Beyond the Halo: Redefining environment with unbound matter in N-body simulations

TL;DR

This paper investigates the distribution and accretion of unbound matter around dark matter halos in cosmological simulations, revealing its relation to galaxy environments and evolution from high to low redshift.

Contribution

It introduces a detailed analysis of unbound matter distribution around halos and its dependence on redshift, scale, and observable galaxy properties, expanding understanding beyond traditional halo boundaries.

Findings

Halos at z=2 accrete more unbound matter than at z=0.

78% of particles near halos at z=2 are accreted by z=0.

Galaxy environment correlates with nearby unbound matter.

Abstract

Approximately half of the matter in the Universe is "unbound" at z = 0, according to N-body simulations such as the Millennium Run. Here, we use the milli-Millennium simulation to examine the distribution of unbound matter in relation to the dark matter halos which host galaxies. We measure the unbound matter within two types of windows, using a halo dependent radius and a fixed radius at several different scales. We also consider the timescales over which a halo can accrete the local unbound matter at z = 2 and z = 0. Finally, we compare the unbound matter to observable properties of galaxies, such as local galaxy count environment and stellar mass. We find that halos at z = 2 can accrete far more of the nearby unbound matter over a Hubble time than halos at z = 0 and that 78% of particles within 5 $R_{vir}$ of a halo at z = 2 will be accreted by z = 0, compared to 36% of particles within 5 $h^{-1}$ Mpc of the halo. We also find that galaxy count environment is closely related to the amount of nearby unbound matter when measured on the same scale.