Orbital parameters of infalling satellite haloes in the hierarchical $\Lambda$CDM model

TL;DR

This paper analyzes the initial orbital parameters of infalling satellite haloes in the mbda CDM model, revealing trends with host and satellite mass, and provides fitting functions for their distributions.

Contribution

It offers the first detailed statistical characterization of satellite infall orbits across a wide mass range in mbda CDM, including fitted distribution functions.

Findings

Satellites with larger infall mass fractions have more eccentric, radial orbits.

High mass haloes tend to accrete satellites on more radial orbits.

Provided simple fits to bivariate orbital parameter distributions.

Abstract

We present distributions of orbital parameters of infalling satellites of $\Lambda$CDM haloes in the mass range $10^{12}-10^{14}$M$_\odot$, which represent the initial conditions for the subsequent evolution of substructures within the host halo. We use merger trees constructed in a high resolution cosmological N-body simulation to trace satellite haloes, and identify the time of infall. We find signficant trends in the distribution of orbital parameters with both the host halo mass and the ratio of satellite-to-host halo masses. For all host halo masses, satellites whose infall mass is a larger fraction of the host halo mass have more eccentric, radially biased orbits. At fixed satellite-to-host halo mass ratio, high mass haloes are biased towards accreting satellites on slightly more radial orbits. To charactise the orbital distributions fully requires fitting the correlated bivariate distribution of two chosen orbital parameters (e.g. radial and tangential velocity or energy and angular momentum). We provide simple fits to one choice of the bivariate distributions, which when transformed faithfully, captures the behaviour of any of the projected one-dimensional distributions.