Using hydrodynamical cosmological simulations we
investigate the effect of
baryonic dissipation on halo shapes.
We show that dissipational simulations
produce significantly rounder halos than those
formed in equivalent dissipationless simulations.
Gas cooling causes an average increase in halo
principal axis ratios of ~0.2−0.4
in the inner regions and a systematic shift
that persists out to the virial radius, alleviating
any tension between theory and observations.
Although the magnitude of the effect may be
overestimated due to overcooling,
cluster formation simulations designed
to reproduce the observed fraction of
cold baryons still produce substantially
rounder halos. Subhalos also exhibit a trend of increased axis ratios
in dissipational simulations. Moreover, we demonstrate that subhalos are
generally rounder than corresponding field halos
even in dissipationless simulations.
All of these results highlight the vital role of baryonic
processes in comparing theory with observations and warn against
over-interpreting discrepancies with collisionless simulations
on small scales.