A Curious New Milky Way Satellite in Ursa Major

Details A Curious New Milky Way Satellite in Ursa Major A Curious New Milky Way Satellite in Ursa Major

2006-06-26

arxiv.org/abs/astro-ph/0606633v2

Astrophys.J.650:L41-L44,2006

Astronomy and Astrophysics

Astrophysics

4 pages, 4 figures, submitted to ApJ Letters

Scientific paper

10.1086/508628

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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