# The connection between halo concentrations and assembly histories: a probe of gravity?

## TL;DR

This study compares dark matter halo concentrations and formation histories in general relativity and $f(R)$ gravity using high-resolution simulations, revealing significant differences that challenge existing models.

## Contribution

It demonstrates that $f(R)$ gravity alters the concentration--formation time relation of dark matter haloes, indicating the need to revise current models for these relations.

## Key findings

- Haloes in $f(R)$ gravity are more concentrated than in GR at fixed mass.
- The concentration--formation time relation differs significantly between $f(R)$ gravity and GR.
- Existing models for mass--concentration--redshift relations require revision for $f(R)$ gravity.

## Abstract

We use two high resolution N-body simulations, one assuming general relativity and the other the Hu-Sawicki form of $f\left(R\right)$ gravity with $\vert \bar{f}_{R} \vert = 10^{-6}$, to investigate the concentration--formation time relation of dark matter haloes. We stack haloes in logarithmically spaced mass bins to fit median density profiles and extract median formation times. At fixed mass, haloes in modified gravity are more concentrated than those in GR, especially at low masses and at low redshift, and do not follow the concentration--formation time relation seen in GR. We assess the sensitivity of the relation to how concentration and formation time are defined, as well as to the segregation of the halo population by the amount of gravitational screening. We find a clear difference between halo concentrations and assembly histories displayed in modified gravity and those in GR. Existing models for the mass--concentration--redshift relation that have gained success in cold and warm dark matter models require revision in $f\left(R\right)$ gravity.