We assess models for the assembly of supermassive black holes (SMBHs) at the center of galaxies that trace their hierarchical build-up far up in the dark halo 'merger tree'. Motivated by the recent discovery of luminous quasars around redshift z ≈ 6 -suggesting a very early assembly epoch -and by numerical simulations of the fragmentation of primordial molecular clouds in cold dark matter cosmogonies, we assume that the first 'seed' black holes (BHs) had intermediate masses and formed in (mini)halos collapsing at z ∼ 20 from high-σ density fluctuations. As these pregalactic holes become incorporated through a series of mergers into larger and larger halos, they sink to the center owing to dynamical friction, accrete a fraction of the gas in the merger remnant to become supermassive, form a binary system, and eventually coalesce. The merger history of dark matter halos and associated BHs is followed by cosmological Monte Carlo realizations of the merger hierarchy from early times until the present in a ΛCDM cosmology. A simple model, where quasar activity is driven by major mergers and SMBHs accrete at the Eddington rate a mass that scales with the fifth power of the circular velocity of the host halo, is shown to reproduce the observed luminosity function of optically-selected quasars in the redshift range 1 < z < 5. A scheme for describing the hardening of a BH binary in a stellar background with core formation due to mass ejection is applied, where the stellar cusp ∝ r −2 is promptly regenerated after every major merger event, replenishing the mass displaced by the binary. Triple BH interactions will inevitably take place at early times if the formation route for the assembly of SMBHs goes back to the very first generation of stars, and we follow them in our merger tree. The assumptions underlying our scenario lead to the prediction of a population of massive BHs wandering in galaxy halos and the intergalactic medium at the present epoch, and contributing ∼ < 10% to the total BH mass density, ρ SMBH = 4 × 10 5 M ⊙ Mpc −3 (h = 0.7). The fraction of binary SMBHs in galaxy nuclei is of order 10% today, and it increases with redshift so that almost all massive nuclear BHs at early epochs are in binary systems. The fraction of binary quasars (both members brighter than 0.1 L * ) instead is less than 0.3% at all epochs. The nuclear SMBH occupation fraction is unity (0.6) at the present epoch if the first seed BHs were as numerous as the 3.5-σ (4-σ) density peaks at z = 20.

Scaling relations between central black hole (BH) mass and host galaxy properties are of fundamental importance to studies of BH and galaxy evolution throughout cosmic time. Here we investigate the relationship between BH mass and host galaxy total stellar mass using a sample of 262 broad-line active galactic nuclei (AGN) in the nearby Universe (z < 0.055), as well as 79 galaxies with dynamical BH masses. The vast majority of our AGN sample is constructed using Sloan Digital Sky Survey spectroscopy and searching for Seyfert-like narrow-line ratios and broad Hα emission. BH masses are estimated using standard virial techniques. We also include a small number of dwarf galaxies with total stellar masses M stellar 10 9.5 M ⊙ and a sub-sample of the reverberation-mapped AGNs. Total stellar masses of all 341 galaxies are calculated in the most consistent manner feasible using colordependent mass-to-light ratios. We find a clear correlation between BH mass and total stellar mass for the AGN host galaxies, with M BH ∝ M stellar , similar to that of early-type galaxies with dynamicallydetected BHs. However, the relation defined by the AGNs has a normalization that is lower by more than an order of magnitude, with a BH-to-total stellar mass fraction of M BH /M stellar ∼ 0.025% across the stellar mass range 10 8 ≤ M stellar /M ⊙ ≤ 10 12 . This result has significant implications for studies at high redshift and cosmological simulations in which stellar bulges cannot be resolved.

We describe a mechanism by which supermassive black holes (SMBHs) can form directly in the nuclei of protogalaxies, without the need for ‘seed’ black holes left over from early star formation. Self‐gravitating gas in dark matter haloes can lose angular momentum rapidly via runaway, global dynamical instabilities, the so‐called ‘bars within bars’ mechanism. This leads to the rapid build‐up of a dense, self‐gravitating core supported by gas pressure – surrounded by a radiation pressure‐dominated envelope – which gradually contracts and is compressed further by subsequent infall. We show that these conditions lead to such high temperatures in the central region that the gas cools catastrophically by thermal neutrino emission, leading to the formation and rapid growth of a central black hole. We estimate the initial mass and growth rate of the black hole for typical conditions in metal‐free haloes with Tvir∼ 104 K, which are the most likely to be susceptible to runaway infall. The initial black hole should have a mass of ≲20 M⊙, but in principle could grow at a super‐Eddington rate until it reaches ∼104–106 M⊙. Rapid growth may be limited by feedback from the accretion process and/or disruption of the mass supply by star formation or halo mergers. Even if super‐Eddington growth stops at ∼103–104 M⊙, this process would give black holes ample time to attain quasar‐size masses by a redshift of 6, and could also provide the seeds for all SMBHs seen in the present Universe.

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 10 10 M ⊙ . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

Evidence shows that massive black holes reside in most local galaxies. Studies have also established a number of relations between the MBH mass and properties of the host galaxy such as bulge mass and velocity dispersion. These results suggest that central MBHs, while much less massive than the host (∼ 0.1%), are linked to the evolution of galactic structure. In hierarchical cosmologies, a single big galaxy today can be traced back to the stage when it was split up in hundreds of smaller components. Did MBH seeds form with the same efficiency in small proto-galaxies, or did their formation had to await the buildup of substantial galaxies with deeper potential wells? I briefly review here some of the physical processes that are conducive to the evolution of the massive black hole population. I will discuss black hole formation processes for 'seed' black holes that are likely to place at early cosmic epochs, and possible observational tests of these scenarios.

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on ‘Black holes, Gravitational waves and Fundamental Physics’.

We review the expected science performance of the New Gravitational-Wave Observatory (NGO, a.k.a. eLISA), a mission under study by the European Space Agency for launch in the early 2020s. eLISA will survey the low-frequency gravitational-wave sky (from 0.1 mHz to 1 Hz), detecting and characterizing a broad variety of systems and events throughout the Universe, including the coalescences of massive black holes brought together by galaxy mergers; the inspirals of stellar-mass black holes and compact stars into central galactic black holes; several millions of ultra-compact binaries, both detached and mass transferring, in the Galaxy; and possibly unforeseen sources such as the relic gravitational-wave radiation from the early Universe. eLISA's high signal-tonoise measurements will provide new insight into the structure and history of the Universe, and they will test general relativity in its strong-field dynamical regime.

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.