  Observational evidence indicates a mismatch between the shapes of
collisionless dark matter (DM) halos and those of observed systems. 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. Lastly, we analyze a series of
binary, equal-mass merger simulations of disk galaxies. Collisionless mergers
reveal a strong correlation between DM halo shape and stellar remnant
morphology. In dissipational mergers, the combination of strong gas inflows and
star formation leads to an increase of the DM axis ratios in the remnant. 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.
