The stellar and hot gas content of low-mass galaxy clusters
  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
3E14<M/Msun<6E14, 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, AAT and CFHT 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 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.
