Julianne Dalcanton,

The Panchromatic Hubble Andromeda Treasury (PHAT) is a Hubble Multi-Cycle Treasury program that will conduct a wide-area imaging survey of the stellar populations of Andromeda (M31). The goal is to establish a new foundation for interpreting observations of stellar populations across the universe and back through cosmic time.

The Importance of Nearby Galaxies

Our quest to understand the universe is anchored in our knowledge of the Local Group of galaxies. Within these galaxies, Hubble has the potential to resolve millions to billions of stars, all with common distances and foreground extinction. Those stars, along with their ancestors and descendants (e.g., molecular clouds, H ii regions, variable stars, X-ray binaries, supernova remnants), provide opportunities for strengthening the foundation on which all knowledge of the distant universe is based. Local Group constraints on stellar evolution, the initial mass function, the extinction law, and the distance scale are required for interpreting observations of more distant galaxies. Most of those parameters are best constrained outside the Milky Way (MW), thus minimizing line-of-sight reddening, uncertain distances, and background/foreground confusion. External galaxies further allow one to relate the properties of the observed stars to the properties of the surrounding environment (i.e., the interstellar medium, metallicity, and star formation rate). For galaxies beyond ~1 Mpc, however, severe crowding makes the detection of the most age-sensitive stellar populations possible only in their outskirts, preventing resolved population studies of the main bodies of massive galaxies. The Local Group is thus the place where the complex processes that govern star and galaxy evolution can best be revealed in their full galactic context.

The Local Group contains four prominent galaxies that can serve as non-MW proxies for more distant galaxies: the two Magellanic Clouds, M33 (Triangulum), and M31. Of these, M31 provides the best match to the metallicity, morphology, and luminosity of the massive galaxies that dominate redshift surveys, making a wide-area systematic survey of its populations compelling. M31 shows true spiral structure, contains populations that extend to super-solar metallicity, and hosts a traditional spheroidal component. Across the universe, fully 84% of stars lie in spiral disks (58%) or bulges (26%; Driver et al. 2007), and more than 3/4 of all stars today have metallicities within a factor of 2 of solar (Gallazzi et al. 2008). These factors make M31 a far more relevant laboratory than the Magellanic Clouds. Moreover, M31 contains the vast majority of the stars in the Local Group outside the MW, offering the opportunity to generate samples of sufficient size that Poisson statistical errors are negligible, and even rare phenomena are well represented. Finally, the stars in M31 are bright enough to be accessible spectroscopically. Therefore, we can augment photometric observations with an extensive program of spectroscopy, to provide measurements of the kinematics, metallicities, spectral types, and physical parameters of star clusters and stars on the upper main sequence, asymptotic giant branch, and red giant branch.