Alexia R. Lewis

THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY

The Panchromatic Hubble Andromeda Treasury is an ongoing Hubble Space Telescope Multi-Cycle Treasury program to image ~1/3 of M31s star-forming disk in six filters, spanning from the ultraviolet (UV) to the near-infrared (NIR). We use the Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) to resolve the galaxy into millions of individual stars with projected radii from 0 to 20 kpc. The full survey will cover a contiguous 0.5 deg^(2)area in 828 orbits. Imaging is being obtained in the F275W and F336W filters on the WFC3/UVIS camera, F475W and F814W on ACS/WFC, and F110W and F160W on WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The data produce photometry with a signal-to-noise ratio of 4 at m F_(275W) = 25.1, m_(F336W) = 24.9, m_(F475W) = 27.9, m_(F814W) = 27.1, m_(F110W) = 25.5, and m_(F160W) = 24.6 for single pointings in the uncrowded outer disk; in the inner disk, however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 mag brighter. Observations are carried out in two orbits per pointing, split between WFC3/UVIS and WFC3/IR cameras in primary mode, with ACS/WFC run in parallel. All pointings are dithered to produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products available for the survey, along with extensive testing of photometric stability, crowding errors, spatially dependent photometric biases, and telescope pointing control. We also report on initial fits to the structure of M31s disk, derived from the density of red giant branch stars, in a way that is independent of assumed mass-to-light ratios 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.

The SDSS–2MASS–WISE 10-dimensional stellar colour locus

We present the fiducial main sequence stellar locus traced by 10 photometric colors observed by SDSS, 2MASS, and WISE. Median colors are determined using 1,052,793 stars with rband extinction less than 0.125. We use this locus to measure the dust extinction curve relative to the r-band, which is consistent with previous measurements in the SDSS and 2MASS bands. The WISE band extinction coefficients are larger than predicted by standard extinction models. Using 13 lines of sight, we find variations in the extinction curve in H, Ks, and WISE bandpasses. Relative extinction decreases towards Galactic anti-center, in agreement with prior studies. Relative extinction increases with Galactic latitude, in contrast to previous observations. This indicates a universal mid-IR extinction law does not exist due to variations in dust grain size and chemistry with Galactocentric position. A preliminary search for outliers due to warm circumstellar dust is also presented, using stars with high signal-to-noise in the W3-band. We find 199 such outliers, identified by excess emission in Ks W3. Inspection of SDSS images for these outliers reveals a large number of contaminants due to nearby galaxies. Six sources appear to be genuine dust candidates, yielding a fraction of systems with infrared excess of 0:12 0:05%.

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 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 × 100 pc in projected size, covering a 0.5 square degree area (∼380 kpc2, 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 M31s 10 kpc star-forming ring. We find that the 10 kpc ring is at least 400 Myr old, showing ongoing star formation (SF) 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−1, although there is evidence for broadening of the ring due to the diffusion of stars into the disk. Based on existing models of M31s ring features, the lack of evolution in the ring’s position makes a purely collisional ring origin highly unlikely. Besides the well-known 10 kpc ring, we observe two other ring-like features. There is an outer ring structure at 15 kpc with concentrated SF starting ∼80 Myr ago. The inner ring structure at 5 kpc has a much lower star formation rate (SFR) and therefore lower contrast against the underlying stellar disk. It was most clearly defined ∼200 Myr ago, but is much more diffuse today. 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 has occurred 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 ± 0.03 with an average deprojected intensity of , which yields a total SFR of ∼0.7 when extrapolated to the entire area of M31s disk. This SFR is consistent with measurements from broadband estimates.

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 Andromedas 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.

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 (), intermediate mass star clusters (103?104 M?), observed as part of the Panchromatic Hubble Andromeda Treasury program. We fit each cluster?s CMD to measure its mass function (MF) slope for stars ?2 M?. By modeling the ensemble of clusters, we find the distribution of MF slopes is best described by with a very small intrinsic scatter and no drastic outliers. This model allows the MF slope to depend on cluster mass, size, and age, but the data imply no significant dependencies within this regime of cluster properties. The lack of an age dependence suggests that the MF slope has not significantly evolved over the first ?25 Myr and provides direct observational evidence that the measured MF represents the IMF. Taken together, this analysis?based on an unprecedented large sample of young clusters, homogeneously constructed CMDs, well-defined selection criteria, and consistent principled modeling?implies that the high-mass IMF slope in M31 clusters is universal. The IMF has a slope (; statistical uncertainties) that is slightly steeper than the canonical Kroupa () and Salpeter () values, and our measurement of it represents a factor of ?20 improvement in precision over the Kroupa IMF (+1.30 ? 0.7). Using our inference model on select Milky Way (MW) and LMC high-mass IMF studies from the literature, we find and , 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 the literature of 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. To facilitate practical use over the full stellar mass spectrum, we have grafted the M31 high-mass IMF slope onto widely used sub-solar mass Kroupa and Chabrier IMFs. The increased steepness in the M31 high-mass IMF slope implies that commonly used UV- and H?-based star formation rates should be increased by a factor of ?1.3?1.5 and the number of stars with masses M? is ?25% fewer than expected for a Salpeter/Kroupa IMF.

Panchromatic Hubble Andromeda Treasury. XVI. Star Cluster Formation Efficiency and the Clustered Fraction of Young Stars

We use the Panchromatic Hubble Andromeda Treasury (PHAT) survey dataset to perform spatially resolved measurements of star cluster formation efficiency (

A GLOBAL STAR-FORMING EPISODE IN M31 2–4 GYR AGO

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The Influence of Outer Solar System Architecture on the Structure and Evolution of the Oort Cloud

), the fraction of stellar mass formed in long-lived star clusters. We use robust star formation history and cluster parameter constraints, obtained through color-magnitude diagram analysis of resolved stellar populations, to study Andromedas cluster and field populations over the last

THE PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XV. THE BEAST: BAYESIAN EXTINCTION AND STELLAR TOOL*

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PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XII. MAPPING STELLAR METALLICITY DISTRIBUTIONS in M31

300 Myr. We measure