Some of the more efficient accretion processes in astrophysics are associated with the presence of accretion disks. Such disks are found in protostars; in various types of binary stars, including low- and high-mass X-ray binaries; in dwarf novae or cataclysmic variables; and in classical novae. They are also believed to be present in galactic nuclei, around the central supermassive BH, during periods of fast accretion.

Accretion disks in galactic centers are naturally formed by infalling gas that sinks into the central plane of the galaxy while retaining most of its angular momentum. The assumption is that the viscosity in the disk is sufficient to provide the necessary mechanism to transfer outward the angular momentum of the gas and to allow it to spiral into the center, losing a considerable fraction of its gravitational energy on the way. The energy lost in the process can be converted into electromagnetic radiation with extremely high efficiency, from about 4 percent and up to 42 percent. It can also be converted to kinetic energy of gas, which is blown away from the disk, or in other cases, it can heat the gas to very high temperatures, which causes much of the energy to be advected into the BH.

AGN disks, and accretion disks in general, are classified according to their shape into thin, slim, and thick disks. Each one of these can be optically thin or thick, depending on the column density (or surface density) and the level of ionization of the gas. The optical depth of AGN disks, during periods of fast accretion, is very large. The disks that receive most attention are optically thick, geometrically thin accretion disks. Such systems are easier to treat analytically and numerically. The next section describes the basic assumptions and the analytical solution of such disks. A full solution of this type can be used to calculate the emergent disk spectrum and to compare it with observations. The additional sections address other types of accretion disks and accretion flows.

The physical properties of the gas in AGNs depend on its location, density, column density, and composition. These determine the ionization parameter, the relative importance of gravity and radiation pressure force, the dust content, the gas velocity, and more. In this chapter, we consider several possible locations around the central BH and the gas and dust properties in each.

The broad-line region

Consider large-column-density (~ 1023 cm−2), high-density (~ 1010 cm−3) clouds situated around an accreting BH with L/LEdd~ 0.1, at alocation where L/4πr2≃ 109 erg s−1 cm−2. For a very luminous AGN, this is at about 0.1–1 pc from the BH. Assume also that the clouds can survive over many dynamical times either because they are confined or because they are the extensions of large self-gravitating bodies such as stars. For large enough column density clouds, the system is bound because gravity completely dominates over radiation pressure force. The typical Keplerian velocity at this location is ~ 3000 km s−1, which will be reflected in the widths of the emitted lines. We also assume a global (4π) covering factor of order 0.1. This means that we can neglect the effect of the radiation emitted by one cloud on its neighbors.

The above physical properties result in Uhydrogen ~ 10−2. This means that only the illuminated surfaces of the clouds are highly ionized. The most abundant ions in the ionized parts are He II–III, 0 IV–VI, C III–IV, and so on. The strongest predicted emission lines are, therefore, Hα,Lyα,C IV λ1549, and O VI λ1035.

Observations of host galaxies

The earliest discovered quasars, in the 1960s, were so bright that their optical images showed no signs of the host galaxies. This resulted in the names quasistellar objects (QSOs) and quasars and caused a lot of confusion and even some unusual explanations and models. Interestingly, the much earlier discovery of the first Seyfert galaxies, by Seyfert in 1943, raised no such questions. The luminosity of the central sources in these galaxies was about 2 orders of magnitude below the luminosity of the first discovered quasars, and the galaxy was clearly seen in all cases.

The host galaxies of the earlier quasars were soon discovered by ground-based telescopes in sites with good seeing conditions. Faint nebulosities were discovered in all objects with redshift less than about 0.5, where conditions allowed such detection. The launch of the Hubble Space Telescope (HST), in 1990, resulted in superb resolution observations and the extension of such studies to redshifts 2 and 3. Some of the information was detailed enough to enable a systematic study of the morphology, color, and even stellar population of the hosts. Such information is now available for numerous low-redshift AGNs where the study is made easier because of the lower luminosity of low-z AGNs and the vast improvement in the performance of adaptive optics (AO) systems on giant ground-based telescopes. Similar observations of type-II AGNs, where the optical–UV radiation of the central source is completely obscured, allowed us to extend such studies to more sources and to higher redshift. Figure 8.1 shows several examples of the hosts of luminous type-I AGNs observed with the HST. Several such hosts show clear signs of distortion and interaction.

The names “active galaxies” and “active galactic nuclei” (AGNs) are related to the main feature that distinguishes these objects from inactive (normal or regular) galaxies: the presence of supermassive accreting black holes (BHs) in their centers. As of 2011, there were approximately a million known sources of this type selected by their color and several hundred thousand by basic spectroscopy and accurate redshifts. It is estimated that in the local universe, at z ≤ 0.1, about 1 out of 50 galaxies contains a fast-accreting supermassive BH, and about 1 in 3 contains a slowly accreting supermassive BH.

