Galaxy shapes and intrinsic alignments in the MassiveBlack-II simulation

The intrinsic alignment of galaxy shapes with the large-scale density field is a contaminant to weak lensing measurements, as well as being an interesting signature of galaxy formation and evolution (albeit one that is difficult to predict theoretically). Here we investigate the shapes and relative orientations of the stars and dark matter of haloes and subhaloes (central and satellite) extracted from the MassiveBlack-II simulation, a state-of-the-art high-resolution hydrodynamical cosmological simulation which includes stellar and active galactic nucleus feedback in a volume of (100 h⁻¹ Mpc)³. We consider redshift evolution from z = 1 to 0.06 and mass evolution within the range of subhalo masses, 10¹⁰–6.0 × 10^14.0 h⁻¹ M_⊙. The shapes of the dark matter distributions are generally more round than the shapes defined by stellar matter. The projected root-mean-square ellipticity per component for stellar matter is measured to bee_rms = 0.28 at z = 0.3 for M_subhalo > 10^12.0 h⁻¹ M_⊙, which compares favourably with observational measurements. We find that the shapes of stellar and dark matter are more round for less massive subhaloes and at lower redshifts. By directly measuring the relative orientation of the stellar matter and dark matter of subgroups, we find that, on average, the misalignment between the two components is larger for less massive subhaloes. The mean misalignment angle varies from ∼30° to 10° forM ∼ 10¹⁰–10¹⁴ h⁻¹ M_⊙ and shows a weak dependence on redshift. We also compare the misalignment angles in central and satellite subhaloes at fixed subhalo mass, and find that centrals are more misaligned than satellites. We present fitting formulae for the shapes of dark and stellar matter in subhaloes and also the probability distributions of misalignment angles.