ON HELIUM MIXING IN QUASI-GLOBAL SIMULATIONS OF THE INTRACLUSTER MEDIUM

ABSTRACT The assumption of a spatially uniform helium distribution in the intracluster medium (ICM) can lead to biases in the estimates of key cluster parameters if composition gradients are present. The helium concentration profile in galaxy clusters is unfortunately not directly observable. Curren...

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Bibliographic Details
Published in:The Astrophysical journal 2016-12, Vol.833 (2), p.164
Main Authors: Berlok, Thomas, Pessah, Martin E.
Format: Article
Language:English
Subjects:
Astronomical models
Astrophysics
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Computer simulation
Concentration gradient
CONCENTRATION RATIO
DIFFUSION
Galactic clusters
GALAXIES
galaxies: clusters: intracluster medium
GALAXY CLUSTERS
HEAT FLUX
HELIUM
INCLINATION
Inclination angle
instabilities
INSTABILITY
INTERGALACTIC SPACE
MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
magnetohydrodynamics (MHD)
One dimensional models
ONE-DIMENSIONAL CALCULATIONS
Parameter estimation
PLASMA
Plasma dynamics
Plasmas (physics)
SEDIMENTATION
Sedimentation & deposition
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Summary:ABSTRACT The assumption of a spatially uniform helium distribution in the intracluster medium (ICM) can lead to biases in the estimates of key cluster parameters if composition gradients are present. The helium concentration profile in galaxy clusters is unfortunately not directly observable. Current models addressing the putative sedimentation are one-dimensional and parametrize the presence of magnetic fields in a crude way, ignoring the weakly collisional, magnetized nature of the medium. When these effects are considered, a wide variety of instabilities can play an important role in the plasma dynamics. In a series of recent papers, we have developed the local, linear theory of these instabilities and addressed their nonlinear development with a modified version of Athena. Here, we extend our study by developing a quasi-global approach that we use to simulate the mixing of helium as induced by generalizations of the heat-flux-driven buoyancy instability (HBI) and the magnetothermal instability, which feed off thermal and composition gradients. In the inner region of the ICM, mixing can occur over a few gigayears, after which the average magnetic field inclination angle is ∼30°-50°, resulting in an averaged Spitzer parameter higher by about 20% than the value obtained in homogeneous simulations. In the cluster outskirts the instabilities are rather inefficient, due to the shallow gradients. This suggests that composition gradients in cluster cores might be shallower than one-dimensional models predict. More quantitative statements demand more refined models that can incorporate the physics driving the sedimentation process and simultaneously account for the weakly collisional nature of the plasma.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/833/2/164