The effects of clumping and substructure on intracluster medium mass measurements

We examine an ensemble of 48 simulated clusters to determine the effects of small-scale density fluctuations and large-scale substructure on X-ray mea surements of the intracluster medium (ICM) mass. We measure rms density flu ctuations in the ICM that can be characterized by a mean mass-weighted clum ping factor C = [rho(2])/[rho](2) between 1.3 and 1.4 within a density cont rast of 500. These fluctuations arise from the cluster history of accretion shocks and major mergers, and their presence enhances the cluster's lumino sity relative to the smooth case. We expect, therefore, that ICM mass measu rements utilizing models that assume uniform density at a given radius carr y a bias of order root C approximate to 1.16. We verify this result by perf orming ICM mass measurements on X-ray images of the simulations and finding the expected level of bias. The varied cluster morphologies in our ensembl e also allow us to investigate the effects of departures from spherical sym metry on our measurements. We find that the presence of large-scale substru cture does not further bias the resulting gas mass unless it is pronounced enough to produce a second peak in the image of at least 1% the maximum sur face brightness. We analyze the subset of images with no secondary peaks an d find a bias of 9% and a Gaussian random error of 4% in the derived mass.