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

TL;DR

This study conducted a wide-area search using DES data for faint dwarf galaxies beyond the Milky Way, discovering a new ultra-diffuse dwarf galaxy near NGC 55, and setting constraints on the population of such galaxies.

Contribution

First systematic search targeting faint resolved dwarf galaxies beyond the Milky Way in DES data, resulting in the discovery of a new ultra-diffuse dwarf galaxy candidate.

Findings

No new field dwarfs found in the DES footprint.

Discovery of a candidate dwarf galaxy at 2.2 Mpc with extremely low surface brightness.

The candidate is one of the most diffuse galaxies at this luminosity.

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 six years 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 respectively. 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 $2.2\substack{+0.05\\-0.12}$ Mpc, a potential satellite of the Local Volume galaxy NGC 55, separated by $47$ arcmin (physical separation as small as 30 kpc). We estimate this dwarf galaxy to have an absolute V-band magnitude of $-8.0\substack{+0.5\\-0.3}$ mag and an azimuthally averaged physical half-light radius of $2.2\substack{+0.5\\-0.4}$ kpc, making this one of the lowest surface brightness galaxies ever found with $\mu = 32.3$ mag ${\rm arcsec}^{-2}$. This is the largest, most diffuse galaxy known at this luminosity, suggesting possible tidal interactions with its host.