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

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 M_V = -12.3 mag (M_K = -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 × 10⁷M_solar. 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 L_X = 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.