A Curious Milky Way Satellite in Ursa Major^*

D. B. Zucker¹, V. Belokurov¹, N. W. Evans¹, J. T. Kleyna², M. J. Irwin¹, M. I. Wilkinson¹, M. Fellhauer¹, D. M. Bramich¹, G. Gilmore¹, H. J. Newberg³, B. Yanny⁴, J. A. Smith^5,6, P. C. Hewett¹, E. F. Bell⁷, H.-W. Rix⁷, O. Y. Gnedin⁸, S. Vidrih¹, R. F. G. Wyse⁹, B. Willman¹⁰, E. K. Grebel¹¹, D. P. Schneider¹², T. C. Beers¹³, A. Y. Kniazev^7,14, J. C. Barentine¹⁵, H. Brewington¹⁵, J. Brinkmann¹⁵, M. Harvanek¹⁵, S. J. Kleinman¹⁶, J. Krzesinski^15,17, D. Long¹⁵, A. Nitta¹⁸, and S. A. Snedden¹⁵

Published 2006 September 21 • © 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
The Astrophysical Journal, Volume 650, Number 1 Citation D. B. Zucker et al 2006 ApJ 650 L41 DOI 10.1086/508628

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Author affiliations

¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822

³ Rensselaer Polytechnic Institute, Troy, NY 12180

⁴ Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510

⁵ Los Alamos National Laboratory, ISR-4, MS D448, Los Alamos, NM 87545

⁶ Department of Physics and Astronomy, Austin Peay State University, P.O. Box 4608, Clarksville, TN 37044

⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

⁸ Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210

⁹ Johns Hopkins University, 3701 San Martin Drive, Baltimore, MD 21218

¹⁰ Center for Cosmology and Particle Physics, Department of Physics, New York University, 4 Washington Place, New York, NY 10003

¹¹ Astronomical Institute of the University of Basel, Department of Physics and Astronomy, Venusstrasse 7, CH-4102 Binningen, Switzerland

¹² Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802

¹³ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824

¹⁴ South African Astronomical Observatory, P.O. Box 9, Observatory 7935, Cape Town, South Africa

¹⁵ Apache Point Observatory, P.O. Box 59, Sunspot, NM 88349

¹⁶ Subaru Telescope, 650 North A`ohoku Place, Hilo, HI 96720

¹⁷ Mount Suhora Observatory, Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland

¹⁸ Gemini Observatory, 670 North A`ohoku Place, Hilo, HI 96720

Dates

Received 2006 July 4
Accepted 2006 August 24
Published 2006 September 21

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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 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 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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Footnotes

*
Based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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