Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2011-11-24
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
17 pages, 9 figures. Submitted to MNRAS
Scientific paper
I review the relation between virial mass and concentration for groups and clusters of galaxies as measured in a number of recent works. As previously noted by several authors, low-mass clusters and groups of galaxies display systematically larger concentrations than simple prescriptions based on pure $n$-body simulations would predict. This implies an observed concentration-mass relation with a substantially larger slope/normalization than expected from theoretical investigations. Additionally, this conclusion seems to be quite independent on selection effects, holding for both lensing based and X-ray based cluster samples. I develop a simple structure model containing dark matter, stars, and hot diffuse gas in proportions and with distributions in agreement with the most recent observations. Moreover, I include the contraction effect experienced by dark matter due to the cooling of baryons in the very central part of the structure itself. The resulting modified concentration-mass relation is steeper than the theoretical input one, because star formation is fractionally more efficient in low-mass objects. However, the effect is non-vanishing at all masses, thus resulting also in a larger normalization. Overall the new relation provides a better representation of the observed one for almost all catalogs considered in this work, although the specific details depend quite significantly on the baryon fraction prescription adopted. Specifically, the observed concentration-mass relation seems to favor a scenario where the stellar mass fraction in large clusters of galaxies is substantially lower than several works have found. Finally I use this simple structure model to show how the estimated concentration of cosmic structures is expected to be overestimated as a function of the radial range covered by the analysis. (abridged)
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