We track the coevolution of supermassive black holes (SMBHs) and their host galaxies through cosmic time. The calculation is embedded in the galform semi‐analytic model which simulates the formation and evolution of galaxies in a cold dark matter (CDM) universe. The black hole (BH) and galaxy formation models are coupled: during the evolution of the host galaxy, hot and cold gas are added to the SMBH by flows triggered by halo gas cooling, disc instabilities and galaxy mergers. This builds up the mass and spin of the BH, and the resulting accretion power regulates gas cooling and subsequent star formation. The accretion flow is assumed to form a geometrically thin cool disc when the accretion rate exceeds , and a geometrically thick, radiatively inefficient hot flow when the accretion rate falls below this value. The resulting quasar optical luminosity function matches observations well, and the mass of the SMBH correlates with the mass of the galaxy bulge as in the observed Mbh–Mbulge relation. The BH spin distribution depends strongly on whether we assume that the gas in any given accretion episode remains in the same plane or it fragments into multiple, randomly aligned accretion episodes due to its self‐gravity. We refer to these cases as the ‘prolonged’ and ‘chaotic’ accretion modes, respectively. In the chaotic accretion model there is a clear correlation of spin with SMBH mass (and hence host galaxy bulge mass). Massive BHs (M > 5 × 108 M⊙) are hosted by giant elliptical galaxies and are rapidly spinning, while lower mass BHs are hosted in spiral galaxies and have much lower spin. Using the Blandford–Znajek mechanism for jet production to calculate the jet power, our model reproduces the radio loudness of radio galaxies, low ionization emission regions (LINERS) and Seyferts, suggesting that the jet properties of active galaxy nuclei (AGN) are a natural consequence of both the accretion rate on to and the spin of the central SMBH. This is the first confirmation that a CDM galaxy formation model can reproduce the observed radio phenomenology of AGN.

We use a coupled model of the formation and evolution of galaxies and black holes (BHs) to study the evolution of active galactic nuclei (AGNs) in a cold dark matter universe. The model is embedded in the galaxy formation code galform and predicts the masses, spins and mass accretion histories of BHs in tandem with the formation of their host galaxies. BHs grow by accretion during starbursts, triggered by discs becoming dynamically unstable or by galaxy mergers, and accretion from quasi‐hydrostatic hot gas haloes. Using an empirical law for AGN obscuration, our model matches the observed luminosity functions (LFs) of AGNs over a wide range of redshifts. Due to the suppression of cooling in massive haloes by AGN feedback, at low redshift (z≲ 2), the brightest quasars (Lbol≳ 1046 erg s−1) are predicted preferentially to inhabit haloes with masses . The model predicts a hierarchical buildup of BH mass, with the typical mass of actively growing BHs increasing with decreasing redshift. Nevertheless, the model displays clear ‘downsizing’ as reflected in the differential evolution of the space density of faint and bright AGNs. This arises naturally from the interplay between the starburst and hot gas halo accretion modes. The faint end of the LF is dominated by massive BHs accreting at low rates via a thick disc, primarily during the hot‐halo mode. The bright end is populated by BHs accreting close to or above the Eddington limit during the starburst mode. Obscuration plays a central role in determining the observed abundance of AGNs and, hence, in their implied cosmic evolution.

