Relaxation of dark matter halos: how to match observational data?

Abstract

We show that moderate energy relaxation in the formation of dark matter halos invariably leads to profiles that match those observed in the central regions of galaxies. The density profile of the central region is universal and insensitive to either the seed perturbation shape or the details of the relaxation process. The profile has a central core; the multiplication of the central density by the core radius is almost independent of the halo mass, in accordance with observations. In the core area the density distribution behaves as an Einasto profile with low index (n ∼ 0.5); it has an extensive region with ρ ∝ r −2 at larger distances. This is exactly the shape that observations suggest for the central region of galaxies. On the other hand, this shape does not fit the galaxy cluster profiles. A possible explanation of this fact is that the relaxation is violent in the case of galaxy clusters; however, it is not violent enough when galaxies or smaller dark matter structures are considered. We discuss the reasons for this.

Can the dark matter annihilation signal be significantly boosted by substructures?

“…One may expect that k depends on c vir of the halo. The most probable value of k is c vir /4 [10], which is close to the maximum value of k, at which the relaxation may still be named moderate. Therefore we should consider the case k = c vir /4.…”

“…As we can see, N d depends only on the average density of the clump. Sinceρ depends on the clump mass only logarithmically in the standard ΛCDM model [1,10], the clump destruction by the disc depends almost not at all on the clump mass.…”

“…The relaxation is moderate if k ≤ c vir /4 [10]. This condition is very strict for galaxy clusters, but quite soft for low-mass galaxies with c vir > 20 (though the real density profile may significantly differ from the NFW one, we used here the NFW halo concentration c vir because of its popularity and in view of the fact that characteristic values of c vir for various types of astronomical objects are well known).…”

“…First of all, it is easy to show that a cusp may form only in scenario 1, i.e., if the relaxation is strong [8,10]. If the relaxation is moderate, the density profile obligatory has a core.…”

Cusps in the center of galaxies: a real conflict with observations or a numerical artefact of cosmological simulations?

“…For instance, the overestimation of the energy exchange between the test bodies that may occur in the N-body simulations leads to the cusp formation [12]. If the energy evolution during the halo formation is limited, then the density profile of the formed halo resembles more closely the observed one [13].…”