# Energy equipartition between stellar and dark matter particles in cosmological simulations results in spurious growth of galaxy sizes

## TL;DR

This paper demonstrates that in cosmological simulations, the mismatch in particle mass between stars and dark matter causes artificial energy transfer, leading to exaggerated galaxy sizes, which impacts the interpretation of simulation results.

## Contribution

It reveals how mass segregation due to unequal particle masses causes spurious galaxy size growth in simulations, highlighting a key source of numerical artifact.

## Key findings

- Galaxy sizes increase systematically when dark matter particles are more massive than stellar particles.
- Size growth correlates with the ratio of stellar to dark matter particle mass.
- Spurious effects are significant when the stellar-to-dark matter particle mass ratio exceeds one.

## Abstract

The impact of 2-body scattering on the innermost density profiles of dark matter haloes is well established. We use a suite of cosmological simulations and idealised numerical experiments to show that 2-body scattering is exacerbated in situations where there are two species of unequal mass. This is a consequence of mass segregation and reflects a flow of kinetic energy from the more to less massive particles. This has important implications for the interpretation of galaxy sizes in cosmological hydrodynamic simulations, which nearly always model stars with less massive particles than are used for the dark matter. We compare idealised models as well as simulations from the EAGLE project that differ only in the mass resolution of the dark matter component, but keep sub-grid physics, baryonic mass resolution and gravitational force softening fixed. If the dark matter particle mass exceeds the mass of stellar particles, then galaxy sizes--quantified by their projected half-mass radii, ${\rm R_{50}}$--increase systematically with time until ${\rm R_{50}}$ exceeds a small fraction of the redshift-dependent mean inter-particle separation, $l$ (${\rm R_{50}}\geq 0.05\times l$). Our conclusions should also apply to simulations that adopt different hydrodynamic solvers, subgrid physics or adaptive softening, but in that case may need quantitative revision. Any simulation employing a stellar-to-dark matter particle mass ratio greater than unity will escalate spurious energy transfer from dark matter to baryons on small scales.