Concentration, ellipsoidal collapse, and the densest dark matter haloes
Abstract
The smallest dark matter haloes are the first objects to form in the hierarchical structure formation of cold dark matter (CDM) cosmology and are expected to be the densest and most fundamental building blocks of CDM structures in our Universe. Nevertheless, the physical characteristics of these haloes have stayed illusive, as they remain well beyond the current resolution of Nbody simulations (at redshift zero). However, they dominate the predictions (and uncertainty) in expected dark matter annihilation signal, amongst other astrophysical observables. Using the conservation of total energy and the ellipsoidal collapse framework, we can analytically find the mean and scatter of concentration c and 1D velocity dispersion σ_{1d} for haloes of different virial mass M_{200}. Both c and σ _1d/M_{200}^{1/3} are in good agreement with numerical results within the regime probed by simulations  slowly decreasing functions of mass that approach constant values at large masses. In particular, the predictions for the 1D velocity dispersion of cluster mass haloes are surprisingly robust as the inverse heat capacity of cosmological haloes crosses zero at M_{200} ∼ 10^{14} M_{⊙}. However, we find that current extrapolations from simulations to smallest CDM haloes dramatically depend on the assumed profile (e.g. NFW versus Einasto) and fitting function, which is why theoretical considerations, such as the one presented here, can significantly constrain the range of feasible predictions.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 March 2016
 DOI:
 10.1093/mnras/stv2905
 arXiv:
 arXiv:1510.03868
 Bibcode:
 2016MNRAS.456.3068O
 Keywords:

 galaxies: haloes;
 cosmology: theory;
 dark matter;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 12 pages, 11 figures, Updated to published version