We report the discovery of a new faint dwarf galaxy , GHOSTS I , using HST/ACS data from one of our GHOSTS ( Galaxy Halos , Outer disks , Substructure , Thick disk , and Star clusters ) fields .
Its detected individual stars populate an approximately one magnitude range of its luminosity function ( LF ) .
Using synthetic color-magnitude diagrams ( CMDs ) to compare with the galaxy ’ s CMD , we find that the colors and magnitudes of GHOSTS I ’ s individual stars are most consistent with being young helium-burning and asymptotic giant branch stars at a distance of \sim 12 \pm 2 Mpc .
Morphologically , GHOSTS I appears to be actively forming stars , so we tentatively classify it as a dwarf irregular ( dIrr ) galaxy , although future HST observations deep enough to resolve a larger magnitude range in its LF are required to make a more secure classification .
GHOSTS I ’ s absolute magnitude is M _ { V } \sim - 9.85 ^ { +0.40 } _ { -0.33 } , making it one of the least luminous dIrr galaxies known , and its metallicity is lower than [ \mathrm { Fe / H } ] = -1.5 dex .
The half-light radius of GHOSTS I is 226 \pm 38 pc and its ellipticity is 0.47 \pm 0.07 , similar to Milky Way and M31 dwarf satellites at comparable luminosity .
There are no luminous massive galaxies or galaxy clusters within \sim 4 Mpc from GHOSTS I that could be considered as its host , making it a very isolated dwarf galaxy in the Local Universe .