A Recently Quenched Isolated Dwarf Galaxy Outside of the Local Group Environment

We report the serendipitous identification of a low-mass (M_* ∼ 2 × 10⁶ M_⊙), isolated, likely quenched dwarf galaxy in the "foreground" of the COSMOS-CANDELS field. From deep Hubble Space Telescope (HST) imaging we infer a surface brightness fluctuation distance for COSMOS-dw1 of D_SBF = 22 ± 3 Mpc, which is consistent with its radial velocity of cz = 1222 ± 64 km s^-1 via Keck/LRIS. At this distance, the galaxy is 1.4 Mpc in projection from its nearest massive neighbor. We do not detect significant Hα emission (EW(Hα)= - 0.4 ± 0.5 Å), suggesting that COSMOS-dw1 is likely quenched. Very little is currently known about isolated quenched galaxies in this mass regime. Such galaxies are thought to be rare, as there is no obvious mechanism to permanently stop star formation in them; to date there are only four examples of well-studied quenched field dwarfs, only two of which appear to have quenched in isolation. COSMOS-dw1 is the first example outside of the immediate vicinity of the Local Group. COSMOS-dw1 has a relatively weak D_n4000 break and the HST data show a clump of blue stars indicating that star formation ceased only recently. We speculate that COSMOS-dw1 was quenched due to internal feedback, which was able to temporarily suspend star formation. In this scenario the expectation is that quenched isolated galaxies with masses M_* = 10⁶-10⁷ M_⊙ generally have luminosity-weighted ages ≲1 Gyr.