The virialized mass of dark matter haloes

TL;DR

This paper introduces the static mass, defined by the zero mean radial velocity region in dark matter haloes, as a more accurate measure than the traditional virial mass, revealing significant underestimations and self-similar behaviors across redshifts.

Contribution

It proposes the static mass as a better estimator for dark matter halo mass and analyzes its relation to virial mass using cosmological simulations, highlighting differences and self-similar scaling.

Findings

Static mass is significantly larger than virial mass for galaxy-sized haloes.

The static mass function aligns with Press & Schechter at z=0 but diverges at higher redshifts.

The static radius can be 2-3 times larger than the overdensity-based estimate.

Abstract

(Abridged) Virial mass is used as an estimator for the mass of a dark matter halo. However, the commonly used constant overdensity criterion does not reflect the dynamical structure of haloes. Here we analyze dark matter cosmological simulations in order to obtain properties of haloes of different masses focusing on the size of the region with zero mean radial velocity. Dark matter inside this region is stationary, and thus the mass of this region is a much better approximation for the virial mass. We call this mass the static mass to distinguish from the commonly used constant overdensity mass. We also study the relation of this static mass with the traditional virial mass, and we find that the matter inside galaxy-size haloes is underestimated by the virial mass by nearly a factor of two. At redshift zero the virial mass is close to the static mass for cluster-size haloes. The same pattern - large haloes having M_vir > M_static - exists at all redshifts, but the transition mass M_0 = M_vir = M_static decreases dramatically with increasing redshift. When rescaled to the same M_0 haloes clearly demonstrate a self-similar behaviour, which in a statistical sense gives a relation between the static and virial mass. To our surprise we find that the abundance of haloes with a given static mass, i.e. the static mass function, is very accurately fitted by the Press & Schechter approximation at z=0, but this approximation breaks at higher redshifts. Instead, the virial mass function is well fitted as usual by the Sheth & Tormen approximation. We find an explanation why the static radius can be 2-3 times larger as compared with the constant overdensity estimate. Applying the non-stationary Jeans equation we find that the role of the pressure gradients is significantly larger for small haloes.