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We have conducted a systematic study of X-ray emission from ultra-compactdwarf (UCD) galaxies and extended star clusters (ESCs), based on archival {\slChandra} observations. Among a sample of 511 UCDs and ESCs complied from theliterature, 17 X-ray counterparts with 0.5-8 keV luminosities above$\sim$$5\times10^{36} {\rm~erg~s^{-1}}$ are identified, which are distributedin eight early-type host galaxies. To facilitate comparison, we also identifyX-ray counterparts of 360 globular clusters (GCs) distributed in four of theeight galaxies. The X-ray properties of the UCDs and ESCs are found to bebroadly similar to those of the GCs. The incidence rate of X-ray-detected UCDsand ESCs, $(3.3\pm0.8)$\%, while lower than that of the X-ray-detected GCs[($7.0\pm0.4)$\%], is substantially higher than expected from the fieldpopulations of external galaxies. A stacking analysis of the individuallyundetected UCDs/ESCs further reveals significant X-ray signals, whichcorresponds to an equivalent 0.5-8 keV luminosity of $\sim$$4\times10^{35}{\rm~erg~s^{-1}}$ per source. Taken together, these provide strong evidencethat the X-ray emission from UCDs and ESCs is dominated by low-mass X-raybinaries having formed from stellar dynamical interactions, consistent with thestellar populations in these dense systems being predominantly old. For themost massive UCDs, there remains the possibility that a putative centralmassive black hole gives rise to the observed X-ray emission.

Chandra Detection of X-ray Emission from Ultra-compact Dwarf Galaxies and Extended Star Clusters