A Search for Faint Resolved Galaxies Beyond the Milky Way in DES Year 6: A New Faint, Diffuse Dwarf Satellite of NGC 55

We report results from a systematic wide-area search for faint dwarf galaxies at heliocentric distances from 0.3 to 2 Mpc using the full 6 yr of data from the Dark Energy Survey (DES). Unlike previous searches over the DES data, this search specifically targeted a field population of faint galaxies located beyond the Milky Way virial radius. We derive our detection efficiency for faint, resolved dwarf galaxies in the Local Volume with a set of synthetic galaxies and expect our search to be complete to M V similar to (-7, -10) mag for galaxies at D = (0.3, 2.0) Mpc. We find no new field dwarfs in the DES footprint, but we report the discovery of one high-significance candidate dwarf galaxy at a distance of 2.2-0.12+0.05Mpc , a potential satellite of the Local Volume galaxy NGC 55, separated by 47 ' (physical separation as small as 30 kpc). We estimate this dwarf galaxy to have an absolute V-band magnitude of -8.0-0.3+0.5mag and an azimuthally averaged physical half-light radius of 2.2-0.4+0.5kpc , making this one of the lowest surface brightness galaxies ever found with mu=32.3magarcsec-2 . This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.