The Origin of the Orbital Parameter Distribution of Merging Halos

TL;DR

This paper presents a simple model explaining the distribution of orbital velocities of merging dark matter halos, aligning well with simulations and predicting effects of different cosmological parameters.

Contribution

The model offers a new, simplified approach to understanding orbital parameter distributions of merging halos, connecting it to the power spectrum and its modifications.

Findings

Qualitative and quantitative agreement with N-body simulations.

Predicts increased velocities in lower mass systems.

Shows minimal impact of warm dark matter truncation on distributions.

Abstract

We describe a simple model which explains the qualitative and (approximate) quantitative features of the distribution of orbital velocities of merging pairs of dark matter halos. Our model considers a primary dark matter halo as a perturber in a background of secondary halos with velocities described by linear theory. By evaluating the ensemble of secondary halos on orbits within the perturbing halo's "loss cone" we derive the distribution of orbital parameters of these captured halos. This model is able provide qualitative explanations for the features of this distribution as measured from N-body simulations, and is in approximate quantitative agreement with those measurements. As the velocity dispersion of the background halos is larger on smaller scales our model predicts an overall increase in the characteristic velocities of merging halos, relative to the virial velocities of those halos, in lower mass systems. Our model also provides a simple explanation for the measured independence of the orbital velocity distribution function on redshift when considered at fixed peak height. By connecting the orbital parameter distribution to the underlying power spectrum our model also allows for estimates to be made of the effect of modifying that power spectrum, for example by including a truncation at large wavenumber. For plausible warm dark matter models we find that this truncation has only a small effect on the predicted distributions.