Effects of helium sedimentation on late star formation in galaxy clusters

TL;DR

Helium sedimentation in cluster cores can modify the ICM and cooling, potentially triggering late-time star formation in BCGs. The authors solve a 1D Burgers-based diffusion model for a multi-species plasma (H, He, e) in an NFW halo and couple it to a simplified, AGN-free semi-analytic model of cooling and star formation. They find negligible effects for $M_{ m halo} \lesssim 10^{14} M_\odot$, mild effects up to $10^{15} M_\odot$, and potentially strong rejuvenation for $M_{ m halo} \gtrsim 10^{15} M_\odot$, with gravitational heating offsetting some cooling. Transport suppression and realistic temperature profiles can significantly modify the outcome, and the Phoenix cluster may represent a late-star-formation example if sedimentation is efficient.

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.

Figures (6)

• Figure 1: Stellar mass as a function of halo mass at redshift $z=0$, both in solar mass units. The stellar-halo mass relation without helium sedimentation, with helium sedimentation but without heating, and with helium sedimentation and heating are given by the green solid, blue dashed, and red dotted lines, respectively. Here we have taken $\alpha = 0.05$.
• Figure 2: Ratio between stellar mass with helium sedimentation and stellar mass without helium sedimentation as a function of halo mass, at redshift $z=0$, without including heating (blue solid curves) and including heating (red dotted curves). Each panel corresponds to a different value of $\alpha$, $\alpha=0.01, 0.05$, and $0.2$, from bottom to top. The left plot is for $t_{\rm avail}=t$ and the right plot for $t_{\rm avail}=t/3$.
• Figure 3: Ratio between stellar mass with helium sedimentation and stellar mass without helium sedimentation as a function of halo mass, for a suppression factor $f_B=1/3$, at redshift $z=0$, without including heating (blue solid curves) and including heating (red dotted curves). Each panel corresponds to a different value of $\alpha$, $\alpha=0.01, 0.05$, and $0.2$, from bottom to top. The left plot is for $T=T_{\text{vir}}$ and the right plot for $T=1.3 \, T_{\text{vir}}$.
• Figure 4: Stellar mass and cold gas mass (in units of solar mass) as a function of time (normalized to the present time $t_0$), for a galaxy cluster with $M_{\text{vir}}=5 \times 10^{15} M_{\rm sun}$ at the present time. The stellar masses without helium sedimentation, with helium sedimentation but without heating, and with helium sedimentation and heating are given by the thick green solid, blue dashed, and red dotted lines, respectively. In turn, the cold gas masses without helium sedimentation and with helium sedimentation (not including heating) are given by the thin green solid and blue dashed curves, respectively. As a reference, we have indicated with a vertical line the time corresponding to the redshift of the Phoenix cluster ($z \simeq 0.6$). For this plot we have taken $\alpha=0.05$.
• Figure 5: Gas density (in ${\rm cm}^{-3}$) as a function of radius (normalized to the virial radius at the corresponding redshift) for a cluster with $M_{\text{vir}}= 5 \times 10^{15} M_{\rm sun}$ at the present time. The green solid curve gives the density without sedimentation and the blue dashed curve gives the density with sedimentation. The left plot shows the density profiles at redshift $z=0.6$, when the cluster was younger and had a mass $M_{\text{vir}} \simeq 2 \times 10^{15} M_{\rm sun}$ (i.e., for a redshift and mass similar to the ones of the Phoenix cluster), while the right plot shows the density profiles at $z=0$.