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 _ { 500 c } .
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 .