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

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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 rᵥ. On the other hand a large scatter is observed in the cluster core properties. We fitted a (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 rc and the slope parameter beta. We quantified the shape of each gas density profile by the variation with radius of the logarithmic slope, αₙ. The bi-weight dispersion of αₙ 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 rc 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 rᵥ. 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.