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A most notable advancement should be the building of fully steerable 40m class telescopes in the optical and near infrared: the GMT and the ESO ELT. The first will consist of seven 8.4 m diameter segments with the collecting area equivalent to a 22.0 m single-mirror telescope (McCarthy, 2006). The E-ELT is a 40-m class with included adaptive optics (Hook et al., 2009). Both will be equipped with integral field units for spatially-resolved spectroscopy (ESO, 2018).

The James Webb Space Telescope (JWST) is expected to be another milestone, providing a ten fold improvement in sensitivity and in spatial resolution with respect to the Hubble Space Telescope (HST, Gardner et al., 2006). The JWST being optimized for optical and infrared, the UV domain (not accessible from ground) needs to be covered by a new generation of space based observatory. At present WSO-UV and LUVOIR may have some chances to become operational in the 2020s (France et al., 2017; WSO-UV Consortium, 2018). It is difficult to undermine the importance of these new developments. The present danger is that the UV range may not be any more accessible after the decommissioning of HST, a loss without precedent, since the UV has been accessible since the late 1978 after the launch of the International Ultraviolet explorer (Macchetto and Penston, 1978). The perspective is on the converse to have instruments with unprecedented resolving power and sensitivity, able to yield a full map star cluster and dwarf galaxies in the local Universe (apart from the heavily-extinguished regions in the Galactic plane).

The Euclid space mission, designed to study the baryonic acoustic oscillations (BAOs) from the large scale distribution of galaxies and quasars (see section 3.2.8 for a brief discussion) is expected to yield millions of moderate-resolution quasar spectra over the visual wavelength range (ESA, 2018). The Euclid spatial resolution of 0.2 arcsec is comparable to the Hubble Space Telescope and should provide an accurate redshift for the majority of the new sources identified by future imaging observatories, from radio to X-rays, at z ≲ 2. The Euclid output is especially welcome as ground-based survey telescopes such as the Large Synoptic Survey Telescope (LSST) will make it possible to discover millions of quasar candidates. LSST telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 square-degree field of view, and a 3.2 Gigapixel camera. This system can image about 10,000 square degrees of sky in three clear nights using pairs of 15-s exposures twice per night, with typical depth for point sources of r ≈ 24.5 (AB) (Ivezić, 2017). About 90% of the LSST time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg² region about 800 times (summed over six photometric bands). For 10 years of operations, the stacked observations will yield a coadded map up a limiting magnitude r ≲ 27.5. These data will result in databases including 20 billion galaxies and a similar number of stars (Ivezić, 2017). Most of the candidate quasars identified by LSST will be too faint for spectroscopic studies with 4m class telescope, yielding only a handful of telescopes from which spectroscopy could be obtained. The SDSS V should offer the equivalent to LSST in terms of spectroscopic monitoring: all-sky, multi-epoch spectroscopic survey of over six million objects (Kollmeier et al., 2017), able to reveal spectral changes on timescales from 20 min to 20 years. It will be based on two 2.5 m telescopes that should yield a sky-survey rate of ≈ 40°² per hour, and complement high-energy surveys in the soft- and hard-X domains (eROSITA and Athena, introduced below), at least for the brightest sources, i ≲ 20.

The SDSS V should still leave the faint end of the quasar luminosity function sampled by LSST. It may however serve as a testing experiment for dedicated, larger aperture telescopes. The need to cover spectroscopically faint sources (i ≳ 21) is already felt (see section 3.2.2 and e.g., the discussion by Sulentic et al., 2014), and will be even more felt in the next decades. It may become a major drive toward the development of more 30 m class and even larger aperture telescopes. Apart from costs, a 100 m telescope is already technologically feasible, and a phase I study for an “Overwhelmingly Large Telescope” was completed at ESO (ESO, 2006). A 100 m telescope working at diffraction limit of 1 milli-arcsec can reach V ≈ 37. This means the ability to sample and study objects in the Universe at the epoch of reionization with high efficiency. We are confident that one or more telescopes of the 100 m class will be eventually built around the 2050s.

The obscured Universe is an important part of extra-galactic studies. The word “obscured” refers here to the optical/UV domain that is subject to extinction by interstellar dust⁴. At present, astronomers looking at an all sky survey in the soft-X-ray domain have to relay on the ROSAT All Sky Survey (RASS) carried out over 6 months between 1990 and 1991. The RASS was tremendously successful and detected more than 60,000 X-ray sources over the whole celestial sphere. There is no all-sky survey in the hard X-ray domain above 2 keV presently available, even if archival observations with XMM and Chandra now include several hundred thousands of exposures. New-generation instruments should be able to probe obscured sources (among them most quasars) in the next decades. eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is expected to perform a deep survey of the entire X-ray sky. In the soft X-ray band (0.5–2 keV) eROSITA is expected to be about 20 times more sensitive than the ROSAT all sky survey with a resolution of 15–30 arcsec with a field of view of 0.8 deg², while in the hard band (2–10 keV) it would provide the first ever true imaging survey of the sky at those energies (Predehl et al., 2010). Athena (over two orders of magnitude more powerful than current facilities) should also provide imaging capabilities in the 0.2–15 keV energy band over a 40 arcmin² field of view with angular resolution of 5 arcsec on-axis, simultaneously with spectrally and time-resolved photon counting (Guainazzi and Athena Study Team, 2017; Rau et al., 2017). The NuSTAR (Nuclear Spectroscopic Telescope Array) focuses light in the hard X-ray domain (3–79 keV) with an angular resolution of ≈ 1′ (HPD) (Harrison et al., 2013). It is conceivable that angular resolution will be improved in forthcoming missions.

