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

TL;DR

The paper reports the discovery of Eridanus IV, an ultra-faint dwarf galaxy candidate near the Milky Way, characterized by its faintness, size, elliptical shape, old and metal-poor stellar population, and evidence of tidal disruption.

Contribution

This is the first detection of Eridanus IV using DECam data and Gaia proper motions, providing detailed properties of a new ultra-faint satellite galaxy candidate.

Findings

Eridanus IV is a faint, extended, elliptical system at ~77 kpc.

Its stellar population is old and metal-poor, consistent with known ultra-faint dwarfs.

Evidence suggests the system may be undergoing tidal disruption.

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 \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.