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

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.

The role of mergers and halo spin in shaping galaxy morphology

“…Dubois et al 2016), except that κrot does not distinguish the direction of rotation about the zaxis (i.e. co-rotating or counter-rotating), as discussed later in this appendix.…”

The jet feedback mechanism (JFM) in stars, galaxies and clusters

“…The first one is that in cooling flows in clusters of galaxies the main role of the jets is to heat the ICM, while during galaxy formation the jets are expected to expel gas from the young galaxy (e.g., Bower et al 2008), e.g., as observed in some young radio galaxies (e.g., Holt et al 2008). Dubois et al (2016), as another example, claim that AGN feedback is essential in order to produce massive galaxies that resemble elliptical galaxies.…”

The relation between galaxy morphology and colour in the EAGLE simulation

“…Despite recent progress in numerical simulation predictions of galaxy morphologies (Snyder et al 2015;Dubois et al 2016;Rodriguez-Gomez et al 2017;Bottrell et al 2017) and increasing observational data (Willett et al 2013;Häußler et al 2013), the origin of the distribution of galaxy morphologies and its correlation with starforming/quenching galaxies is still under debate. Cosmological simulations of galaxy formation aim to reproduce and provide an explanation for the origin of the global properties and scaling relations revealed by galaxy surveys.…”