Flat-cored Dark Matter in Cuspy Clusters of Galaxies

Abstract

Sand and coworkers have measured the central density profile of cluster MS 2137Ϫ23 with gravitational lensing and velocity dispersion and removed the stellar contribution with a reasonable M/L. The resulting dark matter (DM) distribution within was fitted by a density cusp of with , in an apparent Ϫ1 Ϫb r ! 50 h kpc r b p 0.35 contradiction to the cold dark matter prediction of . The disagreement worsens if adiabatic compression of b ∼ 1 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 for a cluster like MS 2137Ϫ23) when the b ≈ 0.35 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.

Early flattening of dark matter cusps in dwarf spheroidal galaxies

“…El Zant et al (2001 proposed that a similar mechanism (baryonic clumps heating DM halos) could be responsible for turning DM cusps into cores. Subsequent work elaborated on this proposal, by considering specific applications to clusters of galaxies (El-Zant et al 2004;Nipoti et al 2004) or exploring the space of parameters by varying the density and velocity distribution of the host system and the clump mass, size and orbits (Arena & Bertin 2007;Jardel & Sellwood 2009;Goerdt et al 2010;Cole et al 2011). The results of these works clearly indicated, as a proof of concept, that dynamical friction heating by clumps can have an important role in halos on different scales and that, quantitatively, the relevance of this process depends on the properties of the clumps and of the host halo.…”

Probing the Dark Matter and Gas Fraction in Relaxed Galaxy Groups with X‐Ray Observations fromChandraandXMM‐Newton

“…A single NFW model to investigate scenarios like the ones of Loeb & Peebles (2003) and El-Zant et al (2004) and the effect of the omission of the stellar mass, if present, on the derived concentration parameter.…”

Halo Density Reduction by Baryonic Settling?

“…They estimated the magnitude of the effect on a galaxy halo using the Chandrasekhar dynamical friction formula. They followed up (El-Zant et al 2004) with simulations of a galaxy cluster, in which the heavy particles were the individual galaxies, finding some density decrease.…”

“…This work was supported by grants AST-0507323 from the NSF and NNG05GC29G from NASA. El-Zant et al (2004) presented results from simulations using a nonadaptive Cartesian grid covering only the inner part of the dark matter halo of a cluster of galaxies. They reported that the density of dark matter declined significantly at radii < r s /3 of the NFW halo profile after ∼ 10 Gyr.…”