Eridanus IV: an Ultra-faint Dwarf Galaxy Candidate Discovered in the DECam Local Volume Exploration Survey

We present the discovery of a candidate ultra-faint Milky-Way satellite, Eridanus IV (DELVE J0505-0931), detected in photometric data from the DECam Local Volume Exploration survey (DELVE). Eridanus IV is a faint (M-V = - 4.7 +/- 0.2), extended (r(1/2)=75(-13)(+16) pc ), and elliptical (epsilon = 0.54 +/- 0.1) system at a heliocentric distance of 76.7(-6.1)(+4.0)kpc, with a stellar population that is well described by an old, metal-poor isochrone (age of tau similar to 13.0 Gyr and metallicity of [Fe/H] less than or similar to - 2.1 dex). These properties are consistent with the known population of ultra-faint Milky-Way satellite galaxies. Eridanus IV is also prominently detected using proper-motion measurements from Gaia Early Data Release 3, with a systemic proper motion of (mu(alpha) cos delta,mu(delta))=(+0.25 +/- 0.06,-0.10 +/- 0.05) mas yr(-1) measured from its horizontal branch and red-giant-branch member stars. We find that the spatial distribution of likely member stars hints at the possibility that the system is undergoing tidal disruption.