The Panchromatic Hubble Andromeda Treasury

Abstract

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going Hubble Space Telescope (HST) Multicycle Treasury program to image ∼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 F275W and F336W filters on the WFC3/UVIS camera, F475W and F814W on ACS/WFC, and F110W and F160W 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 F275W = 25.1, m F336W = 24.9, m F475W = 27.9, m F814W = 27.1, m F110W = 25.5, and m F160W = 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 Nyquistsampled images in F475W, F814W, and F160W. 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.

The Panchromatic Hubble Andromeda Treasury. Ii. Tracing the Inner M31 Halo With Blue Horizontal Branch Stars

“…As part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey (Dalcanton et al 2012), we obtained HST/ACS data in F475W and F814W covering a large fraction of the M31 disk. All of the PHAT data for this paper were acquired and analyzed as part of the survey, as detailed in Dalcanton et al (2012).…”

“…All of the PHAT data for this paper were acquired and analyzed as part of the survey, as detailed in Dalcanton et al (2012). In short, photometry was performed using the package DOLPHOT (Dolphin 2000).…”

“…In short, photometry was performed using the package DOLPHOT (Dolphin 2000). The output was filtered to reject non-point sources and low-quality measurements, as detailed in Dalcanton et al (2012). Artificial star tests were performed on each field to allow measurement of completeness as a function of color and magnitude.…”

Recent SFR calibrations and the constant SFR approximation

“…We illustrate both situations using the work by Simones et al (2014), who analyzed the CMDs obtained from the Panchromatic Hubble Andromeda Treasury data (Dalcanton et al 2012) to obtain the corresponding SFH in the past 500 Myr, and the extinction of 33 FUV-bright regions in M31 and used them to test the reliability of the FUV as an SFR index at small scales.…”

The Panchromatic Hubble Andromeda Treasury. Iii. Measuring Ages and Masses of Partially Resolved Stellar Clusters

“…We demonstrate the effectiveness of this method using synthetic clusters. We then apply this method to a cluster in M31 from the Panchromatic Hubble Andromeda Treasury (PHAT; Dalcanton et al 2012). …”