The XXL Survey XLV. Linking the ages of optically selected groups to
  their X-ray emission
  We investigate the properties of 232 optical spectroscopically selected
groups from the Galaxy And Mass Assembly (GAMA) survey that overlap the XXL
X-ray cluster survey. X-ray aperture flux measurements combined with GAMA group
data provides the largest available sample of optical groups with detailed
galaxy membership information and consistently measured X-ray fluxes and upper
limits. 142 of these groups are divided into three subsets based on the
relative strength of X-ray and optical emission, and we see a trend in galaxy
properties between these subsets: X-ray overluminous groups contain a lower
fraction of both blue and star forming galaxies compared with X-ray
underluminous systems. X-ray overluminous groups also have a more dominant
central galaxy, with a magnitude gap between first and second ranked galaxies
on average 0.22 mag larger than in underluminous groups. The central galaxy in
overluminous groups also lies closer to the 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 trends with X-ray
emission in some of these properties despite the high stochastic noise from the
limited number of group galaxies. We attribute the trends we see 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, a minority of X-ray underluminous groups have
properties that suggest them to be dynamically mature. 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.
