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

TL;DR

This study systematically analyzes X-ray emission from ultra-compact dwarf galaxies and extended star clusters, revealing that their X-ray properties are similar to globular clusters and likely originate from low-mass X-ray binaries or possibly central black holes.

Contribution

It provides the first comprehensive comparison of X-ray emission in UCDs and ESCs with globular clusters, highlighting the role of stellar interactions and potential black holes.

Findings

17 X-ray counterparts identified among 511 UCDs and ESCs.

X-ray detection rate of 3.3%, lower than GCs but higher than field populations.

Stacking analysis indicates a typical X-ray luminosity of 4×10^{35} erg/s per source.

Abstract

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.