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

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

*Corresponding author for this work

Research output: Contribution to journalLetterpeer-review

49 Citations (Scopus)


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 × 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 × 1038 ergs-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.

Original languageEnglish
Pages (from-to)L97–L101
Number of pages5
JournalMonthly Notices of the Royal Astronomical Society: Letters
Issue number1
Publication statusPublished - Mar 2009
Externally publishedYes


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