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 (τ ≳ 10 Gyr, [Fe/H] ≲−1.4). Carina IV (MV = −2.8; r1/2 = 40 pc) and Phoenix III (MV = −1.2; r1/2 = 19 pc) have half-light radii that are consistent with the known population of dwarf galaxies, while DELVE 7 (MV = 1.2; r1/2 = 2 pc) is very compact and seems more likely to be a star cluster, though its nature remains ambiguous without spectroscopic follow-up. The Gaia proper motions of stars in Carina IV ({M}_{\star }=225{0}_{-830}^{+1180}\,{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}_{\star }=52{0}_{-290}^{+660}\,{M}_{\odot }) is the faintest known satellite in the extreme outer stellar halo (DGC > 100 kpc), while DELVE 7 ({M}_{\star }=6{0}_{-40}^{+120}\,{M}_{\odot }) is the faintest known satellite with DGC > 20 kpc.