  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 M_V = -12.3 mag
(M_K = -15.1 mag) and an internal velocity dispersion of 25.0 +/- 5.6 km/s.
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 [alpha/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 10E7 Msun. 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 x 10E38 erg/s 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.
