The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) I. Ultraviolet to Infrared Photometry of 22 Million Stars in M33

TL;DR

This paper presents a comprehensive, high-resolution, multi-wavelength stellar catalog of 22 million stars in M33, utilizing advanced HST photometry techniques to analyze stellar populations and structures.

Contribution

It provides the largest, most detailed multi-band stellar catalog for M33, with improved photometry methods and extensive data quality analysis, enabling new insights into galaxy structure.

Findings

Young stars trace tight spiral arms

Old stars show a looser two-armed structure

Catalog reaches deep photometric limits in crowded regions

Abstract

We present panchromatic resolved stellar photometry for 22 million stars in the Local Group dwarf spiral Triangulum (M33), derived from Hubble Space Telescope (HST) observations with the Advanced Camera for Surveys (ACS) in the optical (F475W, F814W), and the Wide Field Camera 3 (WFC3) in the near ultraviolet (F275W, F336W) and near-infrared (F110W, F160W) bands. The large, contiguous survey area covers $\sim$14 square kpc and extends to 3.5 kpc (14 arcmin, or 1.5-2 scale lengths) from the center of M33. The PHATTER observing strategy and photometry technique closely mimic those of the Panchromatic Hubble Andromeda Treasury (PHAT), but with updated photometry techniques that take full advantage of all overlapping pointings (aligned to within $<$5-10 milliarcseconds) and improved treatment of spatially-varying point spread functions. The photometry reaches a completeness-limited depth of F475W$\sim$28.5 in the lowest surface density regions observed in M33 and F475W$\sim$26.5 in the most crowded regions found near the center of M33. We find the young populations trace several relatively tight arms, while the old populations show a clear, looser two-armed structure. We present extensive analysis of the data quality including artificial star tests to quantify completeness, photometric uncertainties, and flux biases. This stellar catalog is the largest ever produced for M33, and is publicly available for download by the community.