X-ray versus infrared selection of distant galaxy clusters: A case study
  using the XMM-LSS and SpARCS cluster samples
  We present a comparison of two samples of z>0.8 galaxy clusters selected
using different wavelength-dependent techniques and examine the physical
differences between them. We consider 18 clusters from the X-ray selected
XMM-LSS distant cluster survey and 92 clusters from the optical-MIR selected
SpARCS cluster survey. Both samples are selected from the same approximately 9
square degree sky area and we examine them using common XMM-Newton,
Spitzer-SWIRE and CFHT Legacy Survey data. Clusters from each sample are
compared employing aperture measures of X-ray and MIR emission. We divide the
SpARCS distant cluster sample into three sub-samples: a) X-ray bright, b) X-ray
faint, MIR bright, and c) X-ray faint, MIR faint clusters. We determine that
X-ray and MIR selected clusters display very similar surface brightness
distributions of galaxy MIR light. In addition, the average location and
amplitude of the galaxy red sequence as measured from stacked colour histograms
is very similar in the X-ray and MIR-selected samples. The sub-sample of X-ray
faint, MIR bright clusters displays a distribution of BCG-barycentre position
offsets which extends to higher values than all other samples. This observation
indicates that such clusters may exist in a more disturbed state compared to
the majority of the distant cluster population sampled by XMM-LSS and SpARCS.
This conclusion is supported by stacked X-ray images for the X-ray faint, MIR
bright cluster sub-sample that display weak, centrally-concentrated X-ray
emission, consistent with a population of growing clusters accreting from an
extended envelope of material.
