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Register a new userA Curious New Milky Way Satellite in Ursa Major
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Authors
D.B. Zucker
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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 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 IIs 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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In this Letter, we report the discovery of a new dwarf satellite to the Milky Way, located at ($alpha_{2000}, delta_{2000}$) $=$ (158.72,51.92) in the constellation of Ursa Major. This object was detected as an overdensity of red, resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest surface brightness Milky Way companion known, but with approximately an order of magnitude fewer stars. Deeper follow-up imaging confirms this object has an old and metal-poor stellar population and is $sim$ 100 kpc away. We roughly estimate M$_V =$ -6.75 and $r_{1/2} =$ 250 pc for this dwarf. Its luminosity is several times fainter than the faintest known Milky Way dwarf. However, its physical size is typical for dSphs. Even though its absolute magnitude and size are presently quite uncertain, Ursa Major is likely the lowest luminosity and lowest surface brightness galaxy yet known.
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