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We report the discovery of an ultrafaint dwarf (UFD) galaxy, Pegasus W,located on the far side of the Milky Way-M31 system and outside the virialradius of M31. The distance to the galaxy is 915 (+60/-91) kpc, measured usingthe luminosity of horizontal branch (HB) stars identified in Hubble SpaceTelescope optical imaging. The galaxy has a half-light radius (r_h) of 100(+11/-13) pc, M_V = -7.20 (+0.17/-0.16) mag, and a present-day stellar mass of6.5 (+1.1/-1.4) x 10^4 Msun. We identify sources in the color-magnitude diagram(CMD) that may be younger than ~500 Myr suggesting late-time star formation inthe UFD galaxy, although further study is needed to confirm these are bona fideyoung stars in the galaxy. Based on fitting the CMD with stellar evolutionlibraries, Pegasus W shows an extended star formation history (SFH). Using thetau_90 metric (defined as the timescale by which the galaxy formed 90% of itsstellar mass), the galaxy was quenched only 7.4 (+2.2/-2.6) Gyr ago, which issimilar to the quenching timescale of a number of UFD satellites of M31 butsignificantly more recent than the UFD satellites of the Milky Way. Suchlate-time quenching is inconsistent with the more rapid timescale expected byreionization and suggests that, while not currently a satellite of M31, PegasusW was nonetheless slowly quenched by environmental processes.

Pegasus W: An Ultra-Faint Dwarf Galaxy Outside the Halo of M31 Not Quenched by Reionization