Effects of helium sedimentation on late star formation in galaxy clusters

Abstract

We discuss how helium sedimentation in galaxy clusters can affect the history of star formation in the central cluster galaxy. As helium sediments, the gas density in the inner regions of the cluster increases and there is also a non-trivial, radially dependent redistribution of the atomic nuclei and electrons. As a result, the cooling rate in the center increases and this can enhance star formation. On the other hand, there is a slow contraction of the intracluster gas, which may induce gravitational heating and therefore has an opposite effect on star formation. In this work we present these effects and aim to estimate their relevance. For this we have performed a 1-dimensional numerical simulation of helium sedimentation and applied it to a simple semi-analytical model of star formation. We find that for clusters with a halo mass $M_{\rm halo} \lesssim 10^{14} M_{\rm sun}$, helium sedimentation effects on the star formation rate are negligible, even under idealized conditions. In the intermediate range, $10^{14} M_{\rm sun} \lesssim M_{\rm halo} \lesssim 10^{15} M_{\rm sun}$, the effects are at most mild, below a factor ~ 2 in the isothermal model we consider, even for idealized conditions. For clusters with a halo mass $M_{\rm halo} \gtrsim 10^{15} M_{\rm sun}$, helium sedimentation effects can potentially be very important and renew star formation activity in the central galaxy.