Zhuo Chen (University of Washington)

The PHAST survey is a large 195-orbit Hubble Space Telescope program imaging ~0.45 square degrees of the southern half of M31’s star-forming disk at optical and near-ultraviolet (NUV) wavelengths. The PHAST survey area extends the northern coverage of the Panchromatic Hubble Andromeda Treasury (PHAT) through the southern half of the M31 disk, covering out to a radius of ~13 kpc along the southern major axis. In total, PHAT+PHAST now covers ~2/3 of M31’s star forming disk. This new legacy imaging yields stellar photometry of over 90 million resolved stars using the Advanced Camera for Surveys (ACS) in the optical (F475W, F814W), and the Wide Field Camera 3 (WFC3) in the NUV (F275W, F336W). The photometry achieves a 50% completeness-limited depth of F475W ~27.7 in the lowest surface density regions of the outer disk and F475W ~26.0 in the most crowded regions near M31’s bulge. PHAST covers multiple structurally unique regions within M31’s southern disk, including the intersections with M32 and the giant southern stream, the split portion of the star formation ring, and the southern bar. These observations enable us to test models and simulations of the merger and accretion history of M31, as well as its disk formation and evolution. In this talk, I will present an overview of the PHAST survey, including combining it with the PHAT survey to produce the largest HST mosaic of the entire M31 disk, seamless population maps, various data products and spectroscopic follow-up. The combined PHAST+PHAT photometry catalog of ~0.2 billion stars is the largest ever produced for equidistant sources and is available for the public.