The pseudo-evolution of halo mass

TL;DR

This paper investigates how the apparent growth of dark matter halos over time can be largely attributed to pseudo-evolution caused by changing reference densities, rather than physical changes in halo structure.

Contribution

It quantifies pseudo-evolution in halo mass from simulations and demonstrates its significant impact on interpreting halo growth and related scaling relations.

Findings

Pseudo-evolution accounts for most mass growth in halos around 10^12 solar masses.

A simple model assuming static physical density profiles accurately predicts pseudo-evolution.

Pseudo-evolution explains the non-evolution of the low-mass end of the halo mass function.

Abstract

A dark matter halo is commonly defined as a spherical overdensity of matter with respect to a reference density, such as the critical density or the mean matter density of the Universe. Such definitions can lead to a spurious pseudo-evolution of halo mass simply due to redshift evolution of the reference density, even if its physical density profile remains constant over time. We estimate the amount of such pseudo-evolution of mass between z=1 to 0 for halos identified in a large N-body simulation, and show that it accounts for almost the entire mass evolution of the majority of halos with M200 of about 1E12 solar masses and can be a significant fraction of the apparent mass growth even for cluster-sized halos. We estimate the magnitude of the pseudo-evolution assuming that halo density profiles remain static in physical coordinates, and show that this simple model predicts the pseudo-evolution of halos identified in numerical simulations to good accuracy, albeit with significant scatter. We discuss the impact of pseudo-evolution on the evolution of the halo mass function and show that the non-evolution of the low-mass end of the halo mass function is the result of a fortuitous cancellation between pseudo-evolution and the absorption of small halos into larger hosts. We also show that the evolution of the low mass end of the concentration-mass relation observed in simulations is almost entirely due to the pseudo-evolution of mass. Finally, we discuss the implications of our results for the interpretation of the evolution of various scaling relations between the observable properties of galaxies and galaxy clusters and their halo masses.