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The mass distribution in galaxy clusters from their internal dynamics
Munari, Emiliano
2014-03-20
Contributor(s)
Borgani, Stefano
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.
Insegnamento
Publisher
Università degli studi di Trieste
Languages
en
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