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Title: The mass distribution in galaxy clusters from their internal dynamics
Authors: Munari, Emiliano
Keywords: CosmologyGalaxyClustersDynamics
Issue Date: 20-Mar-2014
Publisher: Università degli studi di Trieste
Abstract: We analyse the relation between the masses of cluster- and group-sized halos, extracted from $\Lambda$CDM cosmological N-body and hydrodynamic simulations, and their velocity dispersions, at different redshifts from $z=2$ to $z=0$. The main aim of this analysis is to understand how the implementation of baryonic physics in simulations affects such relation, i.e. to what extent the use of the velocity dispersion as a proxy for cluster mass determination is hampered by the imperfect knowledge of the baryonic physics. In our analysis we use several sets of simulations with different physics implemented: one dark matter (DM hereafter) -- only simulation, one simulation with non-radiative gas, and two radiative simulations, one of which with feedback from Active Galactic Nuclei. Velocity dispersions are determined using three different tracers, DM particles, subhalos, and galaxies. We confirm that DM particles trace a relation that is fully consistent with the theoretical expectations based on the virial theorem, stating that the velocity dispersion is proportional to $M^\alpha$ with $\alpha = 1/3$, $M$ being the mass of the cluster, and with previous results presented in the literature. On the other hand, subhalos and galaxies trace steeper relations, with velocity dispersion scaling with mass with $\alpha>1/3$, and with larger values of the normalization. Such relations imply that galaxies and subhalos have a $\sim10$ per cent velocity bias relative to the DM particles, which can be either positive or negative, depending on halo mass, redshift and physics implemented in the simulation. We explain these differences as due to dynamical processes, namely dynamical friction and tidal disruption, acting on substructures and galaxies, but not on DM particles. These processes appear to be more or less effective, depending on the halo masses and the importance of baryon cooling, and may create a non-trivial dependence of the velocity bias and the velocity dispersion--cluster mass relation on the tracer, the halo mass and its redshift. The method, based on the scaling relations, to infer the mass distribution is an excellent way to deal with a large quantity of data even if the quality is not excellent. On the other hand, when high quality data is available, more sophisticated methods can be applied, that can provide more information. This is the case of the galaxy cluster Abell 2142. High quality photometric and spectroscopic information are available for this cluster, and we compute the mass and velocity anisotropy profiles of it. Once we have this information, it is possible to investigate the pseudo phase space density profile $Q(r)$ and the density slope - velocity anisotropy $\beta - \gamma$ relation, and compare them with theoretical expectations. The mass profiles have been obtained by using three techniques based on member galaxy kinematics, namely the caustic method, the method of Dispersion - Kurtosis and MAMPOSSt. Through the inversion of the Jeans equation it has been possible to compute the velocity anisotropy profiles. The mass profiles, as well as the virial values of mass and radius, computed with the different techniques are in agreement with one another and with the estimates coming from X-ray and weak lensing studies. A concordance mass profile is obtained by averaging the lensing, X-ray and kinematics determinations. The population of red and blue galaxies appear to have a different velocity anisotropy configuration, red galaxies being almost isotropic while blue galaxies are radially anisotropic, with a weak dependence on radius. The $Q(r)$ profile for the red galaxy population agrees with the theoretical results found in cosmological simulations. The $\beta - \gamma$ relation matches the theoretical relation only in the inner region when considering the red galaxies. The deviations might be due to the theoretical relations not taking into account the presence of baryons and using DM particles as tracers.
Description: 2012/2013
NBN: urn:nbn:it:units-12251
Appears in Collections:Scienze fisiche

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