The effect of helium sedimentation on galaxy cluster masses and scaling relations

Abstract

Context. Recent theoretical studies predict that the inner regions of galaxy clusters may have an enhanced helium abundance due to sedimentation over the cluster lifetime. If sedimentation is not suppressed (e.g., by tangled magnetic fields), this may significantly affect the cluster mass estimates. Aims. We use Chandra X-ray observations of eight relaxed galaxy clusters to investigate the upper limits to the effect of helium sedimentation on the measurement of cluster masses and the best-fit slopes of the Y X − M 500 and Y X − M 2500 scaling relations. Methods. We calculated gas mass and total mass in two limiting cases: a uniform, unenhanced abundance distribution and a radial distribution from numerical simulations of helium sedimentation on a timescale of 11 Gyr. Results. The assumed helium sedimentation model, on average, produces a negligible increase in the gas mass inferred within large radii (r < r 500 ) (1.3 ± 1.2%) and a 10.2 ± 5.5% mean decrease in the total mass inferred within r < r 500 . Significantly stronger effects in the gas mass (10.5 ± 0.8%) and total mass (25.1 ± 1.1%) are seen at small radii owing to a larger variance in helium abundance in the inner region, r ≤ 0.1 r 500 . Conclusions. We find that the slope of the Y X − M 500 scaling relation is not significantly affected by helium sedimentation.

The Stability of Weakly Collisional Plasmas With Thermal and Composition Gradients

“…It could be argued that the timescales involved in viscous and diffusion processes should be of the same order because it is mainly the ion dynamics what determines both of them. A detailed analysis of the diffusion coefficient for a binary mixture of ions (see Appendix A) shows that τ −1 d ≃ 9τ −1 v for primordial composition (c ≃ 0.25 or µ ≃ 0.6) and decreases toward τ −1 d ≃ 3τ −1 v for the compositions expected at the inner core of galaxy clusters according to recent models for helium sedimentation (Bulbul et al 2011). Since we will be mostly concerned with the two regimes τ −1 c ≫ ω dyn or ω dyn ≫ τ −1 c , as long as the ratio τ −1 d /τ −1 v is not too small, its particular value will not affect our main conclusions, and we will thus consider that τ −1 d ≃ τ −1 v .…”

“…While the landscape of thermal instabilities that render homogeneous, dilute plasmas unstable has been well explored (Kunz 2011), and even extended to account for the effects of cosmic rays (Chandran & Dennis 2006;Sharma et al 2010), very little is known about the effects that composition gradients can have on the stability of the dilute ICM. If magnetic fields do not prevent the efficient diffusion of ions (Narayan & Medvedev 2001;Chuzhoy & Nusser 2003;Chuzhoy & Loeb 2004) then the gradients in mean molecular weight can be as important as the gradients in tempera-ture (see Section 7 and Qin & Wu 2000;Peng & Nagai 2009;Shtykovskiy & Gilfanov 2010;Bulbul et al 2011) and provide another source of free energy to feed instabilities. In order to obtain a more complete picture of the stability properties of the ICM, it is thus important to relax the assumption of a homogeneous medium.…”

Outskirts of Galaxy Clusters

“…More details can be found, e.g., in Ettori & Fabian (2006);Markevitch (2007); Peng & Nagai (2009);Bulbul et al (2011), and references therein.…”

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

“…If helium sedimentation occurs, however, the expected impact is very limited: an enhancement by 10% of the helium abundance has been shown to affect only the metal abundances by 0.02–0.03 over the Hitomi SXS band (Hitomi Collaboration et al ). On the integrated quantities, like gas and total mass, an effect is potentially measurable only in very inner regions and is negligible when large cluster's volume is considered (∼ R 500 ; see, e.g., Bulbul et al ).…”