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 \times 10 ^ { 14 } < M / M _ { \odot } < 6 \times 10 ^ { 14 } ( 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 , 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 .