Disc Heating: Comparing the Milky Way with Cosmological Simulations

TL;DR

This study compares particle and grid-based cosmological simulations with Milky Way observations, revealing broad agreement but also highlighting differences in disc heating and formation histories, with implications for modeling galaxy evolution.

Contribution

First comprehensive comparison of particle and grid-based hydrodynamical codes in galaxy simulations against Milky Way data, analyzing disc heating and stellar kinematics.

Findings

Simulations broadly agree but differ in disc heating profiles.

Models with less recent merger activity better match Milky Way data.

All simulations are hotter than the Milky Way disc, indicating model limitations.

Abstract

We present the analysis of a suite of simulations run with different particle-and grid-based cosmological hydrodynamical codes and compare them with observational data of the Milky Way. This is the first study to make comparisons of properties of galaxies simulated with particle and grid-based codes. Our analysis indicates that there is broad agreement between these different modelling techniques. We study the velocity dispersion - age relation for disc stars at z=0 and find that four of the simulations are more consistent with observations by Holmberg et al. (2008) in which the stellar disc appears to undergo continual/secular heating. Two other simulations are in better agreement with the Quillen & Garnett (2001) observations that suggest a "saturation" in the heating profile for young stars in the disc. None of the simulations have thin discs as old as that of the Milky Way. We also analyse the kinematics of disc stars at the time of their birth for different epochs in the galaxies' evolution and find that in some simulations old stars are born cold within the disc and are subsequently heated, while other simulations possess old stellar populations which are born relatively hot. The models which are in better agreement with observations of the Milky Way's stellar disc undergo significantly lower minor-merger/assembly activity after the last major merger - i.e. once the disc has formed. All of the simulations are significantly "hotter" than the Milky Way disc; on top of the effects of mergers, we find a "floor" in the dispersion that is related to the underlying treatment of the heating and cooling of the interstellar medium, and the low density threshold which such codes use for star formation. This finding has important implications for all studies of disc heating that use hydrodynamical codes.