Regularity in the X-ray surface brightness profiles of galaxy clusters and the M-T relation

Abstract

We used archival ROSAT observations to investigate the X-ray surface brightness profiles of a sample of 26 clusters in the redshift range (0.04-0.06). For 15 of these clusters accurate temperature (kT>3.5 keV) were available from the literature. The scaled emission measure profiles look remarkably similar above ~0.2 times the virial radius r_V. On the other hand a large scatter is observed in the cluster core properties. We fitted a \betamodel (with and without excising the central part) to all the ROSAT profile to quantify the structural variations in the cluster population, unraveling a robust quadratic correlation between the core radius r_c and the slope parameter beta. We quantified the shape of each gas density profile by the variation with radius of the logarithmic slope, α_n. The bi-weight dispersion of α_n among the clusters is less than 20% for any given scaled radii above x=0.2. There is a clear minimum spread at x=0.3, which is related to the existence of a correlation between r_c and β. These ensemble properties are insensitive to the exact treatment of a possible central excess when fitting the profiles. On the other hand the scatter is decreased when the radii are scaled to r_V. The regularity we found in the gas profiles at x>0.2 supports the existence of an universal underlying dark matter profile. It suggests that non gravitational heating is negligible for clusters with temperature above ~3.5keV. The very large scatter observed in the core properties favor scenario where Cooling Flows are periodically erased by merger events. Our results are consistent with the classical scaling M-T relation. Accordingly the spread in the reduced mass profiles derived from the hydrostatic isothermal betamodel is small.