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

We compare the shapes and intrinsic alignments of galaxies in the MassiveBlack-II (MBII) cosmological hydrodynamic simulation to those in an identical dark matter-only (DMO) simulation. Understanding the impact of baryonic physics on galaxy shapes should prove useful for creating mock galaxy catalogues based on DMO simulations that mimic intrinsic alignments in hydrodynamic simulations. The dark matter subhaloes are typically rounder in MBII, and the shapes of stellar matter in low-mass galaxies are more misaligned with the shapes of dark matter of the corresponding subhaloes in the DMO simulation. At z = 0.06, the fractional difference in the mean misalignment angle between MBII and DMO simulations varies from ∼28to12 per cent in the mass range 10^10.8–6.0 × 10¹⁴ h⁻¹ M_⊙. We study the dark matter halo shapes and alignments as a function of radius, and find that while galaxies in MBII are more aligned with the inner parts of their dark matter subhaloes, there is no radial trend in their alignments with the corresponding subhalo in the DMO simulation. This result highlights the importance of baryonic physics in determining galaxy alignments with inner parts of their halo. Finally, we find that the stellar-dark matter misalignment suppresses the ellipticity–direction (ED) correlation of galaxies in comparison to that of dark matter haloes. In the projected shape–density correlation (w_{δ +}), higher mean ellipticities of the stellar component reduce this effect, but differences of the order of 30–40 per cent remain on scales >1 Mpc.