Low X-ray surface brightness clusters: implications on the scatter of the M-T and L-T relations

Abstract

We aim at studying scaling relations of a small but well-defined sample of galaxy clusters that includes the recently discovered class of objects that are X-ray faint for their mass. These clusters have an average low X-ray surface brightness, a low gas fraction, and are underrepresented (by a factor of 10) in X-ray surveys or entirely absent in Sunyaev-Zeldovich (SZ) surveys. With the inclusion of these objects, we find that the temperature-mass relation has an unprecedentedly large scatter, 0.20 ± 0.03 dex at fixed mass, as wide as allowed by the temperature range, and the location of a cluster in this plane depends on its surface brightness. Clusters obey a relatively tight luminosity-temperature relation independently of their brightness. We interpret the wide difference in scatter around the two relations as due to the fact that X-ray luminosity and temperature are dominated by photons coming from small radii (in particular for T we used a 300 kpc aperture radius) and strongly affected by gas thermodynamics (e.g. shocks and cool cores), whereas mass is dominated by dark matter at large radii. We measure a slope of 2.0 ± 0.2 for the L₅₀₀-T relation. Given the characteristics of our sample, this value is free from collinearity (degeneracy) between evolution and slope and from hypothesis on the undetected population, which both affect the analysis of X-ray-selected samples, and can therefore be profitably used both as reference and to break the aforementioned degeneracy of X-ray-selected samples.