Why are some galaxy clusters underluminous? The very low concentration of the CL2015 mass profile
Journal
Date Issued
2019
Author(s)
Abstract
Our knowledge of the variety of galaxy clusters has been increasing in the
last few years thanks to our progress in understanding the severity of
selection effects on samples. To understand the reason for the observed
variety, we study CL2015, a cluster easily missed in X-ray selected
observational samples. Its core-excised X-ray luminosity is low for its mass
M500, well below the mean relation for an X-ray selected sample, but only ~1.5
sigma below that derived for an X-ray unbiased sample. We derived thermodynamic
profiles and hydrostatic masses with the acquired deep Swift X-ray data, and we
used archival Einstein, Planck, and SDSS data to derive additional
measurements, such as integrated Compton parameter, total mass, and stellar
mass. The pressure and the electron density profiles of CL2015 are
systematically outside the +/- 2 sigma range of the universal profiles; in
particular the electron density profile is even lower than the one derived from
Planck-selected clusters. CL2015 also turns out to be fairly different in the
X-ray luminosity versus integrated pressure scaling compared to an X-ray
selected sample, but it is a normal object in terms of stellar mass fraction.
CL2015's hydrostatic mass profile, by itself or when is considered together
with dynamical masses, shows that the cluster has an unusual low concentration
and an unusual sparsity compared to clusters in X-ray selected samples. The
different behavior of CL2015 is caused by its low concentration. When
concentration differences are accounted for, the properties of CL2015 become
consistent with comparison samples. CL2015 is perhaps the first known cluster
with a remarkably low mass concentration for which high quality X-ray data
exist. Objects similar to CL2015 fail to enter observational X-ray selected
samples because of their low X-ray luminosity relative to their mass.
Volume
630
Start page
A78
Issn Identifier
0004-6361
Rights
open.access
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