A powerful (albeit costly) strategy is to detect the track of γ-rays (as well as cosmic rays) through dedicated arrays of telescopes sensitive to the Cerenkov radiation emitted in the earth atmosphere. The Cerenkov Telescope Array (CTA), based on this approach, is expected to detect Cerenkov radiation from very high-energy (VHE) cosmic rays and γ-rays. The CTA, an array of ~100 built-for-purpose optical telescopes, should be able to cover a huge range in photon energy from 20 GeV to 300 TeV, and be a factor ~10³ more sensitive on hour timescales than the space-based Fermi Large Area Telescope (LAT) at 30 GeV. The angular resolution of CTA will approach 1 arc-minute allowing detailed imaging of a large number of γ-ray sources. CTA will be the first VHE observatory that will reach the angular resolution needed for easy cross-identification of optical counterparts (Cherenkov Telescope Array Consortium et al., 2017).

The Atacama Large Millimeter/submillimeter Array (ALMA) is an aperture synthesis array with 66 radio telescopes for sub-/millimeter astronomy. ALMA bands cover from 30 to 1,000 GHz (300 μm), with typical resolutions of a few hundredths of arcsec (Testi and Walsh, 2013). The telescope has been operational since 2011, and it will remain operational in the next decades, providing an unprecedented view of the cold Universe. Some of the ongoing and potential research will be described in the next sections. After the huge success of the FIR/mm space observatories (ISO, WISE, Spitzer, Herschel), the future might involve new telescopes for surveys such as WFIRST or mainly dedicated for spectroscopy, for example SPICA (Space Infrared Telescope for Cosmology and Astrophysics) is targeted for launch in the late 2020s. With SPICA, the goals are to reveal “metal and dust enrichment through galaxy evolution” (Spinoglio, 2016) through low-to-high resolution capabilities in the MIR/FIR spectral range. Planned FIR observatories are single dish although it is conceivable that an ALMA-like heterodyne interferometer could eventually become in part space-based.

Currently, the Very Large Array-based FIRST (Becker et al., 1995) is the main radio survey of the Northern hemisphere carried out with a resolution of about ≈ 5 arcsec. Its catalog is overwhelmingly dominated by AGNs (at a detection limit of ≈ 1 mJy), and only a small fraction of sources are star-forming galaxies. At radio wavelengths, advances in digital signal processing and cost reductions are making it possible to build arrays composed of a large number of relatively small antenna elements, giving large fields of view with large collecting areas. Most sources detectable by the next-generation surveys (which will reach sensitivities of the order of 10μJy) are expected to be star-forming galaxies, so that these surveys are dominated by the same galaxies that are studied by optical and infrared (IR) surveys (Padovani, 2017). The Square Kilometer Array (SKA) with its unprecedented collecting area and baseline, will offer improvements over the JVLA, LOFAR, for resolving power (5 milliarcsec at 6cm for the widest array configuration, 3,000 km s⁻¹), sensitivity and survey speed due to its relatively large field of view.

The RadioAstron telescope is an array of ground based telescopes with an antenna of 10m diameter in space (with a baseline of 350,000 km) that achieved the unprecedented spatial resolution of ≈ 35μarcsec at 6cm. RadioAstron is a successful effort that demonstrated the feasibility of VLBI with space antennæ. It is reasonable to think that by 2050 more sensitive arrays with more antennæ in space will be operational. Space based radio-antennæ obviously yields an extension of the baseline, but can also take advantage of a much lower background noise that allows for a much wider dynamic range (especially helpful when trying to map a boosted jet and much fainter unboosted, extended features, a typical condition in extragalactic radio sources).

A poorly sampled radio frequency domain corresponds to the meter wavelength domain. There are several radio telescopes operating at low frequency, among them the Giant Metrewave Radio Telescope (Ananthakrishnan, 2005) that covers several bands between 50 and 1420 MHz, and the Murchison Widefield Array that covers the frequency range 80–300 MHz. The GMRT completed a survey at 150 MHz with resolution ~ 25 arcsec and noise level ≲ 5mJy (Intema et al., 2017). The VLA Low-Frequency Sky Survey Redux (VLSSr) provides a Northern view at 74 MHz with resolution about 80” and sensitivity 0.1 Jy rms (Cohen et al., 2007; Lane et al., 2012). With LOFAR and SKA low-frequency arrays of unprecedented sensitivity and resolution will become operational (≈ 0.1 arcsec at 100 MHz with SKA2 and ≈ 10 arcsec for LOFAR). LOFAR exploits an array of simple omni-directional antennas in place of a dish antenna (van Haarlem et al., 2013). A space interferometer would allow to extend the frequency range down to about 0.3 MHz, a frequency domain where earth's ionosphere opacity makes ground-based observations unfeasible (Basart et al., 1997).

Since spatial changes induced by gravitational waves occur with opposing sign for orthogonal directions, a Michelson interferometer has a well-suited geometry to maximize the tiny effect on the detector. Laser interferometry is employed for motion sensing (Hough et al., 2005). The next generation of ground-based gravitational observatories will be more than ten times more sensitive than Advanced LIGO that made possible the first detection of gravitational waves ever. Several laser-interferometric telescopes are under development or planned. Of them, the third-generation Einstein telescope in Germany has the potential to dramatically increase the detection rate of gravitational wave sources. The current design posits two independent interferometers located underground with vertices separates by 10 km in a triangular configuration. With this baseline the interferometer may detect merging of intermediate mass black holes, below the smallest masses that are found in the nearest AGN, ~10⁶ M_⊙. Some of them may however be detected in the nuclei of non-active galaxies and be precursors of more massive BHs detected as AGN. LISA is expected to adopt a triangular configuration in space but the separation is planned to be 2.5 million km, making the instrument unique to cover a low-frequency domain not accessible from the ground (Figure 2). It will be necessary to wait even beyond the first generation of space-borne observatories such as LISA to detect merging super massive black holes (SMBH ≳ 10⁹ M_⊙, see Figure 2).

A complementary technique to detect low-frequency gravitational waves is to consider an array of millisecond pulsars and measure the pulse arrival times. Differences in arrival times (the timing residuals) should be correlated if produced by gravitational waves (Hobbs et al., 2010). Pulsar timing observations are used to place constraints on the rate of coalescence of supermassive black-hole (SMBH) binaries as a function of mass and redshift from the GW background in the nHz domain (Wen et al., 2011). At present the very existence of SMBH binaries is established in only a handful of cases. Larger pulsar timing arrays (i.e., involving a larger number of pulsar and a long temporal baseline will allow for the detection of a SMBH binary in a nearby galaxy, provided that the gravitational wave background can be adequately subtracted (Mingarelli et al., 2017).

