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

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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
JournalAstrophysical Journal
Issue number1 II
Publication statusPublished - 10 Oct 2006



  • Galaxies: dwarf
  • Galaxies: individual (Ursa Major II) - Local group

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Zucker, D. B., Belokurov, V., Evans, N. W., Kleyna, J. T., Irwin, M. J., 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., ... Snedden, S. A. (2006). A curious Milky Way satellite in Ursa Major. Astrophysical Journal, 650(1 II), L41-L44. https://doi.org/10.1086/508628