Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey

Abstract

We report the discovery of two ultra-faint stellar systems found in early data from the DECam Local Volume Exploration survey (DELVE). The first system, Centaurus I (DELVE J1238-4054), is identified as a resolved overdensity of old and metal-poor stars with a heliocentric distance of D-circle dot = 116.3(-0.6)(+0.6) kpc, a half-light radius of r(h) = 2.3(-0.3)(+0.4) arcmin, an age of tau > 12.85 Gyr, a metallicity of Z = 0.0002(-0.0002)(+0.0001), and an absolute magnitude of M-V = -5.55(-0.11)(+0.11) mag. This characterization is consistent with the population of ultra-faint satellites, and confirmation of this system would make Centaurus I one of the brightest recently discovered ultra-faint dwarf galaxies. Centaurus I is detected in Gaia DR2 with a clear and distinct proper motion signal, confirming that it is a real association of stars distinct from the Milky Way foreground; this is further supported by the clustering of blue horizontal branch stars near the centroid of the system. The second system, DELVE 1 (DELVE J1630-0058), is identified as a resolved overdensity of stars with a heliocentric distance of D-circle dot = 19.0(-0.6)(+0.5) kpc, a half-light radius of r(h) = 0.97(-0.17)(+0.24) arcmin, an age of tau = 12.5(-0.7)(+1.0) Gyr, a metallicity of Z = 0.0005(-0.0001)(+0.0002), and an absolute magnitude of M-V = -0.2(-0.6)(+0.8) mag, consistent with the known population of faint halo star clusters. Given the low number of probable member stars at magnitudes accessible with Gaia DR2, a proper motion signal for DELVE 1 is only marginally detected. We compare the spatial position and proper motion of both Centaurus I and DELVE 1 with simulations of the accreted satellite population of the Large Magellanic Cloud (LMC) and find that neither is likely to be associated with the LMC.