Universal at last? The splashback mass function of dark matter halos

TL;DR

This paper investigates the universality of the splashback mass function of dark matter halos across various cosmologies and redshifts, finding it more universal than traditional definitions and providing a new fitting formula.

Contribution

It systematically studies the splashback mass function's universality, demonstrating its advantages over spherical overdensity definitions and offering a new universal fitting formula.

Findings

Splashback mass functions are more universal at z<2.

They show remarkable universality in self-similar cosmologies.

Traditional mass definitions exhibit significant non-universality.

Abstract

The mass function of dark matter halos is one of the most fundamental statistics in structure formation. Many theoretical models (such as Press-Schechter theory) are based on the notion that it could be universal, meaning independent of redshift and cosmology, when expressed in the appropriate variables. However, simulations exhibit persistent non-universalities in the mass functions of the virial mass and other commonly used spherical overdensity definitions. We systematically study the universality of mass functions over a wide range of mass definitions, for the first time including the recently proposed splashback mass, Msp. We confirm that, in LambdaCDM cosmologies, all mass definitions exhibit varying levels of non-universality that increase with peak height and reach between 20% and 500% at the highest masses we can test. Mvir, M200m, and Msp exhibit similar levels of non-universality. There are, however, two regimes where the splashback mass functions are significantly more universal. First, they are universal to 10% at z<2, whereas spherical overdensity definitions experience an evolution due to dark energy. Second, when additionally considering self-similar cosmologies with extreme power spectra, splashback mass functions are remarkably universal (to between 40% and 60%) whereas their spherical overdensity counterparts reach non-universalities between 180% and 450%. These results strongly support the notion that the splashback radius is a physically motivated definition of the halo boundary. We present a simple, universal fitting formula for splashback mass functions that accurately reproduces our simulation data.