The virialized mass of dark matter haloes

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 (M ≈ 1012M⊙) is underestimated by the virial mass by nearly a factor of two. At z ≈ 0 the virial mass is close to the static mass for cluster-size haloes (M ≈ 1014M⊙). The same pattern – large haloes having Mvir > Mstatic – exists at all redshifts, but the transition mass M0 = Mvir = Mstatic decreases dramatically with increasing redshift: M0(z) ≈ 3 × 10 15h−1M⊙(1 + z) . When rescaled to the same M0 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 z ≃ 1. Instead, the virial mass function is well fitted as usual by the Sheth & Tormen approximation even at z . 2. We find an explanation why the static radius can be 2–3 times larger as compared with the constant overdensity estimate. The traditional estimate is based on the top-hat model, which assumes a constant density and no rms velocities for the matter before it collapses into a halo. Those assumptions fail for small haloes, which find themselves in environment, where density is falling off well outside the virial radius and random velocities grow due to other haloes. Applying the non-stationary Jeans equation we find that the role of the pressure gradients is significantly larger for small haloes. At some moment it gets too large and stops the accretion.