$-$100 M$_{odot}$, we find that the total amount of star formation over the past 14 Gyr in the area over which we have fit models is 5${times}$10$^{10}$ M$_{odot}$. Fitting the radial distribution of this star formation and assuming azimuthal symmetry, (1.5$pm$0.2)${times}$10$^{11}$ M$_{odot}$ of stars have formed in the M31 disk as a whole, (9$pm$2)${times}$10$^{10}$ M$_{odot}$ of which has likely survived to the present after accounting for evolutionary effects. This mass is about one fifth of the total dynamical mass of M31.
We have measured stellar photometry with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in near ultraviolet (F275W, F336W), optical (F475W, F814W), and near infrared (F110W, F160W) bands for 117 million resolved stars in M31. As part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, we measured photometry with simultaneous point spread function fitting across all bands and at all source positions after precise astrometric image alignment (<5-10 milliarcsecond accuracy). In the outer disk, the photometry reaches a completeness-limited depth of F475W~28, while in the crowded, high surface brightness bulge, the photometry reaches F475W~25. We find that simultaneous photometry and optimized measurement parameters significantly increase the detection limit of the lowest resolution filters (WFC3/IR) providing color-magnitude diagrams that are up to 2.5 magnitudes deeper when compared with color-magnitude diagrams from WFC3/IR photometry alone. We present extensive analysis of the data quality including comparisons of luminosity functions and repeat measurements, and we use artificial star tests to quantify photometric completeness, uncertainties and biases. We find that largest sources of systematic error in the photometry are due to spatial variations in the point spread function models and charge transfer efficiency corrections. This stellar catalog is the largest ever produced for equidistant sources, and is publicly available for download by the community.
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