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

¹ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
e-mail: bonamem@email.uah.edu
² Department of Physics, University of Alabama, Huntsville, AL 35899, USA
³ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁴ Department of Astronomy, University of Arizona, Tucson, AZ 85721, USA
⁵ Department of Physics, Columbia University, New York, NY 10027, USA
⁶ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA

Received: 24 December 2010
Accepted: 21 June 2011

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 − M₅₀₀ and Y_X − M₂₅₀₀ 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 < r₅₀₀) (1.3  ±  1.2%) and a 10.2  ±  5.5% mean decrease in the total mass inferred within r < r₅₀₀. 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   r₅₀₀.

Conclusions. We find that the slope of the Y_X − M₅₀₀ scaling relation is not significantly affected by helium sedimentation.