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We compare the shapes and intrinsic alignments of galaxies in theMassiveBlack-II cosmological hydrodynamic simulation (MBII) to those in a darkmatter-only (DMO) simulation performed with the same volume(100$h^{-1}$Mpc)$^{3}$, cosmological parameters, and initial conditions.Understanding the impact of baryonic physics on galaxy shapes and alignmentsand their relation to the dark matter distribution should prove useful to mapthe intrinsic alignments of galaxies from hydrodynamic to dark matter-onlysimulations. We find that dark matter subhalos are typically rounder in MBII,and the shapes of stellar matter in low mass galaxies are more misaligned withthe shapes of the dark matter of the corresponding subhalos in the DMOsimulation. At $z=0.06$, the fractional difference in the mean misalignmentangle between MBII and DMO simulations varies from $\sim 28 \% - 12 \%$ in themass range $10^{10.8} - 6.0 \times 10^{14} h^{-1}M_{\odot}$. We study the darkmatter halo shapes and alignments as a function of radius, and find that whilegalaxies in MBII are more aligned with the inner parts of their dark mattersubhalos, there is no radial trend in their alignments with the correspondingsubhalo in the DMO simulation. This result highlights the importance ofbaryonic physics in determining the alignment of the galaxy with respect to theinner parts of the halo. Finally, we compare the ellipticity-direction (ED)correlation for galaxies to that for dark matter halos, finding that it issuppressed on all scales by stellar-dark matter misalignment. In the projectedshape-density correlation ($w_{\delta+}$), which includes ellipticityweighting, this effect is partially canceled by the higher mean ellipticitiesof the stellar component, but differences of order $30-40\%$ remain on scales$> 1$ Mpc over a range of subhalo masses, with scale-dependent effects below$1$ Mpc.

Galaxy shapes and alignments in the MassiveBlack-II hydrodynamic and dark matter-only simulations