NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-02-20 T22:31:53 PST
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For refcode 2016ApJ...819..164H:
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2016ApJ...819..164H Chandra Detection of X-Ray Emission from Ultracompact Dwarf Galaxies and Extended Star Clusters Hou, Meicun; Li, Zhiyuan Abstract. We have conducted a systematic study of X-ray emission from ultracompact dwarf (UCD) galaxies and extended star clusters (ESCs), based on archival 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 ~5 x 10^36^ 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% +/- 0.8%, while lower than that of the X-ray-detected GCs (7.0% +/- 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 ~4 x 10^35^ 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. Key words: galaxies: dwarf, galaxies: star clusters: general, X-rays: galaxies
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