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- On the origin of cores in simulated galaxy clusters
- Mitchell, N. L.; McCarthy, I. G.; Bower, R. G.; Theuns, T.; Crain, R. A.
- The diffuse plasma that fills galaxy groups and clusters (the intracluster medium) is a by-product of galaxy formation. The present thermal state of this gas results from a competition between gas cooling and heating. The heating comes from two distinct sources: gravitational heating associated with the collapse of the dark matter halo and additional thermal input from the formation of galaxies and their black holes. A long-term goal of this research is to decode the observed temperature, density and entropy profiles of clusters and to understand the relative roles of these processes. However, a long-standing problem has been that cosmological simulations based on smoothed particle hydrodynamics (SPH) and Eulerian mesh-based codes predict different results even when cooling and galaxy/black hole heating are switched off. Clusters formed in SPH simulations show near power-law entropy profiles, while those formed in Eulerian simulations develop a core and do not allow gas to reach such low entropies. Since the cooling rate is closely connected to the minimum entropy of the gas distribution, the differences are of potentially key importance. In this paper, we investigate the origin of this discrepancy. By comparing simulations run using the GADGET-2 SPH code and the FLASH adaptive Eulerian mesh code, we show that the discrepancy arises during the idealized merger of two clusters and that the differences are not the result of the lower effective resolution of Eulerian cosmological simulations. The difference is not sensitive to the minimum mesh size (in Eulerian codes) or the number of particles used (in SPH codes). We investigate whether the difference is the result of the different gravity solvers, the Galilean non-invariance of the mesh code or an effect of unsuitable artificial viscosity in the SPH code. Instead, we find that the difference is inherent to the treatment of vortices in the two codes. Particles in the SPH simulations retain a close connection to their initial entropy, while this connection is much weaker in the mesh simulations. The origin of this difference lies in the treatment of eddies and fluid instabilities. These are suppressed in the SPH simulations, while the cluster mergers generate strong vortices in the Eulerian simulations that very efficiently mix the fluid and erase the low-entropy gas. We discuss the potentially profound implications of these results.
- Publication type
- Journal article
- Monthly Notices of the Royal Astronomical Society, Vol. 395, no. 1 (May 2009), pp. 180-196
- Publication year
- FOR Code(s)
- 0201 Astronomical and Space Sciences; 020103 Cosmology and Extragalactic Astronomy
- Cosmology; GADGET-2; Galaxy clusters; Hydrodynamics; N-body simulation methods; Smoothed particle hydrodynamics; SPH
- Publisher URL
- Copyright © 2009 The Authors. Journal compilation copyright © 2009 Royal Astronomical Society. Author version of the paper reproduced here in accordance with the copyright policy of the publisher. The definitive version is available at www.interscience.wiley.com.
- Additional information
- This research was supported by STFC studentships, a NSERC Postdoctoral Fellowship at the ICC in Durham and a Kavli Institute Fellowship at the Kavli Institute for Cosmology, Cambridge. The FLASH software used in this work was in part developed by the DOE-supported ASC/Alliance Center for Astrophysical Thermonuclear Flashes at the University of Chicago.
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