超致密矮星系团和扩展星团X射线辐射的Chandra探测
Chandra Detection of X-ray Emission from Ultra-compact Dwarf Galaxies and Extended Star Clusters
摘要：
基于{ \sl Chandra}的观测资料，我们对超致密矮星系（UCD）和扩展星团（ESC）的X射线辐射进行了系统的研究。在文献中收集的511个UCD和ESC样本中，发现了17个亮度高于$\sim$ $5\times10^{36} {\rm~erg~s^{-1}}$ 0.5-8 keV的X射线对应体，它们分布在8个早型宿主星系中。为了便于比较，我们还确定了X射线对应的360球状星团（GC）分布在八个星系中的四个。发现UCD和ESC的X射线性质与GC的性质大致相似。X射线探测到的UCD和ESC的发生率$(3.3\pm0.8)$虽然低于X射线探测到的GC的发生率[（$7.0\pm0.4)$ \%]，但远高于从外部星系的场群中所预期的发生率。单独未检测到的UCD/ESC的叠加分析进一步揭示了显著的X射线信号，其对应于每个源的$\sim$ $4\times10^{35} {\rm~erg~s^{-1}}$的等效0.5-8 keV光度。总之，这些提供了强有力的证据表明，来自UCD和ESC的X射线发射主要是由恒星动力学相互作用形成的低质量X射线双星，这与这些致密系统中的恒星星族主要是老年人相一致。对于质量最大的UCD，仍然有可能是一个假定的中心大质量黑洞引起了观测到的X射线发射。
英文摘要：
We have conducted a systematic study of X-ray emission from ultra-compact dwarf (UCD) galaxies and extended star clusters (ESCs), based on archival {\sl Chandra} observations. Among a sample of 511 UCDs and ESCs complied from the literature, 17 X-ray counterparts with 0.5-8 keV luminosities above $\sim$$5\times10^{36} {\rm~erg~s^{-1}}$ are identified, which are distributed in eight early-type host galaxies. To facilitate comparison, we also identify X-ray counterparts of 360 globular clusters (GCs) distributed in four of the eight galaxies. The X-ray properties of the UCDs and ESCs are found to be broadly similar to those of the GCs. The incidence rate of X-ray-detected UCDs and ESCs, $(3.3\pm0.8)$\%, while lower than that of the X-ray-detected GCs [($7.0\pm0.4)$\%], is substantially higher than expected from the field populations of external galaxies. A stacking analysis of the individually undetected UCDs/ESCs further reveals significant X-ray signals, which corresponds to an equivalent 0.5-8 keV luminosity of $\sim$$4\times10^{35} {\rm~erg~s^{-1}}$ per source. Taken together, these provide strong evidence that the X-ray emission from UCDs and ESCs is dominated by low-mass X-ray binaries having formed from stellar dynamical interactions, consistent with the stellar populations in these dense systems being predominantly old. For the most massive UCDs, there remains the possibility that a putative central massive black hole gives rise to the observed X-ray emission.
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