Weakening dark-matter cusps by clumpy baryonic infall

TL;DR

This paper demonstrates that the infall of massive baryonic clumps into dark-matter halos can significantly reduce central densities, transforming cusps into cores, and potentially explains observed discrepancies in galaxy density profiles.

Contribution

It introduces a simple model showing how clumpy baryonic infall can efficiently alter dark-matter halo structures, highlighting a possible baryonic mechanism for core formation.

Findings

A 1% mass clump can remove twice its mass from the halo center.

Removing the clump mimics galactic winds, further reducing density.

Lighter clumps are more efficient at removing mass relative to their size.

Abstract

We consider the infall of a massive clump into a dark-matter halo as a simple and extreme model for the effect of baryonic physics (neglected in gravity-only simulations of large-scale structure formation) on the dark-matter. We find that such an infalling clump is extremely efficient in altering the structure of the halo and reducing its central density: a clump of 1% the mass of the halo can remove about twice its own mass from the inner halo and transform a cusp into a core or weaker cusp. If the clump is subsequently removed, mimicking a galactic wind, the central halo density is further reduced and the mass removed from the inner halo doubled. Lighter clumps are even more efficient: the ratio of removed mass to clump mass increases slightly towards smaller clump masses. This process is the more efficient the more radially anisotropic the initial dark-matter velocities. While such a clumpy infall may be somewhat unrealistic, it demonstrates that the baryons need to transfer only a small fraction of their initial energy to the dark matter via dynamical friction to explain the discrepancy between predicted dark-matter density profiles and those inferred from observations of dark-matter dominated galaxies.