草帽星系周围的超致密矮星（M104）：最近的大质量UCD
An Ultra Compact Dwarf around the Sombrero galaxy (M104): the Nearest Massive UCD
摘要：
我们报告了与草帽星系（M104）相关的超紧凑矮星（UCD）的发现。这是已知的最接近的大规模UCD，也是第一个光谱验证的位于低密度环境中的大规模UCD。该物体，我们命名为SUCD 1，在HST/ACS成像中被识别，并通过从Keck光谱获得的后退速度证实与草帽星系有关。光分布由Wilson模型很好地拟合。我们测得半光大小为14.7 ± 1.4秒差距，绝对星等为M_V = -12.3等（M_K = -15.1等），内部速度色散为25.0 ± 5.6公里/秒。这些值是UCD的典型值。根据Lick光谱指数，我们测得亮度加权中心年龄为12.6 +/- 0.9 Gyrs，[Fe/H]为-0.08 +/- 0.08 dex，[alpha/Fe]为0.06 +/- 0.07 dex。缺乏颜色梯度表明这些值代表整个UCD。导出的恒星质量和维里质量在误差范围内是相同的，约为3.3 x 10 E7 Msun。因此，我们没有发现暗物质存在的有力证据，也没有发现需要调用非标准IMF的证据。我们还报告了可以说是第一个真正的UCD的X射线探测，我们归因于低质量X射线双星（LMXB）的存在。X射线光度L_X = 0.56 × 10 E ~（38）erg/s，与同金属丰度GC的观测值一致。总的来说，我们发现SUCD 1具有与其他已知的UCD和大规模GC相似的特性。
英文摘要：
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.
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