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We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACSdata from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thickdisk, and Star clusters) fields. Its detected individual stars populate anapproximately one magnitude range of its luminosity function (LF). Usingsynthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, wefind that the colors and magnitudes of GHOSTS I's individual stars are mostconsistent with being young helium-burning and asymptotic giant branch stars ata distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be activelyforming stars, so we tentatively classify it as a dwarf irregular (dIrr)galaxy, although future HST observations deep enough to resolve a largermagnitude range in its LF are required to make a more secure classification.GHOSTS I's absolute magnitude is $M_V = -9.85^{+ 0.40}_{- 0.33}$, making it oneof the least luminous dIrr galaxies known, and its metallicity is lower than[Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and itsellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites atcomparable luminosity. There are no luminous massive galaxies or galaxyclusters within ~ 4 Mpc from GHOSTS I that could be considered as its host,making it a very isolated dwarf galaxy in the Local Universe.

GHOSTS I: A New Faint very Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc