Intrinsic alignments of disc and elliptical galaxies in the MassiveBlack-II and Illustris simulations

Abstract

We study the shapes and intrinsic alignments of disks and elliptical galaxies in the MassiveBlack-II (MBII) and Illustris cosmological hydrodynamic simulations, with volumes of $(100h^{-1}Mpc)^{3}$ and $(75h^{-1}Mpc)^{3}$ respectively. We find that simulated disk galaxies are more oblate in shape and more misaligned with the shape of their host dark matter subhalo when compared with ellipticals. The disk major axis is found to be oriented towards the location of nearby elliptical galaxies. We also find that the disks are thinner in MBII and misalignments with dark matter halo orientations are smaller in both disks and ellipticals when compared with Illustris. As a result, the intrinsic alignment correlation functions at fixed mass have a higher amplitude in MBII than in Illustris. Finally, at scales above $\sim 0.1$\hmpc, the intrinsic alignment two-point correlation functions for disk galaxies in both simulations are consistent with a null detection, unlike those for ellipticals. Despite significant differences in the treatments of hydrodynamics and baryonic physics in the simulations, we find that the $w_{\delta +}$ correlation function scales similarly with transverse separation. However, the less massive galaxies show different scale dependence in the ED correlation. This result indicates that, while hydrodynamic simulations are a promising tool to study intrinsic alignments, further study is needed to understand the impact of differences in the implementations of hydrodynamics and baryonic feedback.