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 $${\rm D}_{\odot} = 116.3_{-0.6}^{+0.6}$$ 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 $${\rm D}_{\odot} = 19.0_{-0.6}^{+0.5} kpc$$, 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. Here, 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.
Mau, S., et al. "Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey." The Astrophysical Journal (Online), vol. 890, no. 2, Feb. 2020. https://doi.org/10.3847/1538-4357/ab6c67
Mau, S., Cerny, W., Pace, A. B., Choi, Y., Drlica-Wagner, A., Santana-Silva, L., Riley, A. H., Erkal, D., Stringfellow, G. S., Adamów, M., Carlin, J. L., Gruendl, R. A., Hernandez-Lang, D., Kuropatkin, N., Li, T. S., Martínez-Vázquez, C. E., Morganson, E., Mutlu-Pakdil, B., ... Wechsler, R. H. (2020). Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey. The Astrophysical Journal (Online), 890(2). https://doi.org/10.3847/1538-4357/ab6c67
Mau, S., Cerny, W., Pace, A. B., et al., "Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey," The Astrophysical Journal (Online) 890, no. 2 (2020), https://doi.org/10.3847/1538-4357/ab6c67
@article{osti_1597067,
author = {Mau, S. and Cerny, W. and Pace, A. B. and Choi, Y. and Drlica-Wagner, A. and Santana-Silva, L. and Riley, A. H. and Erkal, D. and Stringfellow, G. S. and Adamów, M. and others},
title = {Two Ultra-faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey},
annote = {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 ${\rm D}_{\odot} = 116.3_{-0.6}^{+0.6}$ 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 ${\rm D}_{\odot} = 19.0_{-0.6}^{+0.5} kpc$, 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. Here, 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.},
doi = {10.3847/1538-4357/ab6c67},
url = {https://www.osti.gov/biblio/1597067},
journal = {The Astrophysical Journal (Online)},
issn = {ISSN 1538-4357},
number = {2},
volume = {890},
place = {United States},
publisher = {Institute of Physics (IOP)},
year = {2020},
month = {02}}