A Search for Extended Ultraviolet Disk (XUV-Disk) Galaxies in the Local Universe

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© 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
, , Citation David A. Thilker et al 2007 ApJS 173 538 DOI 10.1086/523853

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Author affiliations

1 Center for Astrophysical Sciences, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218

2 Departamento de Astrofísica, Universidad Complutense de Madrid, Madrid 28040, Spain

3 Laboratoire d'Astrophysique de Marseille, BP 8, Traverse du Siphon, 13376 Marseille Cedex 12, France

4 Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101

5 Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh, Edinburgh, UK

6 School of Physics A29, University of Sydney, NSW 2006, Australia

7 Five College Astronomy Department, Hampshire College, Amherst, MA 01003

8 California Institute of Technology, MC 405-47, 1200 East California Boulevard, Pasadena, CA 91125

9 Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771

10 Department of Astronomy, Columbia University, New York, NY 10027

11 Department of Physics and Astronomy, The Johns Hopkins University, Homewood Campus, Baltimore, MD 21218

12 Center for Space Astrophysics, Yonsei University, Seoul 120-749, Korea

13 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095

14 Space Sciences Laboratory, University of California at Berkeley, 601 Campbell Hall, Berkeley, CA 94720

Dates

  1. Received 2007 April 30
  2. Accepted 2007 September 11
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0067-0049/173/2/538

Abstract

We have initiated a search for extended ultraviolet disk (XUV-disk) galaxies in the local universe. Here we compare GALEX UV and visible-NIR images of 189 nearby (D < 40 Mpc) S0-Sm galaxies included in the GALEX Atlas of Nearby Galaxies and present the first catalog of XUV-disk galaxies. We find that XUV-disk galaxies are surprisingly common but have varied relative (UV/optical) extent and morphology. Type 1 objects (≳20% incidence) have structured, UV-bright/optically faint emission features in the outer disk, beyond the traditional star formation threshold. Type 2 XUV-disk galaxies (~10% incidence) exhibit an exceptionally large, UV-bright/optically low surface brightness (LSB) zone having blue UV–Ks outside the effective extent of the inner, older stellar population, but not reaching extreme galactocentric distance. If the activity occurring in XUV-disks is episodic, a higher fraction of present-day spirals could be influenced by such outer disk star formation. Type 1 disks are associated with spirals of all types, whereas Type 2 XUV-disks are predominantly found in late-type spirals. Type 2 XUV-disks are forming stars quickly enough to double their (currently low) stellar mass in the next Gyr (assuming a constant star formation rate). XUV-disk galaxies of both types are systematically more gas-rich than the general galaxy population. Minor external perturbation may stimulate XUV-disk incidence, at least for Type 1 objects. XUV-disks are the most actively evolving galaxies growing via inside-out disk formation in the current epoch, and may constitute a segment of the galaxy population experiencing significant, continued gas accretion from the intergalactic medium or neighboring objects.

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10.1086/523853