Rhapsody-G simulations: galaxy clusters as baryonic closed boxes and the covariance between hot gas and galaxies

TL;DR

This study uses Rhapsody-G hydrodynamic simulations to show that galaxy clusters approximately behave as baryonic closed boxes, with an anti-correlation between hot gas and galaxy mass fractions, enabling low-scatter mass proxies.

Contribution

It demonstrates that the total baryon content in galaxy clusters is a reliable low-scatter proxy for total mass, revealing a significant anti-correlation between hot gas and galaxy mass fractions.

Findings

Anti-correlation between hot gas and galaxy mass fractions (r = -0.69) within $R_{500c}$.

Total baryon fraction has low scatter (~2%) as a mass proxy, decreasing with radius.

Joint optical and hot gas selection could achieve 5% scatter in cluster mass estimates.

Abstract

Within a sufficiently large cosmic volume, conservation of baryons implies a simple `closed box' view in which the sum of the baryonic components must equal a constant fraction of the total enclosed mass. We present evidence from Rhapsody-G hydrodynamic simulations of massive galaxy clusters that the closed-box expectation may hold to a surprising degree within the interior, non-linear regions of haloes. At a fixed halo mass, we find a significant anti-correlation between hot gas mass fraction and galaxy mass fraction (cold gas + stars), with a rank correlation coefficient of -0.69 within $R_{500c}$. Because of this anti-correlation, the total baryon mass serves as a low-scatter proxy for total cluster mass. The fractional scatter of total baryon fraction scales approximately as $0.02 (\Delta_c/100)^{0.6}$, while the scatter of either gas mass or stellar mass is larger in magnitude and declines more slowly with increasing radius. We discuss potential observational tests using cluster samples selected by optical and hot gas properties; the simulations suggest that joint selection on stellar and hot gas has potential to achieve 5% scatter in total halo mass.