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 ( ${r}_{1/2}={75}_{-13}^{+16}\,\mathrm{pc}$ ), and elliptical (ϵ = 0.54 ± 0.1) system at a heliocentric distance of ${76.7}_{-6.1}^{+4.0}\,\mathrm{kpc}$ , with a stellar population that is well described by an old, metal-poor isochrone (age of τ ~ 13.0 Gyr and metallicity of [Fe/H] ≲ - 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 $({\mu }_{\alpha }\cos \delta ,{\mu }_{\delta })=(+0.25\pm 0.06,-0.10\pm 0.05)$ 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.