The Effects of Clumping and Substructure on Intracluster Medium Mass Measurements

We examine an ensemble of 48 simulated clusters to determine the effects ofsmall-scale density fluctuations and large-scale substructure on X-raymeasurements of the intracluster medium (ICM) mass. We measure RMS densityfluctuations in the ICM which can be characterized by a mean mass-weightedclumping factor C = /^2 between 1.3 and 1.4 within a densitycontrast of 500 times the critical density. These fluctuations arise from thecluster history of accretion shocks and major mergers, and their presenceenhances the cluster's luminosity relative to the smooth case. We expect,therefore, that ICM mass measurements utilizing models which assume uniformdensity at a given radius carry a bias of order sqrt(C) = 1.16. We verify thisresult by performing ICM mass measurements on X-ray images of the simulationsand finding the expected level of bias. The varied cluster morphologies in our ensemble also allow us to investigatethe effects of departures from spherical symmetry on our measurements. We findthat the presence of large-scale substructure does not further bias theresulting gas mass unless it is pronounced enough to produce a second peak inthe image of at least 1% the maximum surface brightness. We analyze the subsetof images with no secondary peaks and find a bias of 9% and a Gaussian randomerror of 4% in the derived mass.Comment: To appear in ApJ