Dissecting the spin distribution of Dark Matter halos

TL;DR

This study investigates the origins of deviations in the dark matter halo spin distribution from a lognormal shape, linking these deviations to correlations between spin-related quantities and large-scale tidal influences.

Contribution

It introduces a statistical approach to identify the causes of spin PDF deviations, highlighting the role of correlations between mass-dependent spin ratios and gravitational energy.

Findings

Deviations from lognormal are consistent across redshifts z <= 1.

The J-M relationship follows a power law close to linear theory prediction.

Large-scale tidal fields may influence the evolution of high-spin halos.

Abstract

(Abridged) We apply a very general statistical theorem introduced by Cramer (1936) to study the origin of the deviations of the halo spin PDF from the reference lognormal shape. We find that these deviations originate from correlations between two quantities entering the definition of spin, namely the ratio $J/M^{5/2}$ (which depends only on mass) and the total gravitational binding energy $E$. To reach this conclusion, we have made usage of the results deduced from two high spatial- and mass resolution simulations. Our simulations cover a relatively small volume and produce a sample of more than 16.000 gravitationally bound halos, each traced by at least 300 particles. We verify that our results are stable to different systematics, by comparing our results with those derived by the GIF2 and by a more recent simulation performed by Maccio' et al. We find that the spin probability distribution function shows systematic deviations from a lognormal, at all redshifts z <= 1. These deviations depend on mass and redshift: at small masses they change little with redshift, and also the best lognormal fits are more stable. The J-M relationship is well described by a power law of exponent $\alpha$ very near to the linear theory prediction (alpha=5/3), but systematically lower than this at z<= 0.3. We argue that the fact that deviations from a lognormal PDF are present only for high-spin halos could point to a role of large-scale tidal fields in the evolution of the spin PDF.