The CTA will detect Cerenkov photons emitted by the cosmic rays before their first interaction with the atmosphere. While the CTA will remain state-of-the-art in the next decades for atmospheric Cerenkov CR detection, it is expected that other type of CR detectors may undergo significant developments. For instance, a large collecting area can be obtained exploiting Cerenkov radiation emitted in dense media. In this case, the detection of Cerenkov flashes is achieved by using photomultipliers submerged in water tanks over a large surface area, as in the case of “HAWC” (Carramiñana and HAWC Collaboration, 2017). Extensive air shower (EAS) arrays presently employ a variety of detecting techniques, according to the energies of the air shower particles that may include, drift chambers, scintillators and Geiger tubes, and other devices. The Auger Observatory employs a hybrid technique monitoring the sky with a fast UV-sensitive camera able to record the track of fluorescent emission (a near-UV line of sodium) associated with the interaction between the atmosphere and particle showers induced by CRs, in addition to water tanks for monitoring Cerenkov flashes.

Cosmic ray detection from extra-galactic sources has been a serious problems in the past decades. Low energy (≲ 1 GeV) cosmic rays are frequent (1 event/s/m²) and their detection straightforward with fog chambers. The cosmic ray energy spectrum (Beringer et al., 2012) is a power law that shows a steepening (a “knee”) at energy ~1PeV, and then a flattening (an “ankle”) at ~1 EeV (in the domain of ultra-high energy cosmic rays, UHECRs). Acceleration from supernovæ to energies ~1 EeV is ruled out. Around and above this energy the UHECR energy distribution is believed to be dominated by extra-galactic CRs. Since the spectral energy distribution of CRs is a steep power law, events due to extra-galactic CRs are exceedingly rare (at energies of the order of ~1 PeV, the expected flux is just 1 event/yr/m²). A major driver in the development of detectors with large collecting areas is therefore the nature of the CR with energies ≳ 1 PeV whose acceleration mechanism is unclear but believed to be at the heart of the inner working of radio jet acceleration.

Neutrinos offer a unique diagnostic of extremely high energy processes, and they can, unlike cosmic rays, travel unimpeded across the magnetic field of the Galaxy. Given the weak interaction of neutrinos with matter, large masses are needed to reveal neutrinos. Mechanisms for the production of high energy neutrinos will also produce γ rays of similar energies. γ-ray telescopes such as CTA are expected to achieve precision pointing and sensitivity to identify populations of accelerators.

A common type of design of a neutrino observatory involves an array of photomultiplier tubes housed in transparent containers which are suspended within a large tank of pure water or ice (or other suitable materials) and aimed at the detecting the Cerenkov radiation due to leptons (typically muons) or to other decay products induced by the interaction with the neutrinos. Over the years, neutrino observatories have employed larger and larger volumes placed underground, to improve the detection rate and lower the background. The IceCube Neutrino Observatory is a cubic-kilometer detector that uses ice as a medium which detects Cerenkov radiation through an array of photomultipliers. At present, IceCube has detected ≲ 100 PeV neutrinos of astrophysical origin. The PeV detections of IceCube might be substantially increased by a second-generation observatory, IceCube-Gen2 which should be based on a 10 km³ volume of ice at the South Pole (IceCube-Gen2 Collaboration et al., 2014). The large collecting volumes are needed in order to detect extra-galactic neutrinos in the PeV energy domain. They are extremely rare and likely the ones produced in active nuclei. The neutrinos detected using reconstructed muon tracks, are unambiguous tracers of hadronic acceleration, up to high redshifts and beyond PeV energies. This means that at present, data rest on several tens of neutrino detections, and that larger volumes are needed to have a good statistics of UHE neutrinos to make them of relevance for the study of relativistic jet formation physics (section 3.2.7). A high-energy neutrino background has been revealed by IceCube since 2013 (Icecube Collaboration, 2013), and blazars have been suggested as a likely source (Padovani et al., 2016). Very recently, IceCube detected an ≈ 290 TeV neutrino for which follow up γ-ray observations have made possible, for the first time, the identification with an electromagnetic counterpart, the flaring γ-ray blazar TXS 0506+056 (Icecube Collaboration, 2018).

Figure 7 depicts the early stages in the evolution of quasars from low to high redshift (0 ≲ z ≲ 6): merging and strong interaction lead to accumulation of gas in the galaxy central regions, inducing a burst of star formation. In this phase, the quasar may remain shrouded by a dense cocoon of gas and dust, opaque to UV and optical radiation. In the most extreme cases, the cocoon may be Compton thick and therefore so opaque that soft-X-ray emission can blanketed (if the column density of absorbing gas is Nc~1025 cm⁻², a source is fully absorbed up to ~10 keV, Comastri, 2004), and even emission at ~10 KeV is significantly absorbed. These sources are expected to be visible only in the hard X-ray domain, by instrument such as Nustar. The “obscured” phase may well occur after the collapse of the first seed black holes at redshift ≳10. Mass loss due to stellar winds and supernova explosions provide a large amount of enriched gas that is accretion fuel for the black hole. The accretion rate is initially expected to be very high, yielding maximum radiative output per unit mass. Radiation force and mechanical energy can then sweep away the dust surrounding the accreting black hole, at least within a cone coaxial with the accretion disk axis. An unobscured (if seen not to far from the cone's axis) quasar is born. This scheme may apply to seed quasars of masses as low as 10³⁻⁴ M_⊙ in the primordial Universe as well as to low-mass local quasars (local quasars that may be quasi-analogous of highly-accreting seed black holes have been identified since long, Mathur, 2000; Sulentic et al., 2000). In the case of primordial seeds, direct black hole collapse and enormous star formation rates may be associated with the build-up of the protogalaxy (instead of the merging phase depicted in Figure 7).

