A Pride of Satellites in the Constellation Leo? Discovery of the Leo VI Milky Way Satellite Galaxy with DELVE Early Data Release 3

Abstract

We report the discovery and spectroscopic confirmation of an ultra-faint Milky Way (MW) satellite in the constellation of Leo. This system was discovered as a spatial overdensity of resolved stars observed with Dark Energy Camera (DECam) data from an early version of the third data release of the DECam Local Volume Exploration survey (DELVE EDR3). The low luminosity ($M_V = -3.56_{-0.37}^{+0.47}$ ; $L_V = 2300_{-700}^{+1200} L_\odot$), large size ($R_{1/2} = 90_{-30}^{+30}$ pc), and large heliocentric distance ($D = 111_{-6}^{+9}$ kpc) are all consistent with the population of ultra-faint dwarf galaxies (UFDs). Using Keck/DEIMOS observations of the system, we were able to spectroscopically confirm nine member stars, while measuring a tentative mass-to-light ratio of $700_{-500}^{+1400} M_\odot/L_\odot$ and a non-zero metallicity dispersion of $\sigma_{[\rm Fe/H]}=0.19_{-0.11}^{+0.14}$, further confirming Leo VI's identity as an UFD. While the system has a highly elliptical shape, $\epsilon = 0.54_{-0.29}^{+0.19}$, we do not find any conclusive evidence that it is tidally disrupting. Moreover, despite the apparent on-sky proximity of Leo VI to members of the proposed Crater-Leo infall group, its smaller heliocentric distance and inconsistent position in energy-angular momentum space make it unlikely that Leo VI is part of the proposed infall group.