A Possible Dearth of Hot Gas in Galaxy Groups at Intermediate Redshift

Abstract

We examine the X-ray luminosity of galaxy groups in the CNOC2 survey at redshifts 0.1<z<0.6. Previous work examining the gravitational lensing signal of the CNOC2 groups has shown that they are likely to be genuine, gravitationally bound objects. Of the 21 groups in the field of view of the EPIC pn camera on XMM-Newton, not one was visible in over 100 ks of observation, even though three of them have velocity dispersions high enough that they would easily be visible if their luminosities scaled with their velocity dispersions in the same way as nearby groups' luminosities scale. We consider the possibility that this is due to the reported velocity dispersions being erroneously high and conclude that this is unlikely. We therefore find tentative evidence that groups at intermediate redshift are underluminous relative to their local cousins.