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

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

来源：Monthly Notices of the Royal Astronomical Society, 2009, 394(1): L97-L101.

DOI：10.1111/j.1745-3933.2009.00618.x

• galaxies: dwarf
• galaxies: formation
• galaxies: individual: M104
• galaxies: individual: SUCD1
• galaxies: star clusters

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 [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 similar to 3.3 x 10(7) M(circle dot). 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 x 10(38) erg s(-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.

• 出版日期2009-3-21