Eight Ultra-faint Galaxy Candidates Discovered in Year Two of the Dark Energy Survey

The DES Collaboration, A. Drlica-Wagner, K. Bechtol, E. S. Rykoff, E. Luque, A. Queiroz, Y. -Y. Mao, R. H. Wechsler, J. D. Simon, B. Santiago, B. Yanny, E. Balbinot, S. Dodelson, A. Fausti Neto, D. J. James, T. S. Li, M. A. G. Maia, J. L. Marshall, A. Pieres, K. Stringer, A. R. Walker, T. M. C. Abbott, F. B. Abdalla, S. Allam, A. Benoit-Levy, G. M. Bernstein, E. Bertin, D. Brooks, E. Buckley-Geer, D. L. Burke, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, M. Crocce, L. N. da Costa, S. Desai, H. T. Diehl, J. P. Dietrich, P. Doel, T. F. Eifler, A. E. Evrard, D. A. Finley, P. Fosalba, J. Frieman, E. Gaztanaga, D. W. Gerdes, D. Gruen, R. A. Gruendl, G. Gutierrez, K. Honscheid, K. Kuehn, N. Kuropatkin, O. Lahav, P. Martini, R. Miquel, B. Nord, R. Ogando, A. A. Plazas, K. Reil, A. Roodman, M. Sako, E. Sanchez, V. Scarpine, M. Schubnell, I. Sevilla-Noarbe, R. C. Smith, M. Soares-Santos, F. Sobreira, E. Suchyta, M. E. C. Swanson, G. Tarle, D. Tucker, V. Vikram, W. Wester, Y. Zhang, J. Zuntz

Published 2015-08-14Version 1

We report the discovery of eight new ultra-faint dwarf galaxy candidates in the second year of optical imaging data from the Dark Energy Survey (DES). Six of these candidates are detected at high confidence, while two additional lower-confidence candidates are identified in regions of incomplete or non-uniform survey coverage. The new stellar systems are found using three independent automated search techniques, and are identified as statistically significant overdensities of individually resolved stars consistent with the isochrone and luminosity function of an old and metal-poor simple stellar population. The new systems are faint (Mv > -4.7 mag) and span a broad range of physical sizes (17 pc < $r_{1/2}$ < 181 pc) and heliocentric distances (25 kpc < D < 214 kpc). All of the new systems have central surface brightnesses (\mu > 27.5 mag arcsec$^2$) consistent with known ultra-faint dwarf galaxies. Roughly half of the DES candidates are more distant, less luminous, and/or have lower surface brightnesses than previously known Milky Way satellite galaxies, and would have had a low probability of detection if observed by the Sloan Digital Sky Survey. A large fraction of satellite candidates are found in the southern half of the DES footprint in proximity to the Magellanic Clouds. We find that the DES data alone exclude (p < 0.001) a spatially isotropic distribution of Milky Way satellites, and that this distribution can be well, although not uniquely, explained by a model in which several of the observed DES satellites are associated with the Magellanic system. Including the current sample, our model predicts that ~100 ultra-faint galaxies with physical properties comparable to the DES satellites might exist over the full sky and that 20-30% of these would be spatially associated with the Magellanic Clouds.