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 HST/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 Gyrs, [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 × 10 M . Thus we find no strong evidence for dark matter or the need to invoke a non-standard IMF. 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 (LMXBs). The X-ray luminosity of LX = 0.56×10 erg s−1 is consistent with the values observed for GCs of the same metallicity. Overall we find SUCD1 has properties similar to other known UCDs and massive GCs.