The impact of radio feedback from active galactic nuclei in cosmological simulations: formation of disc galaxies

In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion on to black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty ill our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion discs and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion discs, we assume that only the RIAF is responsible for the AGN feedback. The focus of this paper is to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that, after an initial episode of bursting star formation, the accretion rate on to the central black hole drops so that the accretion disc switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly Suppresses star formation in the galactic disc and almost completely halts star formation in the bulge. This suppression of the star formation regulates mass accretion on to the black hole and associated AGN feedback. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. During the 'on' events, the AGN produces reasonably powerful jets (radio-loud state) and is less luminous than the host galaxy, while in the 'off' phase, the nucleus is inactive and 'radio quiet'. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the ratios between the central black hole mass and the mass of the host spheroid at z = 0 are similar to 10(-3) regardless of the strength of either supernova feedback or AGN feedback because the radiation drag model directly relates the star formation activity in the Galactic Centre and the mass accretion rate on to the central black hole.