Detailed studies of large samples of AGNs, and the understanding of their connection with inactive galaxies and their redshift evolution, started in the late 1970s, long after the discovery of the first quasi-stellar objects (hereinafter quasars or QSOs) in the early 1960s. Although all objects containing active supermassive BHs are now referred to as AGNs, various other names, relics from the 1960s, 1970s, and even later, are still being used. Some of the names that appear occasionally in the literature, such as “Seyfert 1 galaxies” and “Seyfert 2 galaxies,” in honor of Carl Seyfert, who observed the first few galaxies of this type in the late 1940s (see Chapter 6 for a detailed discussion of the various groups), are the result of an early confusion between different sources that are now known to have similar properties.

Of the many questions addressed in this book, some are still open and require more attention. Answering these questions will lead to a much improved understanding of active and dormant BHs, of the physics in the various regions of AGNs, and of the complex evolutionary connections between massive BHs and their host galaxies. The questions that are considered to be more important are arranged subsequently in four large categories. Most of these issues were discussed in the previous chapters, and the list is only intended to serve as a reminder of the outstanding questions in this area of research and of the direction in which the field is going.

Questions related to the central power house

Black hole mass and spin

BH mass measurements in local type-II AGNs are based on the M–σ* relationship and the known relationships between bulge luminosity and BH size. For type-I AGNs, the estimates are based on RM-based measurements of RBLR and the assumption of virialized BLRs. As of 2011, Hβ-based RBLR estimates are available for about 40 sources, and it is not at all clear how well this sample represents the various types of AGNs, for example, radio loud versus radio quiet, high versus low luminosity, and high versus low L/LEdd. The large, yet limited range of Lbol requires extrapolation to reach the highest-luminosity AGNs. This limits the accuracy of BH mass estimates in the most luminous AGNs.

The redshift sequence of AGNs

AGN are now detected with ground-based telescopes all the way to z ~ 7. This is done by observing thousands of sources discovered by large surveys like SDSS and 2DF. Arranging the objects according to their redshift, like in Figure 9.1, clearly shows the redshift progression of absorption by intergalactic neutral hydrogen starting at an observed wavelength of (1 + z)1215 Å. At z = 7, this is close to the long wavelength limit of ground-based spectroscopy. Detailed study of such sources, and measurement of their mass and accretion rate, can be used to follow BH evolution through cosmic time and to compare it with the evolution of dark matter halos and galaxies. The main tools that are used for this study are the luminosity and mass functions of AGNs. Before investigating these functions, we review, briefly, some aspects of galaxy evolution.

Highlights of galaxy evolution

Hierarchical structure formation

The most successful cosmological model of today is the Λ cold dark matter (ACDM) model with its three ingredients: dark energy, dark matter, and bary-onic matter. Support for this model comes from measurements of the acceleration of the universe, from observations of the cosmic microwave background (CMB), from the abundance of the light elements, and from several other observations. The success of the model in explaining the observed temperature fluctuations at the recombination era, about 380,000 years after the Big Bang, when the radiation and baryon fluids stopped interacting with each other, is perhaps its main strength.

BEFORE passing to an account of Jeans's technical and scientific achievements, it may be of interest to sketch the background of science in the days of his boyhood, more especially as Jeans himself was to write, at the age of fiftysix, a volume entitled The New Background of Science. And, as Jeans devoted the last eighteen years of his life to popular exposition, the best way of doing this would appear to be to take a brief survey of the state of popular science in the last half of the nineteenth century.

Let us take the lectures and writings of two renowned expositors of physics, Helmholtz in Germany and Tyndall in England.

The Popular Lectures on Scientific Subjects of Hermann von Helmholtz were published in an excellent English translation in two volumes in 1893. They consisted of addresses on sundry formal occasions, delivered to educated but not specialist audiences, and covered ground to which Helmholtz himself had made notable contributions. One group of addresses was concerned with the first law of thermodynamics (as it is now called), namely, the law of conservation of energy or, as Helmholtz termed it, the law of conservation of force. This great generalization appealed strongly to Helmholtz. It was the subject of his Carlsruhe address of 1862, ‘On the Conservation of Force’; he had emphasized it in his Konigsberg address of 1854 ‘On the Interaction of Natural Forces’, and he was to dwell on it again in his Innsbruck address of 1869, ‘On the Aim and Progress of Physical Science’. The possibility that the law of conservation of energy applied to all forms of energy had been outlined by Julius Robert Mayer in 1842; but it was the experiments of James Prescott Joule, published in 1843, which first established the strict equivalence of heat and mechanical energy. Joule's classical paper on this subject was dated 1849. It is evident from Helmholtz's insistence on the importance of this law that its power and generality were not fully realized by the audiences which he was addressing: It was necessary for him to pile example upon example.

IT was shown in 1861 by G. Kirchhoff that in an enclosure at temperature T, the state of the field of radiation depends only on this temperature T, and does not depend on the optical properties of the substances that happen to be present in the enclosure. This state of radiation is called complete or equilibrium radiation, or black-body radiation. It was one of the primary objects of theoretical physics in the nineteenth century to determine this characteristic state of radiation by calculation.

