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

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$$5\times 10^{36}\mathrm{erg}{\mathrm{s}}^{-1}$ are identified, which are distributed in eight early-type host galaxies. To facilitate comparison, we also identify X-ray counterparts of 360 globular clusters (GCs) distributed in four of the eight galaxies. The X-ray properties of the UCDs and ESCs are found to be broadly similar to those of the GCs. The incidence rate of X-ray-detected UCDs and ESCs, $(3.3\pm 0.8)$\%, while lower than that of the X-ray-detected GCs [($7.0\pm 0.4)$\%], is substantially higher than expected from the field populations of external galaxies. A stacking analysis of the individually undetected UCDs/ESCs further reveals significant X-ray signals, which corresponds to an equivalent 0.5-8 keV luminosity of $\sim$$4\times 10^{35}\mathrm{erg}{\mathrm{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.

19 pages, 4 figures. Accepted for publication in APJ. Comments welcome

References in corpus (11)

• A Supermassive Black Hole in an Ultracompact Dwarf Galaxy
• The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database
• Chandra Multiwavelength Project X-ray Point Source Catalog
• HST/ACS Wide-Field Photometry of the Sombrero Galaxy Globular Cluster System
• The Next Generation Virgo Cluster Survey. VI. The Kinematics of Ultra-compact Dwarfs and Globular Clusters in M87
• The AIMSS Project III: The Stellar Populations of Compact Stellar Systems
• An Ultra Compact Dwarf around the Sombrero galaxy (M104): the Nearest Massive UCD
• A search for ultra-compact dwarf galaxies in the Centaurus galaxy cluster
• The Most Massive Ultra-Compact Dwarf Galaxy in the Virgo Cluster
• Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey
• Extended Star Clusters in NGC 1023 from HST/ACS Mosaic Imaging

Cited by in corpus (8)

• The Black Hole in the Most Massive Ultracompact Dwarf Galaxy M59-UCD3
• Spatially Resolved Stellar Spectroscopy of the Ultra-diffuse Galaxy Dragonfly 44. III. Evidence for an Unexpected Star-Formation History
• A search for active galactic nuclei in low-mass compact galaxies
• The Second Nucleus of NGC 7727: Direct Evidence for the Formation and Evolution of an Ultracompact Dwarf Galaxy
• Deep Chandra observations of X-ray point sources in M87
• Chandra Detection of Intra-cluster X-ray sources in Virgo
• Chandra Detection of Intra-cluster X-ray Sources in Fornax
• Prospects for Revealing Intermediate-Mass Black Holes in NGC 1399 using SKA