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 \pm 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 \pm 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 \pm 0.9 Gyrs , [ Fe / H ] of -0.08 \pm 0.08 dex and [ \alpha / Fe ] of 0.06 \pm 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 \sim 3.3 \times 10 ^ { 7 } M _ { \odot } .
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 L _ { X } = 0.56 \times 10 ^ { 38 } { erg } \thinspace { 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 .