The red sequence of Abell X-ray underluminous clusters
  We present an analysis of the colour-magnitude relation for a sample of 56
X-ray underluminous Abell clusters, aiming to unveil properties that may
elucidate the evolutionary stages of the galaxy populations that compose such
systems. To do so, we compared the parameters of their colour-magnitude
relations with the ones found for another sample of 50 "normal" X-ray emitting
Abell clusters, both selected in an objective way. The $g$ and $r$ magnitudes
from the SDSS-DR7 were used for constructing the colour-magnitude relations. We
found that both samples show the same trend: the red sequence slopes change
with redshift, but the slopes for X-ray underluminous clusters are always
flatter than those for the normal clusters, by a difference of about 69% along
the surveyed redshift range of 0.05 $\le z <$ 0.20. Also, the intrinsic scatter
of the colour-magnitude relation was found to grow with redshift for both
samples but, for the X-ray underluminous clusters, this is systematically
larger by about 28%. By applying the Cram\'er test to the result of this
comparison between X-ray normal and underluminous cluster samples, we get
probabilities of 92% and 99% that the red sequence slope and intrinsic scatter
distributions, respectively, differ, in the sense that X-ray underluminous
clusters red sequences show flatter slopes and higher scatters in their
relations. No significant differences in the distributions of red-sequence
median colours are found between the two cluster samples. This points to X-ray
underluminous clusters being younger systems than normal clusters, possibly in
the process of accreting groups of galaxies, individual galaxies and gas.
