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

Williams, Benjamin F., Lang, Dustin, Dalcanton, Julianne J., Dolphin, Andrew E., Weisz, Daniel R., Bell, Eric F., Bianchi, Luciana, Byler, Nell, Gilbert, Karoline M., Girardi, Léo, Gordon, Karl, Gregersen, Dylan, Johnson, L. C., Kalirai, Jason, Lauer, Tod R., Monachesi, Antonela, Rosenfield, Philip, Seth, Anil, & Skillman, Eva
2014, The Astrophysical Journal Supplement Series, 215, 9


We have measured stellar photometry with the Hubble Space Telescope Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys 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 survey, we measured photometry with simultaneous point-spread function (PSF) fitting across all bands and at all source positions after precise astrometric image alignment (<5-10 mas 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 (CMDs) that are up to 2.5 mag deeper when compared with CMDs 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 the largest sources of systematic error in the photometry are due to spatial variations in the PSF 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.

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