Pegasus W: An Ultrafaint Dwarf Galaxy Outside the Halo of M31 Not Quenched by Reionization

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 virial radius of M31. The distance to the galaxy is ${915}_{-91}^{+60}$ kpc, measured using the luminosity of horizontal branch stars identified in Hubble Space Telescope optical imaging. The galaxy has a half-light radius ( r _h ) ${100}_{-13}^{+11}$ pc, ${M}_{V}=-{7.20}_{-0.16}^{+0.17}$ mag, and a present-day stellar mass ${6.5}_{-1.4}^{+1.1}\times {10}^{4}$ M _⊙ . We identify sources in the color–magnitude diagram (CMD) that may be younger than ∼500 Myr, suggesting late-time star formation in the UFD galaxy, although further study is needed to confirm these are bona fide young stars in the galaxy. Based on fitting the CMD with stellar evolution libraries, Pegasus W shows an extended star formation history. Using the τ _90 metric (defined as the timescale by which the galaxy formed 90% of its stellar mass), the galaxy was quenched only ${7.4}_{-2.6}^{+2.2}$ Gyr ago, which is similar to the quenching timescale of a number of UFD satellites of M31 but significantly more recent than the UFD satellites of the Milky Way. Such late-time quenching is inconsistent with the more rapid timescale expected by reionization and suggests that, while not currently a satellite of M31, Pegasus W was nonetheless slowly quenched by environmental processes.