We explore the multiwavelength properties of AGN host galaxies for different classes of radio-selected AGN out to z 6 via a multiwavelength analysis of about 7 700 radio sources in the COSMOS field. The sources were selected with the Very Large Array (VLA) at 3 GHz (10 cm) within the VLA-COSMOS 3 GHz Large Project, and cross-matched with multiwavelength ancillary data. This is the largest sample of highredshift (z 6) radio sources with exquisite photometric coverage and redshift measurements available. We constructed a sample of moderate-tohigh radiative luminosity AGN (HLAGN) via spectral energy distribution (SED) decomposition combined with standard X-ray and mid-infrared diagnostics. Within the remainder of the sample we further identified low-to-moderate radiative luminosity AGN (MLAGN) via excess in radio emission relative to the star formation rates in their host galaxies. We show that at each redshift our HLAGN have systematically higher radiative luminosities than MLAGN and that their AGN power occurs predominantly in radiative form, while MLAGN display a substantial mechanical AGN luminosity component. We found significant differences in the host properties of the two AGN classes, as a function of redshift. At z<1.5, MLAGN appear to reside in significantly more massive and less star-forming galaxies compared to HLAGN. At z>1.5, we observed a reversal in the behaviour of the stellar mass distributions with the HLAGN populating the higher stellar mass tail. We interpret this finding as a possible hint of the downsizing of galaxies hosting HLAGN, with the most massive galaxies triggering AGN activity earlier than less massive galaxies, and then fading to MLAGN at lower redshifts. Our conclusion is that HLAGN and MLAGN samples trace two distinct galaxy and AGN populations in a wide range of redshifts, possibly resembling the radio AGN types often referred to as radiative-and jet-mode (or high-and low-excitation), respectively, whose properties might depend on the different availability of cold gas supplies.

We present the spatial clustering properties of 1466 X-ray selected AGN compiled from the Chandra CDF-N, CDF-S, eCDF-S, COSMOS and AEGIS fields in the 0.5 − 8 keV band. The X-ray sources span the redshift interval 0 < z < 3 and have a median value ofz = 0.976. We employ the projected two-point correlation function to infer the spatial clustering and find a clustering length of r 0 = 7.2 ± 0.6h −1 Mpc and a slope of γ = 1.48 ± 0.12, which corresponds to a bias of b(z) = 2.26 ± 0.16. Using two different halo bias models, we consistently estimate an average dark-matter host halo mass of M h ≃ 1.3(±0.3) × 10 13 h −1 M ⊙ . The Xray AGN bias and the corresponding dark-matter host halo mass, are significantly higher than the corresponding values of optically selected AGN (at the same redshifts). The redshift evolution of the X-ray selected AGN bias indicates, in agreement with other recent studies, that a unique dark-matter halo mass does not fit well the bias at all the different redshifts probed. Furthermore, we investigate if there is a dependence of the clustering strength on X-ray luminosity. To this end we consider only 650 sources around z ∼ 1 and we apply a procedure to disentangle the dependence of clustering on redshift. We find indications for a positive dependence of the clustering length on X-ray luminosity, in the sense that the more luminous sources have a larger clustering length and hence a higher dark-matter halo mass. In detail we find for an average luminosity difference of δ log 10 L x ≃ 1 a halo mass difference of a factor of ∼3.These findings appear to be consistent with a galaxy-formation model where the gas accreted onto the supermassive black hole in intermediate luminosity AGN comes mostly from the hot-halo atmosphere around the host galaxy.

Quasars represent the brightest Active Galactic Nuclei (AGN) in the Universe and are thought to indicate the location of prodigiously growing Black Holes (BHs), with luminosities as high as 10 48 erg s −1 . It is often expected though that such an extremely energetic process will take place in the most massive bound structures in the dark matter (DM) distribution. We show that in contrast to this expectation, in a galaxy formation model which includes AGN feedback, quasars are predicted to live in average DM halo environments with typical masses of a few times 10 12 M ⊙ . This fundamental prediction arises from the fact that quasar activity (i.e., BH accretion with luminosity greater than 10 46 erg s −1 ) is inhibited in DM haloes where AGN feedback operates. The galaxy hosts of quasars in our simulations are identified with over massive (in gas and stars) spheroidal galaxies, in which BH accretion is triggered via a galaxy merger or secular processes. We further show that the z = 0 descendants of high redshift (z 6) QSOs span a wide range of morphologies, galaxy and halo masses. The z ∼ 6 BHs typically grow only by a modest factor by the present day. Remarkably, high redshift QSOs never inhabit the largest DM haloes at that time and their descendants are very seldom found in the most massive haloes at z = 0. We also show that observationally it is very likely to find an enhancement in the abundance of galaxies around quasars at z ∼ 5. However, these enhancements are considerably weaker compared to the overdensities expected at the extreme peaks of the DM distribution. Thus, it is very unlikely that a quasar detected in the z 5 Universe pinpoints the location of the progenitors of superclusters in the local Universe.