Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey
Bechtol, K.; Zenteno, A.; Allam, S.; Gruendl, R. A.; James, D. J.; Kuehn, K.; Kuropatkin, N.; Li, T. S.; Soares-Santos, M.; Walker, A. R.; Carlin, J. L.; Crnojević, D.; Martínez-Vázquez, C. E.; Palmese, A.; Bell, E. F.; van der Marel, R. P.; Monachesi, A.; Sanchez, J.; Erkal, D.; Drlica-Wagner, A.; Wechsler, R. H.; Yanny, B.; Vivas, A. K.; Gallart, C.; Nidever, D. L.; Stringfellow, G. S.; Olsen, K. A. G.; Ferguson, P. S.; Adamów, M.; Cerny, W.; Choi, Y.; Esteves, J.; Hernandez-Lang, D.; Hughes, A. K.; Massana, P.; Mau, S.; McNanna, M.; Mutlu-Pakdil, B.; Neilsen, E. H.; Noël, N. E. D.; Pace, A. B.; Pieres, A.; Riley, A. H.; Sand, D. J.; Santana-Silva, L.; Simon, J. D.; Tavangar, K.; Tollerud, E. J.; Tucker, D. L.; Johnson, L. C.; Mao, Y. -Y.; Nadler, E. O.; Shipp, N.; Morganson, E.; Barkhouse, W. A.; Balaji, P.; Jethwa, P.; Majewski, S.; Paz-Chinchon, F.
United States, Brazil, United Kingdom, Chile, Spain, Austria, Australia
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 ${\text{}}{D}_{\odot }={116.3}_{-0.6}^{+0.6}\,\mathrm{kpc}$ , a half-light radius of ${r}_{h}={2.3}_{-0.3}^{+0.4}\,\mathrm{arcmin}$ , an age of $\tau \gt 12.85\,\mathrm{Gyr}$ , a metallicity of $Z={0.0002}_{-0.0002}^{+0.0001}$ , and an absolute magnitude of ${M}_{V}=-{5.55}_{-0.11}^{+0.11}\,\mathrm{mag}$ . 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 ${\text{}}{D}_{\odot }={19.0}_{-0.6}^{+0.5}\,\mathrm{kpc}$ , a half-light radius of ${r}_{h}={0.97}_{-0.17}^{+0.24}\,\mathrm{arcmin}$ , an age of $\tau ={12.5}_{-0.7}^{+1.0}\,\mathrm{Gyr}$ , a metallicity of $Z={0.0005}_{-0.0001}^{+0.0002}$ , and an absolute magnitude of ${M}_{V}=-{0.2}_{-0.6}^{+0.8}\,\mathrm{mag}$ , 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.