Why are some galaxy clusters underluminous? The very low concentration of the CL2015 mass profile

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.