The sketch of Figure 7 identifies several key elements that are at the root of the complex physical processes in quasars:

The connection between black hole growth and the build-up and evolution of galaxies, which involves the interplay between accretion and star formation. The first side of the issue is what the physical conditions (e.g., fueling mode, triggering mechanism) that initiate major black hole accretion events should be. The second side involves the mechanical and radiative output of the quasar (understood as an accreting black hole). What is the nature of AGN feedback?

These issues are addressed by detailed studies of both nearby and distant SMBHs, and will of course benefit from the wide array of instruments providing very high spatial resolution from ground (active optics; interferometers) and space.

ALMA can locate star-formation activity hidden by dust, and identify spectroscopically the cooling of molecular clouds with primordial chemical composition. ALMA is a powerful tool with the potential of clarifying the inter-relation between star formation, metal enrichment and SMBH accretion-induced activity. As an example of an application of the ALMA data, we can consider the [CII] 158 μm line that is strong in star-forming galaxies, and is the dominant cooling mechanism for cold interstellar gas. Kimball et al. (2015) presented an analysis of an unusual [CII] emission line observed with ALMA of a very luminous quasi-stellar object (QSO) SDSS J155426.16+193703.0 at z ≈ 4.6. The line is extremely broad, with FWHM ≈ 700 km s⁻¹ and a flat-topped or double-peaked line profile. These results suggest the presence of a massive rotating disc that may be the ultimate accretion fuel for the SMBH. Findings like these are hinting at the potential of sensitive, high resolution sub-mm spectroscopy for relating gas dynamics and metal content to a quasar dark matter halo also at high redshift. X-rat observatories such as Athena should maximize the synergies with ALMA and reveal hidden nuclear activity in star forming galaxies. The issue of the relation between nuclear activity and circumnuclear star formation may be solved by the sinergy of radio, mm, and X observations which circumvent the effect of dust and provide a less biased view of the “obscured” Universe than the one we have now over a broad range of cosmic epochs. Gravitational wave observations from LISA (section 2.2.1) may reveals BHs that are invisible even in the X-ray domain because deeply enshrouded by a Compton thick cocoon. The Einstein telescope may reveal intermediate mass BHs (~10³ M_⊙) that may be the progenitor of more massive BHs, perhaps filling the gap between the stellar mass domain and the massive black hole domin.

The radio surveys with SKA (Diamond, 2008) and the next-generation VLA will play an important role of multi-wavelength studies of galactic evolution, as they will detect sources whose radio emission is associated with stellar evolution processes up to the epoch of reionization (z ≳ 6; Nyland et al., 2018). The 21 cm line will becomes detectable up to high redshift: an all-sky survey detecting ~10⁹ galaxies up to redshift ≈ 2 will become feasible with SKA (for comparison, at present there are only few detections beyond z ≈ 0.2, Abdalla et al., 2015).

The existence of a SMBH (J1342+0928, M_BH ≈ 8·10⁸ M_⊙) at redshift 7.5 (when the Universe age was just 690 Myr) reinforces black hole growth models that assume seed black holes with significant initial masses (≲ 10⁴ M_⊙) and super-Eddington accretion (Lodato and Natarajan, 2007). There is time to build up an M_BH that large without challenge the ΛCDM cosmology? Even if there has been a large delay between the previous record-holder and the last one and only ~100 quasars are known above redshift 6 (Matsuoka et al., 2018, and references therein), it is legitimate to surmise that more quasars at z > 6 will be discovered in the next decades through dedicated surveys with ad-hoc photometric bandpasses. Very high M_BH values may turn out to be more challenging for ΛCDM cosmology if they are found at higher redshift. It is not only the high mass, but also the relatively high metal content (especially iron) of the line emitting gas that needs to be accounted for (Juarez et al., 2009). Clearly, metal enrichment in the proximity of a black hole may be decoupled from the host galaxy via circumnuclear star formation. The details of the enrichment process are however still not known.

In the standard big bang cosmology, the Universe baryonic matter, following the hot phase after the big bang, should have undergone a rapid cooling, and have become mostly neutral. It may have remained so until the first accreting black holes and the first shining massive stars may have produced enough radiation to reionize it (Jiang et al., 2016; Yoshiura et al., 2017, and references therein). The evidence of reionization (or better, of the existence of a significant fraction of neutral gas able to absorb Lyα photons from quasars) was associated with a deep, contiguous absorption through leading to zero flux on the blue side of Lyα (Gunn and Peterson, 1965). The first detection of the Gunn-Peterson effect (Becker et al., 2001) started the exploration of the dark ages when the Universe was still partly neutral. We see strong evidence of the quasar Lyα emission line being absorbed by a Gunn-Peterson damping wing from the intergalactic medium, as would be expected if the intergalactic hydrogen surrounding is significantly neutral, indicating that the quasar allows to probe well within the reionization epoch (Bañados et al., 2016). The redshift frontier is now at z = 9−12, within 500 million years of the Big Bang. Lyman-break galaxies are detected in increasing number from z = 10 to z = 6, consistently with the predicted mass growth of their parent dark matter halos (Kashikawa et al., 2011). On the converse, Hydrogen Lyα emission from these early galaxies appears to decline rapidly at z > 6, which suggests that gas at early epochs is becoming more and more opaque to the Lyα photons (Stark et al., 2011).

Determining the relation of star formation (a Population III of stars) and accretion (onto a direct collapse black hole?) and reionization during early cosmic epochs will connect the first light sources to the processes that assembled galaxies after reionization. Quasars are apparently not enough for the re-ionization of hydrogen: the number of ionizing photons from the luminosity function of z ≈ 6 is apparently insufficient to keep the Universe ionized, given also that the soft X-ray background sets limits on accretion power at high redshift (McQuinn, 2016). In a cosmological context, X-rays are essential for addressing the issue, as they uniquely probe AGN at both the early heavily obscured stage and the later blow-out phase. X-rays can identify the “buried” evidence of heavily obscured black hole growth (e.g., the iron Kα line at 6.4 keV). Planned large-aperture X-ray observatories will trace the cosmic history back to the time when the first luminous sources ignited, and the subsequent evolution of galaxies and their supermassive black holes (Georgakakis et al., 2013).

