XXL Survey. X-ray emission in opt. select. group

X-ray overluminous groups also have a more dominant central galaxy, with a magnitude gap between first and second ranked galaxies on average 0.22mag larger than in underluminous groups. Moreover, the central galaxy in overluminous groups lies closer to the luminosity-weighted centre of the group. We examine a number of other structural properties of our groups, such as axis ratio, velocity dispersion, and group crossing time, and find evidence of trends with X-ray emission in some of these properties despite the high stochastic noise arising from the limited number of group galaxies.

We attribute the trends we see primarily to the evolutionary state of groups, with X-ray overluminous systems being more dynamically evolved than underluminous groups. The X-ray overluminous groups have had more time to develop a luminous intragroup medium, quench member galaxies, and build the mass of the central galaxy through mergers compared to underluminous groups. However, an interesting minority of X-ray underluminous groups have properties that suggest them to be dynamically mature.

We find that the lack of hot gas in these systems cannot be accounted for by high star formation efficiency, suggesting that high gas entropy resulting from feedback is the likely cause of their weak X-ray emission.