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We report the serendipitous identification of a low mass ($M_* \sim 2\times10^6 \, \mathrm{M}_\odot$), 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-dw1of $D_{\mathrm{SBF}} = 22 \pm 3$ Mpc, which is consistent with its radialvelocity of $cz = 1222 \pm 64$ km s$^{-1}$ via Keck/LRIS. At this distance, thegalaxy is 1.4 Mpc in projection from its nearest massive neighbor. We do notdetect significant H$\alpha$ emission (EW(H$\alpha$)$ = -0.4 \pm 0.5$angstroms), suggesting that COSMOS dw1 is likely quenched. Very little iscurrently known about isolated quenched galaxies in this mass regime. Suchgalaxies are thought to be rare, as there is no obvious mechanism topermanently stop star formation in them; to date there are only four examplesof well-studied quenched field dwarfs, only two of which appear to havequenched in isolation. COSMOS-dw1 is the first example outside of the immediatevicinity of the Local Group. COSMOS-dw1 has a relatively weakD$_\mathrm{n}$4000 break and the HST data show a clump of blue stars indicatingthat star formation ceased only recently. We speculate that COSMOS-dw1 wasquenched due to internal feedback, which was able to temporarily suspend starformation. In this scenario the expectation is that quenched isolated galaxieswith masses $M_*=10^6 - 10^7$ M$_{\odot}$ generally have luminosity-weightedages $\lesssim 1$ Gyr.

A recently quenched isolated dwarf galaxy outside of the Local Group environment