Near- and mid IR spectroscopy in addition to X-ray observations (discussed in mode detail below) are crucial to understand the quasar accretion properties. JWST and E-ELT will be suitable instruments to characterize the first luminous sources, in order to reconstruct the ionization history of the early universe, and to analyze how AGN and star formation evolved from the epoch of reionization to the present day (Gardner et al., 2006).

The redshift frontiers depend on luminosity. Relatively low mass black holes radiating at modest Eddingon ratio remain undetectable at high z, with the important consequence that our view of nuclear activity evolution suffers from a bias: at intermediate-to-high redshifts (z ≳ 1), we almost fully miss a population of low-luminosity quasars (Sulentic et al., 2014). The ability to collect moderate dispersion visual spectra down to V~30 mag would allow to cover black holes down to 10⁶ M_⊙ radiating at low Eddington ratio up to z ≈ 3. The LSST and the 40m-class telescopes are needed to unveil a much more comprehensive view of nuclear activity than the one available to us until now.

The most important epoch for investigating the relation between accreting black holes and galaxies is the redshift range 1–4, when most black holes gained most of their masses and when most accretion power was released. X-rays are well-suited for studying in detail black hole feedback, although they are only one of the many spectral ranges that need to be covered to get a complete view of the phenomenon. Feedback is a process that ultimately originates in the innermost regions close to the supermassive black hole and is dominated, in terms of energy and mass flow, by material over a wide range of ionization stages. Current studies of the incidence, nature and energetics of AGN feedback are mainly restricted to the local Universe (with only very limited knowledge on the most deeply enshrouded (Compton-thick) black hole population), but systematic studies of AGN feedback to z~4 via the identification and measurement of blue-shifted absorption and emission lines in the X-ray, optical and UV domains should become possible. We still have no clear global view of the prevalence and evolution of outflows and their relation to the growth of black holes as well as of their effects on galaxy evolution. Most galaxies host a SMBH at their center, with their mass correlated with that of their host galaxy (Ferrarese and Merritt, 2000; Gebhardt et al., 2000). This correlation may suggest that the evolution of AGN and their galaxy hosts follow a parallel track (at least on a broad temporal average and for massive galaxies), although it says nothing of how this relation is built in physical term. Some self-regulating process has to connect the growth of the SMBH (accretion-powered) to the growth of the host galaxy (due to star formation) to the point of suggesting coevolution. The observational evidences of such coevolution has been reviewed by Kormendy and Ho (2013) while the theoretical motivation for the link between AGN feedback and galaxy evolution was reviewed by Fabian (2012).

A crucial process is the phase in which quasar winds “invade” the host galaxy (mechanical feedback). According to models, quasar outflow rates may reach thousands of solar masses per year at high z(Barai et al., 2018). A key physical question is: how are the energy and metals accelerated in winds/outflows transferred and deposited into the circum-nuclear medium? The energy of such powerful AGN-driven winds is deposited into the host galaxy ISM, but it is as yet unclear, for instance, under which conditions quasar winds quench or trigger star formation. Fast galactic-scale molecular outflows (e.g., Cicone et al., 2014; Zschaechner et al., 2016) are believed to be ultimately driven by nuclear activity (Morganti, 2017 and refences therein). In a spectacular case, a galactic-scale molecular outflow (Feruglio et al., 2015) sweeps away the molecular gas ultimately suppressing star formation, but there are counterexamples in which the quasar outflows trigger star formation (e.g., Ishibashi and Fabian, 2012). The most compelling evidence is now limited to a few case studies, but it is reasonable to expect that consensus on a global view may be reached by the mid of the century. A more general assessment will come from the next generation of imagining spectrographs operating in the optical and near-IR from ground (with active optics) and space, that may reach sub-0.1 arcsec resolution under ordinary observing conditions.

A second key question is: how do accretion disks around black holes launch winds/outflows, and how much energy do these carry? The answer to this question suffers because of the poor understanding of the structure and dynamics of the broad line emitting regions, within 10⁴ gravitational radii from the central black hole. Certainly, not all quasars show powerful winds able to influence the global evolution of their hosts. Some authors distinguish between wind- and disk-dominated quasars (Richards et al., 2011), a separation that is consistent with the one between high- and low- Eddington ratio quasars (Pop. A and B respectively, Sulentic et al., 2000). The observational results point toward an origin very close to the SMBH for X and UV outflows alike. Measurements in the optical and UV rest frame of the quasars are important tracers of the outflow, but they are only part of the story. In the X-ray bands we observe narrow absorption lines outflowing with moderate velocity of hundreds to few thousands km/s (Halpern, 1984; Longinotti et al., 2015). The warm absorber is detected in ≈ 50% of AGN (Reynolds, 1997; Piconcelli et al., 2005). In the UV band, narrow-absorption lines may be associated with the warm absorber and broad absorption lines are seen in ~20-40% of AGN, and may be present with extreme properties (terminal velocity 0.1–0.2 c) in most Population A AGN (Sulentic et al., 2006; Trump et al., 2006; Scaringi et al., 2009), but detected only if the outflowing material intercepts the line of sight. They probably arise in a radiation driven wind from the accretion disk (Elvis, 2000; Proga and Kallman, 2004) that seems to be a widespread and powerful phenomenon at least among Population A sources (e.g., Sulentic et al., 2007). However, most impressive outflows may appear from gas so highly ionized that the only bound transitions are for Hydrogen- and Helium-like iron. These X-ray winds are relatively frequent at low-z, with a prevalence ≈ 30–40% of local AGN, and outflow velocities reaching 0.3c [Ultra-Fast Outflows (UFOs); Tombesi et al., 2010]. The evidence supporting UFOs has been growing (e.g., Longinotti et al., 2015). Originally it was based on faint absorption features of unclear identification, and UFO phenomenology is as yet poorly known, but the point here is that the mechanical feedback effect from the SMBH involve gas in widely different physical conditions. A multifrequency analysis involving data from sub-mm to the soft and hard X-ray domain is needed to gain a full understanding of a multi-phase, likely highly-turbulent medium.

