The stellar and hot gas content of low-mass galaxy clusters
Abstract
We analyse the stellar and hot gas content of 18 nearby, low-mass galaxy clusters, detected in redshift space and selected to have a dynamical mass 3 × 1014 < M/M⊙ < 6 × 1014 (h= 0.7), as measured from the 2dF Galaxy Redshift Survey. We combine X-ray measurements from both Chandra and XMM with ground-based near-infrared observations from CTIO, Anglo-Australian Telescope and Canada-France-Hawaii Telescope to compare the mass in hot gas and stars to the dynamical mass and state of the clusters. Only 13 of the clusters are detected in X-ray emission, and for these systems we find that a range of 7-20 per cent of their baryonic mass, and <3 per cent of their dynamical mass, is detected in starlight, similar to what is observed in more massive clusters. In contrast, the five undetected clusters are underluminous in X-ray emission, by up to a factor of 10, given their stellar mass. Although the velocity distribution of cluster members in these systems is indistinguishable from a Gaussian, all show subtle signs of being unrelaxed: either they lack a central, dominant galaxy, or the bright galaxy distribution is less concentrated and/or more elongated than the rest of the sample. Thus we conclude that low-mass clusters and groups selected from the velocity distribution of their galaxies exhibit a dichotomy in their hot gas properties. Either they are detected in X-ray, in which case they generally lie on the usual scaling relations, or they are completely undetected in X-ray emission. The non-detections may be partly related to the apparently young dynamical state of the clusters, but it remains a distinct possibility that some of these systems are exceptionally devoid of hot emitting gas as the result of its expulsion or rarefaction.
- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- April 2011
- DOI:
- 10.1111/j.1365-2966.2010.17957.x
- arXiv:
- arXiv:1011.0602
- Bibcode:
- 2011MNRAS.412..947B
- Keywords:
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- galaxies: clusters: general;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- MNRAS, in press. Replacement to correct error caught in proofs. Two of the undetected clusters do not have Chandra or XMM data and the limits are based on ROSAT data