J. J. Dalcanton, B. F. Williams, D. Lang, T. R. Lauer, J. S. Kalirai, A. C. Seth, A. Dolphin, P. Rosenfield, D. R. Weisz, E. F. Bell, L. C. Bianchi, M. L. Boyer, N. Caldwell, H. Dong, C. E. Dorman, K. M. Gilbert, L. Girardi, S. M. Gogarten, K. D. Gordon, P. Guhathakurta, P. W. Hodge, J. A. Holtzman, L. Johnson, S. S. Larsen, A. Lewis, J. L. Melbourne, K. A. G. Olsen, H. -W. Rix, K. Rosema, A. Saha, A. Sarajedini, E. D. Skillman, K. Z. Stanek
The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's 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 M31's 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)
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http://arxiv.org/abs/1204.0010
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