Abstract
To assess the effect of baryonic ``pinching'' of galaxy cluster dark matter (DM) haloes, cosmological (LCDM) TreeSPH simulations of the formation and evolution of two galaxy clusters have been performed, with and without baryons included. The simulations with baryons invoke star formation, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling, strong star-burst, driven galactic super-winds and the effects of a meta-galactic UV field, including simplified radiative transfer. The two clusters have T_X~3 and 6 keV, respectively, and, at z~0, both host a prominent, central cD galaxy. Comparing the simulations without and with baryons, it is found for the latter that the inner DM density profiles, r<50-100 kpc, steepen considerably: Delta(alpha)~0.5-0.6, where -alpha is the logarithmic DM density gradient. This is mainly due to the central stellar cDs becoming very massive, as a consequence of the onset of late time cooling flows and related star formation. Once these spurious cooling flows have been corrected for, and the cluster gravitational potentials dynamically adjusted, much smaller pinching effects are found: Delta(alpha)~0.1. Including the effects of baryonic pinching, central slopes of alpha~1.0 and 1.2 are found for the DM in the two clusters, interestingly close to recent observational findings. For the simulations with baryons, the inner density profile of DM+ICM gas combined is found to be only very marginally steeper than that of the DM, Delta(alpha)<0.05. However, the total matter inner density profiles are found to be Delta(alpha)~0.5 steeper than the inner profiles in the dark matter only simulations.
Highlights
Large N-body cosmological simulations have been carried out for a decade, with the goal of making statistical predictions on dark matter (DM) halo properties
Comparing the simulations without and with baryons, it is found for the latter that the inner DM density profiles, r 50-100 kpc, steepen considerably: ∆α ∼ 0.50.6, where -α is the logarithmic DM density gradient. This is primarily due to the central stellar cDs becoming very massive, as a consequence of the onset of late time cooling flows and related star formation
To enable a precise determination of the galaxy cluster dark matter density profiles at z=0.28, 21 time frames, with a time spacing of 0.1 Gyr and centered on t=10.2 Gyr, were co-added
Summary
Large N-body cosmological simulations have been carried out for a decade, with the goal of making statistical predictions on dark matter (DM) halo properties. Most of these large cosmological simulations contain dark matter particles only They all reliably predict that the 3D density profile ρDM(r) should fall as r−3 at large radii (but within the virial radius). Observations have confirmed these predictions for cluster sized haloes Kneib et al 2003; Pointecouteau et al 2005; Mandelbaum et al 2008; Okabe et al 2009) This agreement is likely to be connected with the fact that at large radius, the density profile of a galaxy cluster is dark matter dominated and the influence of baryons can be neglected. Observing the central part (i.e. the inner ∼ 500 kpc) of a galaxy
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