The alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations

TL;DR

This study analyzes the shapes and alignments of dark matter, stellar, and hot gas distributions in cosmological simulations across a wide range of halo masses, redshifts, and radii, revealing trends in asphericity and misalignment.

Contribution

It provides new quantitative measurements and fitting functions for the shapes and misalignments of baryonic and dark matter components in simulated galaxy haloes.

Findings

Shapes become more aspherical with increasing mass, radius, and redshift.

Galaxies generally align well with their dark matter haloes, but stellar components show significant misalignment.

Half of disc galaxies have a misalignment angle larger than 40 degrees.

Abstract

We report the alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations. The combination of these state-of-the-art hydro-cosmological simulations enables us to span four orders of magnitude in halo mass ($11 < log_{10}(M_{200}/ [h^{-1}M_\odot]) < 15$), a wide radial range ($-2.3 < log_{10}(r/[h^{-1}Mpc ]) < 1.3$) and redshifts $0 < z < 1$. The shape parameters of the dark matter, stellar and hot gas distributions follow qualitatively similar trends: they become more aspherical (and triaxial) with increasing halo mass, radius and redshift. We measure the misalignment of the baryonic components (hot gas and stars) of galaxies with their host halo as a function of halo mass, radius, redshift, and galaxy type (centrals vs satellites and early- vs late-type). Overall, galaxies align well with the local distribution of the total (mostly dark) matter. However, the stellar distributions on galactic scales exhibit a median misalignment of about 45-50 degrees with respect to their host haloes. This misalignment is reduced to 25-30 degrees in the most massive haloes ($13 < log_{10}(M_{200}/ [h^{-1}M_\odot ]) < 15$). Half of the disc galaxies in the EAGLE simulations have a misalignment angle with respect to their host haloes larger than 40 degrees. We present fitting functions and tabulated values for the probability distribution of galaxy-halo misalignment to enable a straightforward inclusion of our results into models of galaxy formations based on purely collisionless N-body simulations.