The Panchromatic Hubble Andromeda Treasury

Abstract: 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)

• The paper presents an extensive survey using multi-filter HST imaging to resolve millions of individual stars over one-third of M31's disk.
• It details precise methodologies including dithered orbits, robust photometric calibration, and artificial star tests to ensure high-quality data.
• Results highlight a 40% overdensity in RGB stars at the 10 kpc ring, offering new insights into galactic dynamics and stellar evolution.

Overview of the Panchromatic Hubble Andromeda Treasury (PHAT)

The paper presents an extensive study conducted under the Panchromatic Hubble Andromeda Treasury (PHAT) program, leveraging the Hubble Space Telescope (HST) to perform high-resolution imaging of the Andromeda Galaxy (M31). This multicycle treasury program endeavors to cover approximately one-third of M31's star-forming disk using a broad range of filters across ultraviolet (UV) to near-infrared (NIR) wavelengths. The PHAT survey remarkably involves resolving the Andromeda Galaxy into millions of individual stars over a contiguous 0.5 square degree area.

Methodology and Observational Strategy

The PHAT program employs HST's Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3), with images obtained in six filters: F#1W{275}, F#1W{336} on the WFC3/UVIS camera; F#1W{475}, F#1W{814} on ACS/WFC; and F#1W{110}, F#1W{160} on WFC3/IR. This setup ensures robust constraints on stellar temperature, bolometric luminosity, and extinction for a range of spectral types. The imaging strategy involves two orbits per pointing, purposely dithered to produce Nyquist-sampled images in key filters, which is crucial for precise photometric and astrometric analyses.

The paper details the methodologies adopted in data acquisition, processing, and calibration. Bias level correction and cosmic ray rejection are noted as vital procedural steps, ensuring high-quality photometric results. Bright stars in selected image frames help in determining the necessary PSF corrections and aperture corrections, refining the photometric accuracy.

Results and Initial Analyses

The PHAT survey produces high-fidelity CMDs, enabling the characterization of a diverse range of stellar populations from young, massive stars to older stellar constituents like the red giant branch (RGB) and the horizontal branch (HB). Notably, the paper observes an RGB stellar density distribution map suggesting a $\sim$40% overdensity at the 10~kpc ring, indicating a dynamic structure beyond a mere recent increase in star formation activity.

A significant emphasis is placed on the artificial star tests to assess completeness and error characteristics, vital for constraining and quantifying the survey depth across varying stellar densities. These analyses reflect comprehensive insights into the interplay between photometric errors, crowding, and systematic uncertainties over M31's intricate stellar environments.

Implications and Potential Research Directions

The implications of PHAT are profound for several research avenues, including but not limited to: constraining the initial mass function as a function of environment, characterizing galaxy and star-forming region dynamics, and improving constraints on stellar evolution models. Notably, the reinterpretation of M31's structure using resolved RGB stars rather than integrated light will significantly reevaluate our understanding of galactic dynamics in a critical exemplar of spiral galaxies.

Future research stemming from PHAT data can richly advance within theoretical modeling and comparative extragalactic studies, especially given the extensive lifetime and diverse applications that the survey promises. The synthesis of the PHAT dataset within broader multi-wavelength and spectroscopic campaigns will likely unravel deeper connections between stellar, galactic, and intergalactic processes, enriching the comprehension of cosmic evolution.

The PHAT initiative exemplifies the transformative potential of high-precision, large-scale stellar population surveys, setting a methodological benchmark for future endeavors in galactic archaeology and stellar astrophysics.