Abstract:

The Panchromatic Hubble Andromeda Treasury is an HST multicycle program to image the north east quadrant of M31 to deep limits in the UV, optical, and near-IR. The HST imaging has resolved the galaxy into over 150 million stars (comparable to ~1/2 the number of stars in SDSS), all with common distances and foreground extinctions. As its legacy, this survey adds M31 to the Milky Way and Magellanic Clouds as a fundamental calibrator of stellar evolution and star-formation processes for understanding the stellar populations of distant galaxies. I will briefly describe the survey strategy, data reduction, and key data products. I will then highlight new work using the NIR stellar populations to constrain the large scale properties of the cold ISM, with 25 pc resolution. These new maps offer the highest resolution available in M31, and point to surprising challenges facing models of dust emission.