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
\pm 0.2$), extended ($r_{1/2} = 75^{+16}_{-13}$ pc), and elliptical ($\epsilon
= 0.54 \pm 0.1$) system at a heliocentric distance of $76.7^{+4.0}_{-6.1}$ kpc,
with a stellar population that is well-described by an old, metal-poor
isochrone (age of $\tau \sim 13.0$ Gyr and metallicity of ${\rm [Fe/H] \lesssim
-2.1}$ 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.
