The Panchromatic Hubble Andromeda Treasury ( PHAT ) is an on-going Hubble Space Telescope ( HST ) Multicycle Treasury program to image \sim 1/3 of M31 ’ s 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 0–20 { kpc } .
The full survey will cover a contiguous 0.5 square degree area in 828 orbits .
Imaging is being obtained in the \ifbool mmodeF275WF275W and \ifbool mmodeF336WF336W filters on the WFC3/UVIS camera , \ifbool mmodeF475WF475W and \ifbool mmodeF814WF814W on ACS/WFC , and \ifbool mmodeF110WF110W and \ifbool mmodeF160WF160W 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 _ { \ifbool { mmode } { \text { F 275 W } } { F 275 W } } = 25.1 , m _ { \ifbool { mmode } { \text { F 336 W } } { F 336 W } } = 24.9 , m _ { \ifbool { mmode } { \text { F 475 W } } { F 475 W } } = 27.9 , m _ { \ifbool { mmode } { \text { F 814 W } } { F 814 W } } = 27.1 , m _ { \ifbool { mmode } { \text { F 110 W } } { F 110 W } } = 25.5 , and m _ { \ifbool { mmode } { \text { F 160 W } } { F 160 W } } = 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 magnitudes 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 \ifbool mmodeF475WF475W , \ifbool mmodeF814WF814W , and \ifbool mmodeF160WF160W .
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 M31 ’ s 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 } .