Even if there is agreement about the existence of an accretion disk and convincing evidence of outflows, the launching mechanism and the physical processes involved are only crudely understood today. Significant progress should come not only from multi-frequency simultaneous observations (optical, UV, and X) but also from SPH hydrodynamics simulations that are expected to improve in numerical sophistication and in the treatment of physical processes (see e.g., Sa̧dowski et al., 2014; Liska et al., 2018).

Variability at all wavelengths is one of the defining properties of AGN. The most rapid variations in γ-rays are on the scale of only a few minutes. The very rapid variability of flares puts strong constraints on the size of the emitting region and its bulk velocity due to light crossing-time arguments. However a fundamental question such as: “what causes the observed variability in AGN from time scales of a few years down to a few minutes?” remains without convincing answers at present. As for the analysis of single-epoch spectra of individual quasars, the potential of spectral variability to constrain quasar models has not been sufficiently explored. The planned “panoptic” SDSS-V is intended to exploit this potential on wide scales. The SDSS-V plans to do spectroscopic reverberation mapping sampling hundreds of epochs for ~10³ quasars (0.1 < z < 4.5) and Lbol~1045-1047 erg/s. This is a 10-fold increase in the number of sources monitored until now. With a more modest number of epochs (a few to a dozen per target), the SDSS-V will also characterize the optical spectral variability of approximately 25,000 quasars. Implications on the SMBH accretion disks, dynamical structure in the broad line region (BLR) including the optical UV outflows, and especially on signatures of binary BHs might be far reaching, perhaps yielding predictive ability on spectral variations of AGN, from broad band luminosity to emission line profiles. Optical spectrographs planned at major telescopes should bring similar scales to other spectrographic surveys.

Changing-look AGN, in which the broad lines in the AGN spectra either appear or disappear (i.e., passing from type-1 to type-2, or viceversa), an extreme case of line profile variability (LaMassa et al., 2015; Yang et al., 2017) may pose that challenges to standard accretion disk theory (Lawrence, 2018). In this respect, the most interesting sources are the ones hinting at a periodic behavior that in turn may suggest the presence of a sub-pc SMBH binary (Bon et al., 2012; Graham et al., 2015). We are forced the word “hint” for both the observational data and inferences, as the temporal baseline is still not long enough to confirm the periods excluding the “red noise” typical of nearby AGN light curves (Vaughan et al., 2016, note that the “red noise” behavior may not be stochastic but due to processes occurring on timescales comparable to the monitoring temporal baseline). The inference of a SMBH binary is bound by several assumptions at present rather ad hoc (Li et al., 2016).

The new capabilities offered by multiplexing spectrographs as well as a significantly longer temporal baseline for the monitoring (periods—often corresponding to the dynamical timescale of the BLR—are of the order of tens of years) will likely lead to an assessment of the prevalence of supermassive binary black holes in the local and in the remote Universe, as well as of the interplay magnetic fields/viscosity/turbulence in accretion disk. Astrometric measurements with resolution ~1μarcsec have the potential to resolve sub-pc binary black hole systems up to high redshifts.

The gas in the accretion disk may lose up to almost half of its energy within 1,000 gravitational radii, resulting in powerful UV and X-ray emission. The strong gravity field implies that general and special relativity effects are detectable from the emitted radiation not only in the hard X-ray domain but in the optical and UV as well. The close proximity to the event horizon is where differences in the spacetime metric due to the black hole rotation become appreciable. Lense-Tirring precession may be at the origin of warped disks in the case the angular momentum of the accreting material is misaligned with the spin angular momentum of the black hole (Bardeen and Petterson, 1975). Effects on optical and UV emission line profiles are expected (Bachev, 1999) although their assessment requires large samples of high-quality spectroscopical data. The DR 14 of the SDSS now lists over half a million quasars (Pâris et al., 2017) and millions of quasars are expected to be discovered and observed by Euclid. The potential of large samples of spectroscopic data are largely unexploited. From single-epoch spectra a wealth of information can be retrieved considering the trends along the quasar “main sequence” (Sulentic et al., 2000), including an estimate of the BLR size, physical and dynamical conditions, and chemical composition of the BLR gas (Negrete et al., 2012, 2013). The methods are in place but still not widely applied also because of the lack of high S/N data that are however expected to grow in availability in the next decades. We expect that the synergy between reverberation mapping and systematic inter-comparison of optical and UV emission lines coming from ionic species of low- and high-ionization potential for large samples of individual spectra organized along the main sequence yields contextualized inputs leading to full physical and dynamical models of the broad line emitting region, as a function of black hole mass, Eddington ratio, spin, and environment (Marziani et al., 2018). This issue—conceptually analogous to the definition and interpretation of the stellar Hertzsprung-Russell diagram—has been an unremitting problem of quasar research that has remained basically unsolved since quasar discovery (see D'Onofrio et al., 2012; Sulentic et al., 2012).

The ISCO of the accreting gas also depends on the black hole spin, being closest to the black hole for maximally rotating black holes. In other words, the determination of the ISCO is a key endeavor because it is an indirect measurement of the black hole spin. This has important consequences for the radiation emitted by the accretion disk which is hotter in the case of high spin (Wang et al., 2014a). It is hard to conceive a black hole that is not spinning at all (everything in the Universe spins, even cometary nuclei or asteroids!), but how many are spinning close to the maximally rotating case? How does the spin evolves with cosmic time? We think these questions are going to find an answer as the next generation of X-ray observatories will be launched. Much is needed even after successful missions such as XMM. Kα line profile—associated with reflection from the accretion disk—remains to be observed in most quasars. While ASCA detected the extended wing in the Kα profile associated with gravitational and transverse redshift (Tanaka et al., 1995), XMM spectra are revealing additional complexity in the Kα profiles that may not be associated with relativistic effects. Several methods have been proposed to estimate the ISCO; the one that has proved viable at least in a large fraction of cases is the Kα profile modeling, but other methods are possible as well: for example, if quasi periodic oscillations are associated with the ISCO angular frequency (Brenneman, 2013). X-ray observatories that are expected be in service after 2025 may provide a view of the spin and ISCO distributions for supermassive black holes over a broad range of z.

