The Horizon-AGN simulation: morphological diversity of galaxies promoted by AGN feedback

TL;DR

This study uses the Horizon-AGN simulation to demonstrate that AGN feedback is crucial for producing realistic galaxy morphologies, especially in massive galaxies, by influencing merger outcomes and star formation.

Contribution

It provides the first clear comparison showing the essential role of AGN feedback in shaping galaxy morphology in cosmological simulations.

Findings

AGN feedback aligns simulated galaxy properties with observations.

Without AGN, massive galaxies tend to reform discs instead of becoming ellipticals.

Merger-driven AGN activity stabilizes galaxy morphology post-merger.

Abstract

The interplay between cosmic gas accretion on to galaxies and galaxy mergers drives the observed morphological diversity of galaxies. By comparing the state-of-the-art hydrodynamical cosmological simulations Horizon-AGN and Horizon-noAGN, we unambiguously identify the critical role of active galactic nuclei (AGN) in setting up the correct galaxy morphology for the massive end of the population. With AGN feedback, typical kinematic and morpho-metric properties of galaxy populations as well as the galaxy-halo mass relation are in much better agreement with observations. Only AGN feedback allows massive galaxies at the centre of groups and clusters to become ellipticals, while without AGN feedback those galaxies reform discs. It is the merger-enhanced AGN activity that is able to freeze the morphological type of the post-merger remnant by durably quenching its quiescent star formation. Hence morphology is shown to be driven not only by mass but also by the nature of cosmic accretion: at constant galaxy mass, ellipticals are galaxies that are mainly assembled through mergers, while discs are preferentially built from the in situ star formation fed by smooth cosmic gas infall.