The Panchromatic Hubble Andromeda Treasury X. Ultraviolet to Infrared Photometry of 117 Million Equidistant Stars

TL;DR

This paper presents a comprehensive photometric catalog of 117 million stars in M31 across ultraviolet to infrared wavelengths, achieved through advanced simultaneous PSF fitting techniques with the Hubble Space Telescope, enabling detailed stellar population studies.

Contribution

It introduces a novel, large-scale, multi-band stellar catalog with improved detection limits and data quality, utilizing simultaneous photometry and precise astrometry in the PHAT survey.

Findings

Photometry reaches F475W~28 in outer disk regions.

Simultaneous multi-band photometry improves detection depth by up to 2.5 magnitudes.

Systematic errors are mainly due to PSF variations and charge transfer efficiency.

Abstract

We have measured stellar photometry with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS) in near ultraviolet (F275W, F336W), optical (F475W, F814W), and near infrared (F110W, F160W) bands for 117 million resolved stars in M31. As part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, we measured photometry with simultaneous point spread function fitting across all bands and at all source positions after precise astrometric image alignment (<5-10 milliarcsecond accuracy). In the outer disk, the photometry reaches a completeness-limited depth of F475W~28, while in the crowded, high surface brightness bulge, the photometry reaches F475W~25. We find that simultaneous photometry and optimized measurement parameters significantly increase the detection limit of the lowest resolution filters (WFC3/IR) providing color-magnitude diagrams that are up to 2.5 magnitudes deeper when compared with color-magnitude diagrams from WFC3/IR photometry alone. We present extensive analysis of the data quality including comparisons of luminosity functions and repeat measurements, and we use artificial star tests to quantify photometric completeness, uncertainties and biases. We find that largest sources of systematic error in the photometry are due to spatial variations in the point spread function models and charge transfer efficiency corrections. This stellar catalog is the largest ever produced for equidistant sources, and is publicly available for download by the community.