Non-thermal pressure support in X-COP galaxy clusters

Abstract

Galaxy clusters are the endpoints of structure formation and are continuously growing through the merging and accretion of smaller structures. Numerical simulations predict that a fraction of their energy content is not yet thermalized, mainly in the form of kinetic motions (turbulence, bulk motions). Measuring the level of non-thermal pressure support is necessary to understand the processes leading to the virialization of the gas within the potential well of the main halo and to calibrate the biases in hydrostatic mass estimates. We present high-quality measurements of hydrostatic masses and intracluster gas fraction out to the virial radius for a sample of 13 nearby clusters with available XMM-Newton and Planck data. We compare our hydrostatic gas fractions with the expected universal gas fraction to constrain the level of non-thermal pressure support. We find that hydrostatic masses require little correction and infer a median non-thermal pressure fraction of ∼ 6% and ∼ 10% at R 500 and R 200 , respectively. Our values are lower than the expectations of hydrodynamical simulations, possibly implying a faster thermalization of the gas. If instead we use the mass calibration adopted by the Planck team, we find that the gas fraction of massive local systems implies a mass bias 1 − b = 0.85 ± 0.05 for SZ-derived masses, with some evidence for a mass-dependent bias. Conversely, the high bias required to match Planck CMB and cluster count cosmology is excluded by the data at high significance, unless the most massive halos are missing a substantial fraction of their baryons.

A tight correlation between the enclosed gravitational mass and hot gas mass in galaxy clusters at intermediate radii

“…However, most of the studies are focusing at large radii only. The recent studies about the details of galaxy clusters at r 2500 did not explicitly discuss the potential correlation [24,25]. Also, the rough relations r 2500 ∼ 0.4r 500 and r 500 ∼ 5r c give r 2500 ∼ 2r c , which are focusing at relatively larger intermediate radii only.…”

“…Although some recent studies have examined the details of galaxy clusters at intermediate radii (e.g. at r 2500 ) [24,25], the explicit correlations between dark matter and baryonic matter at intermediate radii have not been widely discussed to understand their potential coupling. Be-sides, some scaling relations relating gravitational mass and properties of hot gas (e.g.…”

Helium abundance (and H0) in X‐COP galaxy clusters

“…These selection criteria guarantee that a joint analysis of the X‐ray and SZ signals allows the reconstruction of the ICM properties out to R 200 (≈1.5 R 500 ) for all our targets. A complete description of the reduction and analysis of our proprietary X‐ray data and of the Planck SZ data is provided in Ghirardini et al () (see also Eckert et al ; Ettori et al ). Here we want only to remark that a proper treatment of the X‐ray surface brightness profiles, accounting for the median of the distribution of the counts per pixel in a given radial annulus instead of the mean (Eckert et al ; Zhuravleva et al ), guarantees in the X‐COP analysis against a relevant contribution from clumped gas that might systematically bias the estimate of the gas density (Equation (3)).…”

“…We present also the correction propagated to the gas mass fraction f gas = M gas / M hyd through the scaling presented in Equation (2). We can write this correction in a way similar to the form adopted to represent how the fraction α P of the non‐thermal pressure with respect to the total one (and equivalent to the hydrostatic bias b , when α P does not vary with the radius) propagates into the estimate of f gas (see, e.g., Equation 8 in Eckert et al ): where f gas,obs is the observed gas fraction, f gas,true is the expected “true” gas fraction, and f gas,cor is the gas fraction after the correction by its dependence on the quantity x and the Hubble constant h (from M gas and M hyd in Equation (2)). In Figure , we show the constraints we obtain on α c .…”

“…See also the appendix in Ghirardini et al (2019) to R 200 (≈1.5R 500 ) for all our targets. A complete description of the reduction and analysis of our proprietary X-ray data and of the Planck SZ data is provided in Ghirardini et al (2019) (see also Eckert et al 2019;Ettori et al 2019).…”