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

Abstract

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 ( ϵ = 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 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 ( μ α cos δ , μ δ ) = ( + 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.