LISA design is suited to detect the signal for coalescing black holes with masses ≲ 10⁶ M_⊙ in the source frame up to z ≈ 9, and to enable the measurement of the dimensionless spin of the largest M_BH with an absolute error better than 0.1. It would allow the detection of spin misalignment with the orbital angular momentum with a precision better than 10 degrees (Danzmannet al., 2017). These multimessenger abilities should give an unprecedented view of the obscured populations well within the dark ages that preceded full reonization i.e., beyond redshift ≈ 6. Within reach in the next 30–40 years (after LISA) is even the detection of SMBH merging with space-based interferometry (a feat that could be obtained even earlier with the pulsar timing arrays, Mingarelli et al., 2017).

In radio-quiet quasars, X-rays are produced by Comptonization of thermal disk photons in a hot corona. Among radio-loud quasars, photons from the radio jet also contribute as seeds in the inverse Compton scattering process (Pian et al., 1999; Böttcher et al., 2013; Bottacini et al., 2016). Part of the resulting power-law continuum illuminates the disk, where it is reprocessed and reflected, both in the soft and hard X-ray domain, yielding a Compton hump peaking at energy ϵ ≈ 40 keV and Fe Kα emission at 6.4 KeV. The disk may not always extend down to the ISCO; in the Magnetically Arrested Disk (MAD) configuration, the disk is truncated and the truncation makes it possible to launch relativistic jets (Punsly et al., 2009; Rusinek and Sikora, 2017). The next generation of X-ray observatories should enable measurements of reflection features (Compton hump and Kα line), and allow us to measure the ISCO and spin (from the ISCO) of black holes much beyond the local Universe. This feat should be achieved by measuring the general relativistic effects predicted on the Kα line profile, most notably the gravitational redshifting of the line base that is sensitive to the ISCO value.

It is not clear what the hot corona might actually be: a compact “sphere” (as it is often modeled) or clumps above the disk that illuminate the disk? How does the corona depend on the accretion status of the SMBH? The time lag between changes in primary radiation emission from the corona and the reprocessed emission from the disk provides a tool to measure the distance between corona and illuminated disk, as in the case of optical and UV reverberation mapping. Results on the Kα response of several nearby AGN are already available (Kara et al., 2016) and is presumable that they will be extended in the coming decades. The determination of the disk-corona geometry via transfer function fitting requires data of much better quality than presently available (Dovciak et al., 2013). This is a task meant for X-ray observatories such as Athena, with the possibility of long uninterrupted observations on its orbit at L2. Athena should maximize the synergies with the LISA gravitational wave observatory, notably in locating the mergers of massive and seed black holes expected to be detected by LISA.

The radio source Sgr A^* at the center of the Galaxy, and interpreted as the radio emission from a supermassive black hole, shows a compact and diffuse morphology at high radio frequencies above 200 GHz. The prediction is that a global VLBI array of radio telescopes, the global Event Horizon Telescope, could detect the shadow of the event horizon (Falcke, 2017). Ultimately, a space array at THz frequencies, the Event Horizon Imager, could produce much more detailed images of black holes that may allow for a measurement of the ISCO (and hence of the black hole spin). There is no doubt about the large concentration of mass in a restricted volume of space (e.g., Peterson and Wandel, 1999), but the existence of an event horizon has been questioned also on theoretical grounds in the context of quantum gravity. Gravitational wave detections from black hole merging events may finally distinguish between black holes and two main competitors: gravastar (Mazur and Mottola, 2004) and dark-energy stars (Chapline, 2005), classes of compact objects that are innerly sustained by the negative pressure of dark energy, and that avoid some of the paradoxes associated with the black hole singularity and event horizon.

Radio-loud AGN are producing collimated relativistic outflows by a still poorly-understood process. Acceleration occurs extremely close to the SMBH (to explain remarkably short variability timescales), within a few tens gravitational radii, but what are the sufficient conditions for an efficient accelerations to ultra-relativistic speed (Lorentz factors ≳ 10 – 100)? This may indicate indicate a Lorentz factor much larger than previosly thought, or hadronic acceleration. Very and ultra high energy (VHE and UHE) observations are the best tool to probe the physics of jet formation and the interaction of the black-hole magnetosphere with the accretion disk corona.

The SED of bright blazars is well explained by leptonic emission scenarios, where the radiative output throughout the electromagnetic spectrum is assumed to be dominated by electrons and possibly positrons (Celotti and Ghisellini, 2008). Radio-astron observation of the jet base of powerful radio sources (Kardashev et al., 2015) with resolution of 20μarcsec isolated the “nozzle” at a brightness temperature T~10¹⁵ K, a very high temperature in excess to the Compton temperature. A very rapid variability of the high-energy flux (on timescales of a few minutes) as well as the temperature revealed by RadioAstron require extremely high Lorentz factors which are challenging for leptonic models (Böttcher et al., 2013). In hadronic models, both primary electrons and protons are accelerated and produce VHE photons in the γ domain. Protons however exceed the threshold for p-γ photo-pion production, leading to high-energy emission due to several processes involving synchrotron and Compton emission decay products of pions which include the highest energy neutrinos (Kun et al., 2018), among other particles. The synergy between γ-ray such as CTA and neutrino observatories should yield insight on this issue, which is ultimately about the workings of the mechanism that is extracting energy from the black hole spin (Blandford and Znajek, 1977). Alternatively, the energy could be extracted by the rotating accretion disk (Blandford and Payne, 1982). In both cases, it is not clear how an ordered magnetic field can be transported down to a few tens of gravitational radii. The increase in computing power as well a the improved constraints on jet acceleration (a proton component? is the jet heavy?) from γ-ray and neutrino observation may lead to a more detailed description of magnetic field transport over a broad range of spatial scale and of the ultimate energy source of relativistic ejecta. Understanding the momentum flow of relativistic ejecta is related to the effects that they may have on the host galaxy. And CTA may unveil that γ-ray sources are much more frequent than inferred from present detection rates (Costamante et al., 2018).

