A Curious New Milky Way Satellite in Ursa Major

/ Authors

D. Zucker, V. Belokurov, N. Evans, J. Kleyna, M. Irwin, M. Wilkinson, M. Fellhauer, D. Bramich, G. Gilmore, H. Newberg

and 38 more authors

B. Yanny, J. A. Smith, P. Hewett, E. Bell, H. Rix, O. Gnedin, S. Vidrih, R. Wyse, B. Willman, E. Grebel, D. Schneider, T. Beers, A. Kniazev, J. Barentine, H. Brewington, J. Brinkmann, M. Harvanek, S. Kleinman, J. Long, A. Nitta, S. Cambridge, Hawaii, Rpi, Fnal, Lanl, Austin Peay State University, Mpia, Osu, Jhu, Nyu, Basel, P. S. University, M. University, Saao, A. Point, Subaru Telescope, Cracow, G. Observatory

/ Abstract

In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD) shows a well-defined sub-giant branch, main sequence and turn-off, from which we estimate a distance of ~30 kpc and a projected size of ~250 x 125 pc. The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of M_V \~ -3.8, which would make it the faintest known satellite galaxy. UMa II's isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.

DOI: 10.1086/508628