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We present simulations of galaxy formation, based on the gadget -3 code, in which a sub-resolution model for star formation and stellar feedback is interfaced with a new model for active galactic nucleus (AGN) feedback. Our sub-resolution model describes a multiphase interstellar medium (ISM), accounting for hot and cold gas within the same resolution element: we exploit this feature to investigate the impact of coupling AGN feedback energy to the different phases of the ISM over cosmic time. Our fiducial model considers that AGN feedback energy coupling is driven by the covering factors of the hot and cold phases. We perform a suite of cosmological hydrodynamical simulations of disc galaxies (⁠|$M_{\rm halo,DM} \simeq 2 \times 10^{12}$|  M⊙, at z  = 0), to investigate: (i) the effect of different ways of coupling AGN feedback energy to the multiphase ISM; (ii) the impact of different prescriptions for gas accretion (i.e. only cold gas, both cold and hot gas, with the additional possibility of limiting gas accretion from cold gas with high angular momentum); (iii) how different models of gas accretion and coupling of AGN feedback energy affect the coevolution of supermassive black holes (BHs) and their host galaxy. We find that at least a share of the AGN feedback energy has to couple with the diffuse gas, in order to avoid an excessive growth of the BH mass. When the BH only accretes cold gas, it experiences a growth that is faster than in the case in which both cold and hot gas are accreted. If the accretion of cold gas with high angular momentum is reduced, the BH mass growth is delayed, the BH mass at z  = 0 is reduced by up to an order of magnitude, and the BH is prevented from accreting below z ≲ 2, when the galaxy disc forms. [ABSTRACT FROM AUTHOR]

In order to constrain the physical processes that regulate and downsize the active galactic nucleus (AGN) population, the predictions of the model for the rise of galaxies and active nuclei (morgana) are compared to luminosity functions (LFs) of AGNs in the optical, soft and hard X-ray bands, to the local black hole (BH)–bulge mass relation and to the observed X-ray number counts and background. We also give predictions on the accretion rate of AGNs in units of the Eddington rate and on the BH–bulge relation expected at high redshift. We find that it is possible to reproduce the downsizing of AGNs within the hierarchical Lambda cold dark matter cosmogony and that the most likely cause of this downsizing is the stellar kinetic feedback that arises in star-forming bulges as a consequence of the high level of turbulence and leads to a massive removal of cold gas in small elliptical galaxies. At the same time, to obtain good fits to the number of bright quasars we require that quasar-triggered galactic winds self-limit the accretion on to BHs; however, the very high degree of complexity of the physics of these winds, coupled with our poor understanding of it, hampers more robust conclusions. In all cases, the predicted BH–bulge relation steepens considerably with respect to the observed one at bulge masses ; this problem is related to a known excess in the predicted number of small bulges, common to most similar models, so that the reproduction of the correct number of faint AGNs is done at the cost of underestimating their BH masses. This highlights an insufficient downsizing of elliptical galaxies, and hints at another feedback mechanism able to act on the compact discs that form and soon merge at high redshift. The results of this paper reinforce the need for direct investigations of the feedback mechanisms in active galaxies that will be possible with the next generation of astronomical telescopes from submm to X-rays. [ABSTRACT FROM AUTHOR]