A curious Milky Way satellite in Ursa Major

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

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

271 Citations (Scopus)


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 subgiant branch, main sequence, and turnoff, from which we estimate a distance of ∼30 kpc and a projected size of ∼250 × 125 pc2. 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 Mv ∼ -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.

Original languageEnglish
Pages (from-to)L41-L44
Number of pages4
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 10 Oct 2006
Externally publishedYes


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