A search for faint resolved galaxies beyond the Milky Way in DES year 6 : a new faint, diffuse dwarf satellite of NGC 55

Abstract

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 MV ∼ (−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 +0.05 - 0.12 Mpc, 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 +0.5 - 0.3 mag and an azimuthally averaged physical half-light radius of 2.2 +0.5 - 0.4 kpc, making this one of the lowest surface brightness galaxies ever found with m = - 32.3 mag arcsec -2. This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.