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I review scenarios for the assembly of supermassive black holes (MBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark matter halo “merger tree”. Monte Carlo realizations of the merger hierarchy in a CDM cosmology, coupled to semi-analytical recipes, are a powerful tool to follow the merger history of halos and the dynamics and growth of the MBHs they host. X-ray photons from miniquasars powered by intermediate-mass “seed” holes may permeate the universe more uniformly than EUV radiation, make the low-density diffuse intergalactic medium warm and weakly ionized prior to the epoch of reionization breakthrough, and set an entropy floor. The spin distribution of MBHs is determined by gas accretion, and is predicted to be heavily skewed towards fast-rotating Kerr holes, already in place at early epochs, and not to change significantly below redshift 5. Decaying MBH binaries may shape the innermost central regions of galaxies and should be detected in significant numbers by .

The supermassive black holes observed at the centers of almost all present-day galaxies had a profound impact on their environment. I highlight the principle of , by which supermassive black holes grow until they release sufficient energy to unbind the gas that feeds them from their host galaxy. This principle explains several observed facts, including the correlation between the mass of a central black hole and the depth of the gravitational potential well of its host galaxy, and the abundance and clustering properties of bright quasars in the redshift interval of ~ 2–6. At lower redshifts, quasars might have limited the maximum mass of galaxies through the suppression of cooling flows in X-ray clusters. The seeds of supermassive black holes were likely planted in dwarf galaxies at redshifts > 10, through the collapse of massive or supermassive stars. The minimum seed mass can be identified observationally through the detection of gravitational waves from black hole binaries by or . Aside from shaping their host galaxies, quasar outflows filled the intergalactic medium with magnetic fields and heavy elements. Beyond the reach of these outflows, the brightest quasars at >6 have ionized exceedingly large volumes of gas (tens of comoving Mpc) prior to global reionization, and must have suppressed the faint end of the galaxy luminosity function in these volumes before the same occurred through the rest of the universe.

We discuss currently available observational constraints on the reionization history of the intergalactic medium (IGM), and the extent to which accreting black holes (BHs) can help explain these observations. We show new evidence, based on the combined statistics of Lyman and absorption in quasar spectra, that the IGM contains a significant amount of neutral hydrogen, and is experiencing rapid ionization at redshift ~ 6. However, we argue that quasar BHs, even faint ones that are below the detection thresholds of existing optical surveys, are unlikely to drive the evolution of the neutral fraction around this epoch, because they would over–produce the present–day soft X–ray background. On the other hand, the seeds of the ~ 6 quasar BHs likely appeared at much earlier epochs (~ 20), and produced hard ionizing radiation by accretion. These early BHs are promising candidates to account for the high redshift (~ 15) ionization implied by the recent cosmic microwave anisotropy data from WMAP. Using a model for the growth of BHs by accretion and mergers in a hierarchical cosmology, we suggest that the early growth of quasars must include a super-Eddington growth phase, and that, although not yet optically identified, the FIRST radio survey may have already detected several thousand >10M BHs at >6.

Understanding the emergence of the first supermassive black holes, and hence the first quasars, is a crucial ingredient for models of reionization and galaxy formation. I review recent progress in simulating their formation out of primordial, pure hydrogen-helium gas at redshifts ~ 10. The predicted host systems are early dwarf galaxies with total masses ~ 10. To be able to form seed black holes, with typical mass ~ 10, the dwarf galaxy must have avoided previous or concurrent episodes of star formation. The associated feedback effects would otherwise have prevented the effective assembly of gas in the center of the halo potential well. Such a comprehensive suppression of star formation could have been accomplished in rare cosmic environments where a strong UV background had photodissociated any molecular hydrogen which is the only viable coolant in metal-free gas.

We include into a semi-analytic model of galaxy formation a physical description of starbursts and of the feeding of supermassive black holes (BHs), powering the Active Galactic Nuclei (AGNs). Both such processes originate from the destabilization of cold galactic gas in galaxy encounters, occuring mainly at redshifts ≈ 2–4, preferentially in massive objects, during the phase of galaxy formation characterized by frequent merging events. Our model produces at >3 a rise, and at < 2.5 a decline of the bright quasar (QSO) population as steep as observed, coupled to an almost passive evolution of the galactic stellar populations in massive galaxies. The high- star formation rate and B-band luminosity functions, and the luminosity and redshift distribution of galaxies in K-band at < 2 are all in good agreement with the existing observations concerning the bright galaxy population. As for the AGNs, our results closely fit the observed luminosity functions of QSOs, their density from ≈ 5 to ≈ 0, and the local – relation.

The relationship between the mass of a black-hole and the circular velocity of its host dark-matter halo is fundamental to the clustering length of quasars. The slow evolution of the clustering length with redshift inferred in the 2dF quasar redshift survey strongly favors a scenario where the central black-holes comprise a larger fraction of the host galaxy mass at higher redshifts. In a scenario where quasars are triggered by halo mergers, this scaling, in combination with observed number counts imply that quasars have an episodic lifetime that is set by the dynamical time of a galactic disk rather than by the Salpeter time.

We present a physical model for the coevolution of massive spheroidal galaxies and active nuclei at their centers. Supernova heating is increasingly effective in slowing down the star formation and in driving gas outflows in smaller and smaller dark matter halos. Thus the more massive protogalaxies virializing at early times are the sites of faster star formation. The correspondingly higher radiation drag causes a faster angular momentum loss by the gas and induces a larger accretion rate onto the central black hole. In turn, the kinetic energy of the outflows powered by the active nuclei can unbind the residual gas in a time shorter for larger halos. The model accounts for a broad variety of dynamical, photometric and metallicity properties of early-type galaxies, for the – relation and for the local supermassive black-hole mass function.

The hot phase of the ISM in nearby elliptical galaxies is observed to have temperatures in the range of 10 Mio degrees. While stellar wind and mass–loss by planetary nebulae are widely seen as the sources for the interstellar gas, there is no generally established model to explain the heating of the ISM (supernovae, AGN heating and collisions).

We present a summary of the results obtained with a time-dependent, one-zone toy model aimed at exploring the importance of radiative feedback on the co-evolution of massive black holes (MBHs) at the center of stellar spheroids and their stellar and gaseous components. We consider cosmological infall of gas as well as the mass and energy return for the evolving stellar population. The AGN heating and cooling are described by assuming photoionization equilibrium of a plasma interacting with the average quasar SED. Our results nicely support a new scenario in which the AGN accretion phase characterized by a very short duty-cycle (and now common in the Universe) is due to radiative feedback. The establishment of this phase is recorded as a fossil in the Magorrian and – relations.

The discovery of luminous quasars at >6 indicates the existence of billion-solar-mass black holes at the end of cosmic dark ages. They provide the best probes of the early growth of supermassive black holes in the universe, the relation between the formation of early galaxies and black holes, and put constraints on the role of quasars and AGNs to the cosmic reionization. About 1000 quasars have been discovered at >4, including 50 at >5 and eight at >6. In this proceeding, I review the recent observational results on surveys and detailed follow-up observations of the highest redshift quasars, including the evolution of luminosity function, the evolution of spectral properties and chemical enrichment history, distribution of black hole masses and the host galaxy properties.