Holmberg IX: The Nearest Young Galaxy*

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Published 2008 March 6 • © 2008. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation E. Sabbi et al 2008 ApJ 676 L113 DOI 10.1086/587548

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Author affiliations

1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218

2 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Heidelberg, Germany

3 European Space Agency, Research and Scientific Support Department, Baltimore, MD 21218

4 Department of Astronomy, University of Wisconsin, SAL Chamberlin Hall, 1150 University Avenue, Madison, WI 53706

5 Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

6 Observational Cosmology Laboratory, Code 665, Goddard Space Flight Center, Greenbelt, MD 20771

7 Catholic University of America, Washington, DC 20064

8 Johns Hopkins University, Baltimore, MD 21218

Dates

  1. Received 2008 January 27
  2. Accepted 2008 February 15
  3. Published
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1538-4357/676/2/L113

Abstract

We present a study of the M81 companion dwarf irregular galaxy Holmberg IX using deep images taken with the Wide Field Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope. Based on color-magnitude diagrams the resolved stellar population toward Holmberg IX contains numerous stars with ages of ≲200 Myr as well as older red giant stars. By charting the spatial distribution of the red giant stars and considering their inferred metallicities, we conclude that most of these older stars are associated with M81 or its tidal debris. At least 20% of the stellar mass in Holmberg IX was produced in the last ~200 Myr, giving it the youngest stellar population of any nearby galaxy. The location of Holmberg IX, its high gas content, and its youthful stellar population suggest that it is a tidal dwarf galaxy, perhaps formed during the last close passage of M82 around M81.

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Footnotes

  • * 

    Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. These observations are associated with program 10605.

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10.1086/587548