Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations

TL;DR

This study uses Magneticum hydrodynamic simulations to analyze how halo concentration depends on mass, redshift, and fossilness, revealing higher concentrations in fossil systems due to their relaxed state.

Contribution

It provides the first detailed analysis of the dependence of halo concentration on fossilness in hydrodynamic simulations across a wide mass range.

Findings

Negative mass-concentration relation consistent with observations

Fossil systems have systematically higher concentration

Higher concentration linked to dynamical relaxation and satellite accretion history

Abstract

We study the dependency of the concentration on mass and redshift using three large N-body cosmological hydrodynamic simulations carried out by the Magneticum project. We constrain the slope of the mass-concentration relation with an unprecedented mass range for hydrodynamic simulations and find a negative trend on the mass-concentration plane and a slightly negative redshift dependency, in agreement with observations and other numerical works. We also show how the concentration correlates with the fossil parameter, defined as the stellar mass ratio between the central galaxy and the most massive satellite, in agreement with observations. We find that haloes with high fossil parameter have systematically higher concentration and investigate the cause in two different ways. First we study the evolution of haloes that lives unperturbed for a long period of time, where we find that the internal region keeps accreting satellites as the fossil parameter increases and the scale radius decreases (which increases the concentration). We also study the dependency of the concentration on the virial ratio and the energy term from the surface pressure $E_s$. We conclude that fossil objects have higher concentration because they are dynamically relaxed, with no in-fall/out-fall material and had time to accrete their satellites.