Ultra-Faint Milky Way Satellites Discovered in Carina, Phoenix, and Telescopium with DELVE Data Release 3

Abstract

We report the discovery of three Milky Way satellite candidates: Carina IV, Phoenix III, and DELVE 7, in the third data release of the DECam Local Volume Exploration survey (DELVE). The candidate systems were identified by cross-matching results from two independent search algorithms. All three are extremely faint systems composed of old, metal-poor stellar populations ($\tau \gtrsim 10$ Gyr, [Fe/H] $\lesssim -1.4$). Carina IV ($M_V=-2.8;r_{1/2}=40\mathrm{pc}$) and Phoenix III ($M_V=-1.2;r_{1/2}=19\mathrm{pc}$) have half-light radii that are consistent with the known population of dwarf galaxies, while DELVE 7 ($M_V=1.2;r_{1/2}=2\mathrm{pc}$) is very compact and seems more likely to be a star cluster, though its nature remains ambiguous without spectroscopic followup. The Gaia proper motions of stars in Carina IV ($M_{\ast }=2250_{-830}^{+1180}{\mathrm{M}}_{\odot }$) indicate that it is unlikely to be associated with the LMC, while DECam CaHK photometry confirms that its member stars are metal-poor. Phoenix III ($M_{\ast }=520_{-290}^{+660}{\mathrm{M}}_{\odot }$) is the faintest known satellite in the extreme outer stellar halo ($D_{\mathrm{GC}}>100$ kpc), while DELVE 7 ($M_{\ast }=60_{-40}^{+120}{\mathrm{M}}_{\odot }$) is the faintest known satellite with $D_{\mathrm{GC}}>20$ kpc.

22 pages, 7 figures, 2 tables; This version: accepted to ApJ

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