Energy equipartition between stellar and dark matter particles in cosmological simulations results in spurious growth of galaxy sizes. (arXiv:1903.10110v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ludlow_A/0/1/0/all/0/1">Aaron D. Ludlow</a> (ICRAR/UWA), <a href="http://arxiv.org/find/astro-ph/1/au:+Schaye_J/0/1/0/all/0/1">Joop Schaye</a> (Leiden), <a href="http://arxiv.org/find/astro-ph/1/au:+Schaller_M/0/1/0/all/0/1">Matthieu Schaller</a> (Leiden), <a href="http://arxiv.org/find/astro-ph/1/au:+Richings_J/0/1/0/all/0/1">Jack Richings</a> (ICC/IPPP Durham)
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, $l$
(${rm R_{50}}geq 0.05times l$). 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.
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, $l$
(${rm R_{50}}geq 0.05times l$). 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.
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