Radio-loud quasars are one of the likely sites of the acceleration of UHE CRs, with energies up to around 100 EeV. γ-ray and neutrino observations also allow to search for UHECRs. γ-ray imaging observatories such as the CTA are expected to explore with unprecedented sensitivity the γ rays in the energy range from 50 keV-2 MeV which are the best tracers of CRs: low event statistics and deviation of charged particles in extra-galactic and Galactic magnetic fields make it difficult to direct search for UHECR sources. The γ-ray observations are expected to identify beacons (the γ RL quasars) that track the cosmological evolution of black holes down to the epochs of galaxy formation. Gravitational wave, γ and hard X-ray observations could provide a solution of the long standing problem of the energy source of reionization, and of the role of accreting black holes in the formation of protogalaxies.

The quasars spatial distribution has been used as a tracer of large scale structures and BAOs (e.g., Zarrouk et al., 2018, and references therein). The array of optical/IR instruments described in section 2 should provide an enormous gain in statistics (at least two orders of magnitude) of optical SN Ia measurements, allowing the extension of the SN Ia Hubble diagram, but only up to z ≈ 1.5 (Hook, 2013). Also the DESI instruments may not go beyond 3 Gpc. Therefore, a large fraction of the early cosmic epochs will be left uncharted.

Quasars have a tremendous potential for cosmology, but their potential is as yet unexploited since they are not standard candles in conventional terms. Early efforts to establish correlations between luminosity and one or more parameters (for example, the equivalent width of high-ionization lines, the so-called Baldwin effects, Baldwin et al., 1978) did not live up to cosmological expectations (some early and some recent attempts are reviewed in the Chapter by Bartelmann et al., 2009 D'Onofrio and Burigana, 2009). Nonetheless, in the last few years several methods have been proposed for the use of quasars as redshift-independent distance indicators, or as standard rulers. Four methods are summarized in Table 1, and some of them are widely discussed by Czerny et al. (2018). Their aims are to provide independent measures of the cosmic density of matter and dark energy Ω_M and Ω_Λ, as well as to gain constraints on the equation of state of dark energy, and to test alternative cosmological scenarios. Perhaps an accelerating Universe is not anymore an issue, but the nature of dark energy yielding a non-zero Ω_Λ is still mysterious. We choose to focus on these methods since they represent novel lines of investigations which rely on an improved understanding of quasar inner structure and all have the potential to constrain the geometry of the Universe in the redshift range 2 ≲ z ≲ 4, a still-uncharted territory. Needless to say, these methods still need years of testing before they may provide useful data, and not all of them may turn out to be feasible. The main parameters, the basic relations and the sources for which the methods are applicable are summarized in Table 1.

At very high accretion rate, the luminosity-to-black hole mass ratio (L/M_BH ∝ L/L_Edd) tends toward a well-defined value (more precisely it grows with the logarithm of the mass accretion rate). The resulting “slim” accretion disk is expected to emit a steep soft and hard X-ray spectrum, with hard X-ray photon index (computed between 2 and 20 KeV) converging toward Γ_hard ≈ 2.5. The steep slope of their hard X-ray spectrum allows for the identification of super-Eddington accretors (Wang et al., 2013, 2014b). A challenge in this case is the sample size, but the next generation of X-ray instruments could make possible to identify sizable sample of extremely accreting quasars.

Highly accreting quasars can be considered as “Eddington standard candles:” L/L_Edd∝L/M_BH, so that, if M_BH can be retrieved under the virial assumption, an estimate of the luminosity becomes possible (La Franca et al., 2014; Marziani and Sulentic, 2014, Table 1). This approach is conceptually analogous to the use of the link between the rotational velocity of virialized systems such as the disks of spiral galaxies and their luminosity (Tully and Fisher, 1977): in both cases, L∝δv⁴. In the case of quasars, δv is provided by the FWHM of Hβ or Paα. An explorative analysis confirms the conceptual validity of the quasar “virial luminosity” estimates, although the scatter on the distance modulus is still too large to draw meaningful inferences for cosmology (Negrete et al., 2017). Figure 8 shows that the scatter is no match for supernovæ, being around 0.3 dex rms. A deeper understanding of the dynamics and physical conditions of the BLR of extremely accreting quasars may yield models of line broadening that may in turn improve the accuracy and precision for cosmology.

The BLR size has been suggested as standard ruler, in a way that is conceptually analogous to the BAOs. The cross-correlation function between the continuum and the emission line light curve measures a time lag τ, meaning that the distance of the BLR from a central continuum source can be written as: r_BLR = cτ. If rBLR=cτ∝L (Bentz et al., 2013, and references thererin; see also Kaspi et al., 2005), the ratio τ/f5100 (where f_5,100 is the continuum flux at 5100 Å) is proportional to the luminosity distance that depends on the cosmological parameters H₀, Ω_M, Ω_Λ(Watson et al., 2011; Czerny et al., 2013; Melia, 2015). This method may yield important results in the next decades thanks to the SDSS-V and to other reverberation mapping campaigns that exploit the increasing multiplexing and wide field coverage of the forthcoming generation of spectrographs.

The non-linear relation between soft X-ray and UV has also been used to build a Hubble diagram (Risaliti and Lusso, 2015). Much is dependent on our understanding of high-energy continuum emission in quasars that is, as outlined above, not yet satisfactorily modeled. However, the data coming from the next surveys and advancements in our understanding in quasar structure may yield significant constraints for cosmology that are independent from all methods used to-date.