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Flat-cored Dark Matter in Cuspy Clusters of Galaxies

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Amr A. El-Zant1, Yehuda Hoffman2, Joel Primack3, Francoise Combes4 and Isaac Shlosman5

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Sand and coworkers have measured the central density profile of cluster MS 2137-23 with gravitational lensing and velocity dispersion and removed the stellar contribution with a reasonable M/L. The resulting dark matter (DM) distribution within r < 50 h-1 kpc was fitted by a density cusp of r with β = 0.35, in an apparent contradiction to the cold dark matter prediction of β ~ 1. The disagreement worsens if adiabatic compression of the DM by the infalling baryons is considered. Following El-Zant, Shlosman, & Hoffman, we argue that dynamical friction acting on galaxies moving within the DM background counters the effect of adiabatic compression by transfering their orbital energy to the DM, thus heating up and softening the cusp. Using N-body simulations we show that indeed the inner DM distribution flattens (with β ≈ 0.35 for a cluster like MS 2137-23) when the galaxies spiral inward. We find as a robust result that while the DM distribution becomes core-like, the overall mass distribution preserves its cuspy nature, in agreement with X-ray and lensing observations of clusters.


Subject headings

dark matter; galaxies: clusters: general; galaxies: formation; galaxies: halos


Dates

Issue 2 (2004 June 1)

Received 2003 September 15, accepted for publication 2004 April 16

Published 2004 April 30



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