  We use the cosmo-OWLS and BAHAMAS suites of cosmological hydrodynamical
simulations to explore the separate and combined effects of baryon physics
(particularly feedback from active galactic nuclei, AGN) and free-streaming of
massive neutrinos on large-scale structure. We focus on five diagnostics: i)
the halo mass function; ii) halo mass density profiles; iii) the halo
mass-concentration relation; iv) the clustering of haloes; and v) the
clustering of matter; and we explore the extent to which the effects of baryon
physics and neutrino free-streaming can be treated independently. Consistent
with previous studies, we find that both AGN feedback and neutrino
free-streaming suppress the total matter power spectrum, although their scale
and redshift dependencies differ significantly. The inclusion of AGN feedback
can significantly reduce the masses of groups and clusters, and increase their
scale radii. These effects lead to a decrease in the amplitude of the
mass-concentration relation and an increase in the halo autocorrelation
function at fixed mass. Neutrinos also lower the masses of groups and clusters
while having no significant effect on the shape of their density profiles (thus
also affecting the mass-concentration relation and halo clustering in a
qualitatively similar way to feedback). We show that, with only a small number
of exceptions, the combined effects of baryon physics and neutrino
free-streaming on all five diagnostics can be estimated to typically better
than a few percent accuracy by treating these processes independently (i.e., by
multiplying their separate effects).

Cosmology and Nongalactic Astrophysics