A Curious New 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. Grebel; D. 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

doi:10.1086/508628

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A Curious New Milky Way Satellite in Ursa Major

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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.

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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. Grebel, D. 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. 2006-07-04. A Curious New Milky Way Satellite in Ursa Major. https://doi.org/10.1086/508628

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