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Title

An ultra-compact dwarf around the Sombrero galaxy (M104): the nearest massive UCD

Author(s)

Hau, George K. T.;  Spitler, Lee R.;  Forbes, Duncan A.;  Proctor, Robert N.;  Strader, Jay;  Mendel, J. Trevor;  Brodie, Jean P.;  Harris, William E.

Abstract

We report the discovery of an ultra-compact dwarf (UCD) associated with the Sombrero galaxy (M104). This is the closest massive UCD known and the first spectroscopically verified massive UCD which is located in a low-density environment. The object, we name SUCD1, was identified in Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) imaging and confirmed to be associated with the Sombrero galaxy by its recession velocity obtained from Keck spectra. The light profile is well fitted by a Wilson model. We measure a half-light size of 14.7 ± 1.4 pc , an absolute magnitude of MV=−12.3 mag (MK=−15.1 mag) and an internal velocity dispersion of 25.0 ± 5.6 km s−1 . Such values are typical of UCDs. From Lick spectral indices we measure a luminosity-weighted central age of 12.6 ± 0.9 Gyr, [Fe/H] of −0.08 ± 0.08 dex and [α/Fe] of 0.06 ± 0.07 dex . The lack of colour gradients suggests these values are representative of the entire UCD. The derived stellar and virial masses are the same, within errors, at ∼3.3 x 107 M⊙ . Thus, we find no strong evidence for dark matter or the need to invoke a non-standard initial mass function. We also report arguably the first X-ray detection of a bona fide UCD, which we attribute to the presence of low-mass X-ray binaries. The X-ray luminosity of LX= 0.56 x 1038 erg s−1 is consistent with the values observed for globular clusters (GCs) of the same metallicity. Overall, we find SUCD1 has properties similar to other known UCDs and massive GCs.

Publication type

Journal article

Research centre

Swinburne University of Technology. Faculty of Information and Communication Technologies. Centre for Astrophysics and Supercomputing

Source

Monthly Notices of the Royal Astronomical Society: Letters, Vol. 394, no. 1 (Mar 2009), pp. L97-L101

Publication year

2009

FOR Code(s)

0201 Astronomical and Space Sciences

Keyword(s)

Dwarf galaxies;  Galaxy formation;  M104;  SUCD1;  Star clusters;  UCD;  Ultra-compact dwarf galaxies

Publisher

United Kingdom

ISSN

0035-8711

Publisher URL

http://dx.doi.org/10.1111/j.1745-3933.2009.00618.x

Copyright

Copyright © 2009 Royal Astronomical Society. Author's version of the paper reproduced here in accordance with the copyright policy of the publisher.

Additional information

This work is supported by NSF grant AST 05-07729 and the Australian Research Council.

Full text

Peer reviewed