On the redshift evolution of the spin parameter in cosmological simulations

TL;DR

This study investigates how the spin parameter of dark matter halos evolves with redshift using high-resolution cosmological simulations, providing analytical models for its mean and dispersion over 0 < z < 5.

Contribution

It offers the first detailed characterization and analytical fitting functions for the redshift evolution of halo spin parameters, improving modeling accuracy.

Findings

ln(lambda) evolves linearly with redshift

ln(lambda') shows a non-monotonic trend with a turnover at z ~ 1-2

Provides closed-form functions for spin evolution modeling

Abstract

Although the spin parameter of dark matter halos is well known to follow a log-normal distribution at fixed epoch, its quantitative redshift evolution - encompassing both the mean and the dispersion - remains only partially explored. Prior studies either lack the mass resolution required to establish reliable evolutionary trends or do not provide analytical relations that enable forward modelling. Using a suite of LCDM N-body simulations with controlled resolution across the redshift range 0 < z < 5, we characterise the evolution of the mean and dispersion of the Peebles (lambda) and Bullock (lambda') definitions of spin. We find a mild but statistically robust linear evolution for ln(lambda) and a non-monotonic trend with a turnover at z ~ 1 - 2 for ln lambda', which we verify are unaffected by mass resolution of choice of halo definition. We provide closed-form fitting functions for these trends that allow modellers to draw physically motivated spin values at any redshift within our range of validity. This is a practical, redshift-dependent alternative to the common assumption of a constant spin distribution, and provides a useful input to semi-empirical and semi-analytic models of galaxy formation.