Tidal disruption of substructure in galaxy clusters
Date
2000
Authors
Hayashi, Eric Jeffrey
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Abstract
Overmerging in the context of cosmological N-body simulations is defined as the absence of substructure in the dark matter halos of galaxy clusters due to numerical or physical disruption mechanisms. In early simulations of galaxy clusters the rapid disruption of bound clumps of N-body particles produced a smooth, featureless cluster mass distribution that did not resemble a real galaxy cluster, in which luminous matter is concentrated in discrete galaxies. We investigate recent claims (Ghigna et al., 1998; Klypin, Gottlober & Kravtsov, 1999) that the overmerging problem has been resolved in the latest generation of cosmological simulations simply as a result of increased numerical resolution. To this end we perform a series of simulations of the evolution of satellite halos in bound orbits within a static cluster potential in order to investigate the effects of varying mass and force resolution on the mass loss experienced by substructure due to tidal stripping. We find that satellites on radial orbits with apocentre-to-pericentre ratios greater than about 3:1 lose approximately the same fraction of their remaining mass each time they pass through pericentre. Conversely, tidal stripping of satellites on circular orbits is characterized by a sharp decline in mass during the first orbit followed by continuous mass loss at a reduced rate in subsequent orbits.
We use our results to test semi-analytic models for predicting mass loss and to establish a correlation between the disruption timescale of a satellite and the fraction of mass it loses during its first orbit. These tools are used to construct a toy model for the dynamical evolution of a galaxy cluster. We find that tidal disruption of satellites is sufficient to erase most substructure within the central regions of the cluster after a Hubble time. The number density of surviving substructure halos predicted by our model is compared with the results of cosmological simulations and observations from the Canadian Network for Observational Cosmology (CNOC) cluster survey.
The density profile of surviving satellite halos is similar to that of the Virgo cluster simulation of Ghigna et al. (1998), and is not consistent with the observed distribution of galaxies in the CNOC ensemble cluster.
We conclude that overmerging due to physical disruption mechanisms remains a problem in the central regions of cluster simulations. This suggests that a dissipational hydrodynamic component is needed to properly model the dynamics of galaxy cluster, as was originally proposed by White & Rees (1978).