A Curious New Milky Way Satellite in Ursa Major

Zucker, D B
Belokurov, V
Evans, N W
Irwin, M J
Kleyna, J T
Wilkinson, M I
Fellhauer, M
Bramich, D M
Gilmore, G
Newberg, H J
Yanny, B
Smith, J A
Hewett, P C
Bell, E F
Rix, H W
Gnedin, O Y
Vidrih, S
Wyse, R F G
Willman, B
Grebel, E K
Schneider, D P
Beers, T C
Kniazev, A Yu
Barentine, J C
Brewington, H
Brinkmann, J
Harvanek, M
Kleinman, S J
Krzesínski, J
Long, D
Nitta, A
Snedden, S A

Publication date

June 2006

Abstract

In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in the Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram shows a well-defined sub-giant branch, main sequence and turn-off, from which we estimate a distance of about 30 kpc and a projected size of about 250 pc. Based on its extent and its stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named after its constellation as Ursa Major II (UMa II). 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 ev...