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

McNanna, M.; Bechtol, K.; Mau, S.; Nadler, E. O.; Medoff, J.; Drlica-Wagner, A.; Cerny, W.; Crnojević, D.; Mutlu-Pakdıl, B.; Vivas, A. K.; Pace, A. B.; Carlin, J. L.; Collins, M. L. M.; Ferguson, P. S.; Martínez-Delgado, D.; Martínez-Vázquez, C. E.; Noel, N. E. D.; Riley, A. H.; Sand, D. J.; Smercina, A.; Tollerud, E.; Wechsler, R. H.; Abbott, T. M. C.; Aguena, M.; Alves, O.; Bacon, D.; Bom, C. R.; Brooks, D.; Burke, D. L.; Carballo-Bello, J. A.; Carnero Rosell, A.; Carretero, J.; da Costa, L. N.; Davis, T. M.; de Vicente, J.; Diehl, H. T.; Doel, P.; Ferrero, I.; Frieman, J.; Giannini, G.; Gruen, D.; Gutierrez, G.; Gruendl, R. A.; Hinton, S. R.; Hollowood, D. L.; Honscheid, K.; James, D. J.; Kuehn, K.; Marshall, J. L.; Mena-Fernández, J.; Miquel, R.; Pereira, M. E. S.; Pieres, A.; Malagón, A. A. Plazas; Sakowska, J. D.; Sanchez, E.; Sanchez Cid, D.; Santiago, B.; Sevilla-Noarbe, I.; Smith, M.; Stringfellow, G. S.; Suchyta, E.; Swanson, M. E. C.; Tarle, G.; Weaverdyck, N.; Wiseman, P.; Des Collaboration; Delve Collaboration
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The Astrophysical Journal

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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 ~ (-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 ${2.2}_{-0.12}^{+0.05}\,\mathrm{Mpc}$ , 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 $-{8.0}_{-0.3}^{+0.5}\,\mathrm{mag}$ and an azimuthally averaged physical half-light radius of ${2.2}_{-0.4}^{+0.5}\,\mathrm{kpc}$ , making this one of the lowest surface brightness galaxies ever found with $\mu =32.3\,\mathrm{mag}\,{\mathrm{arcsec}}^{-2}$ . This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.