A recently quenched isolated dwarf galaxy outside of the Local Group
  environment
  We report the serendipitous identification of a low mass ($M_* \sim 2\times
10^6 \, \mathrm{M}_\odot$), 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 $D_{\mathrm{SBF}} = 22 \pm 3$ Mpc, which is consistent with its radial
velocity of $cz = 1222 \pm 64$ 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 H$\alpha$ emission (EW(H$\alpha$)$ = -0.4 \pm 0.5$
angstroms), 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$_\mathrm{n}$4000 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 $M_*=10^6 - 10^7$ M$_{\odot}$ generally have luminosity-weighted
ages $\lesssim 1$ Gyr.
