  We examine the impact of baryonic physics on the halo distribution in
hydrodynamic simulations (Illustris, IllustrisTNG, and EAGLE), particularly
with regards to how it differs from that in dark matter only (DMO) simulations.
We find that, in general, DMO simulations produce halo mass functions (HMF)
that are shifted to higher halo masses than their hydrodynamic counterparts,
due to the lack of baryonic physics. However, the exact nature of this mass
shift is a complex function of mass, halo definition, redshift, and
larger-scale environment, and it also depends on the specifics of the baryonic
physics implemented in the simulation. We present fitting formulae for the
corrections one would need to apply to each DMO halo catalogue in order to
reproduce the HMF found in its hydrodynamic counterpart. We provide these
formulae for all three simulations, for five different halo definitions at
redshifts 0, 1, and 2. Additionally, we explore the dependence on environment
of this HMF discrepancy, and find that, in most cases, halos in low density
environments are slightly more impacted by baryonic physics than halos in high
density environments. We thus also provide environment-dependent mass
correction formulae that can reproduce the conditional, as well as global, HMF.
We show that our mass corrections also repair the large-scale clustering of
halos, though the environment-dependent corrections are required to achieve an
accuracy better than 2%. Finally, we examine the impact of baryonic physics on
the halo mass - concentration relation, and find that its slope in hydrodynamic
simulations is consistent with that in DMO simulations. Ultimately, we
recommend that any future work relying on DMO halo catalogues incorporate our
mass corrections to test the robustness of their results to baryonic effects.
