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

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