Cluster Structure in Cosmological Simulations. I. Correlation to Observables, Mass Estimates, and Evolution

Abstract

We use Enzo, a hybrid Eulerian AMR/N-body code including nongravitational heating and cooling, to explore the morphology of the X-ray gas in clusters of galaxies and its evolution in current generation cosmological simulations. We employ and compare two observationally motivated structure measures: power ratios and centroid shift. Overall, the structure of our simulated clusters compares remarkably well to low-redshift observations, although some differences remain that may point to incomplete gas physics. We find no dependence on cluster structure in the mass-observable scaling relations, T X − M and Y X −M, when using the true cluster masses. However, estimates of the total mass based on the assumption of hydrostatic equilibrium, as assumed in observational studies, are systematically low. We show that the hydrostatic mass bias strongly correlates with cluster structure and, more weakly, with cluster mass. When the hydrostatic masses are used, the mass-observable scaling relations and gas mass fractions depend significantly on cluster morphology, and the true relations are not recovered even if the most relaxed clusters are used. We show that cluster structure, via the power ratios, can be used to effectively correct the hydrostatic mass estimates and mass-scaling relations, suggesting that we can calibrate for this systematic effect in cosmological studies. Similar to observational studies, we find that cluster structure, particularly centroid shift, evolves with redshift. This evolution is mild but will lead to additional errors at high redshift. Projection along the line of sight leads to significant uncertainty in the structure of individual clusters: less than 50% of clusters which appear relaxed in projection based on our structure measures are truly relaxed.

Rhapsody-G simulations I: the cool cores, hot gas and stellar content of massive galaxy clusters

“…This result is consistent with earlier analysis in this direction, by e.g. Jeltema et al (2008), who however found a dependence on the dynamical state once the scaling relations are not compared against the true mass but against, e.g., the hydrostatic mass. This is very plausibly the case for our simulations as well, since the hydrostatic mass bias can be large close to R500c (see our analysis in Section 3.3).…”

Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations

“…One -the so-called power ratio method -is based on a multipole analysis of the azimuthal surface brightness distribution (Buote & Tsai 1995Buote 2002;Jeltema et al 2005Jeltema et al , 2008Valdarnini 2006). The other method is based on studying the emission centroid shift as a function of the integration radius (Mohr et al , 1995Poole et al 2006;O'Hara et al 2006;Kay et al 2007;Ventimiglia et al 2008;Yang et al 2008;Maughan et al 2008).…”

“…Mohr et al 1993Mohr et al , 1995Poole et al 2006;O'Hara et al 2006) and the determination of so-called power ratios (e.g. Buote & Tsai 1995Jeltema et al 2005Jeltema et al , 2008Valdarini 2006;Ventimiglia et al 2008).…”

Thermo-dynamic and chemical properties of the intra-cluster medium

“…One should, however, keep in mind that the characterization of the degree of relaxation of clusters through X-ray morphology is limited by projection effects (e.g. Jeltema et al 2008) and that the limited amount of information available at large cluster radii, beyond 0.2 R 180 , provides a further complication. There have been some attempts to compare dynamical and core properties on individual objects (e.g.…”