Understanding X-ray and optical selection of galaxy clusters: A
  comparison of the XXL and CAMIRA cluster catalogues obtained in the common
  XXL-HSC SSP area
  Large samples of galaxy clusters provide knowledge of both astrophysics in
the most massive virialised environments and the properties of the cosmological
model that defines our Universe. However, an important issue that affects the
interpretation of galaxy cluster samples is the role played by the selection
waveband and the potential for this to introduce a bias in the physical
properties of clusters thus selected. We aim to investigate waveband-dependent
selection effects in the identification of galaxy clusters by comparing the
X-ray Multi-Mirror (XMM) Ultimate Extra-galactic Survey (XXL) and Subaru Hyper
Suprime-Cam (HSC) CAMIRA cluster samples identified from a common 22.6 deg2 sky
area. We compare 150 XXL and 270 CAMIRA clusters in a common parameter space
defined by X-ray aperture brightness and optical richness. We find that 71/150
XXL clusters are matched to the location of a CAMIRA cluster, the majority of
which (67/71) display richness values N>15 that exceed the CAMIRA catalogue
richness threshold. We find that 67/270 CAMIRA clusters are matched to the
location of an XXL cluster (defined within XXL as an extended X-ray source). Of
the unmatched CAMIRA clusters, the majority display low X-ray fluxes consistent
with the lack of an XXL counterpart. However, a significant fraction (64/107)
CAMIRA clusters that display high X-ray fluxes are not asociated with an
extended source in the XXL catalogue. We demonstrate that this disparity arises
from a variety of effects including the morphological criteria employed to
identify X-ray clusters and the properties of the XMM PSF.