In the preceding paragraph I have stated the broad facts, so that the reader may see the issue. To give these facts their quantitative form, certain refinements of statement are needed. What Kirchhoff actually showed, by means of thermodynamic arguments, is as follows. Let the enclosure contain substances capable of emitting and absorbing radiation of energy frequency v. At any point P in the enclosure, let the specific intensity of radiation for frequency v be Iv; that is to say, in a short time dt through an element of area dS containing P, in a cone of directions of solid angle dω making an angle with θ the normal to dS, the flow of energy is taken to be IvdvdtdS cos θ dω, dv being a small range of frequencies surrounding v. Further, let kv be the absorption coefficient of the material at P, jv the emission coefficient of the same material. These statements mean that a beam of radiation of intensity Iv traversing a thin layer of the material of thickness dl is weakened by the amount dIv = — kvpIv dl, where p is the density; and that the emission of radiant energy from a small element pdv of volume dv in time dt in directions included in dω is jvp dv dt dω.

From dream to reality

Are there inhabited worlds elsewhere in the Universe? The question is as old as humanity. We can trace such debates back to antiquity, in texts written by Greek philosophers such as Epicurus (341–270 BCE) in particular. At the time of the Copernican revolution, a new dimension was reached, this time more on astronomical and physical grounds: since the Earth was no longer seen as the centre of the Universe, other planetary systems could exist around other stars. Giordano Bruno (1548–1600) was among the first to express his support for this new astronomical theory, in opposition to the Catholic church, a conviction for which he paid with his life. Many scientists such as Galileo (1564–1642) and Huygens (1629–1695) supported this hypothesis. Closer to our times, philosophers such as Fontenelle (1657–1757) and Kant (1724–1804), scientists such as Laplace (1749–1827) and later Flammarion (1842–1925) raised the question of the plurality of worlds.

The search for planets around other stars – also called ‘extrasolar planets’ or ‘exoplanets’ – did not start in earnest, however, until the twentieth century, because of our inability to observe them. Indeed, it is extremely difficult to detect the intrinsic visible light of such a planet, hidden in the blinding brightness of its host star, which is about ten million times brighter. Imaging extrasolar planets directly, in a few very favourable cases, has only become possible during the past decade, thanks to the development of techniques such as coronagraphy (which blocks light from the centre of a telescope in order to image the fainter surroundings) and adaptive optics (see Subsection 2.4.3). During the twentieth century, indirect methods had to be developed. The idea is the following: the light of the exoplanet is too weak to be detectable, but the presence of the planet induces a small motion of the host star around the centre of gravity of the combined star–planet system. The first method used by astronomers to detect this motion was astrometry, the measurement of stellar positions relative to their background. It was successfully applied by Bessel (1784–1846) who first detected a low-mass companion around Sirius A, the brightest star in our skies. The companion turned out to be a white dwarf, named Sirius B. A century later, the same technique was used to search for exoplanets.

The search for habitable worlds in the Universe entails our understanding of the conditions in which life appeared, survived and developed on Earth. This understanding has been growing consistently since the first geological, atmospheric, oceanographic and biological studies. As stated in The Limits of Organic Life in Planetary Systems, put together by the Committee on the Origins and Evolution of Life of the National Research Council (NRC, 2007):

Our planet is not blessed everywhere with conditions favourable to human life, but in spite of the harsh and extreme chemical and temperature ranges that living species have to deal with, we have proof today that life thrives on Earth wherever liquid water and energy sources are available. However, other lifeforms may well exist, as has been suggested by some scientific studies. In what follows in this chapter we try to give an overview of terrestrial life and what it requires, touch upon other possibilities and focus on the environmental conditions necessary for the sustainability of life of the standard definition (Earth-like), before we begin our trip across the Solar System and elsewhere in quest of habitable places.

MY first meeting with Jeans was, I think, in 1912 in Charles Sayle's house in Trumpington Street. I was then quite a junior member of the staff of the University Press and Jeans was little more to me than the author of some of those big, blue mathematical books with which I was beginning to be familiar in the Syndics’ catalogue. When I returned to the Press after the 1914 war, I began to realize more clearly his importance as an author, but it was not until I became Secretary in 1922 that I had personal dealings with him. Reprints and new editions of his earlier books involved a certain amount of discussion and correspondence, but it was the publication of Astronomy and Cosmogony (1928) that led me into more intimate talk with him. I remember very clearly Ralph Fowler coming in to my room at the Press and asking me whether I had read Jeans's latest book. I took the enquiry to be a jocular one and reminded Fowler, in reply, that I was not obliged to read every book that I published. Then, more seriously, Fowler said: ‘Ah, yes, but you should look at the last chapter.’ It was good advice and I realized, especially after promptings from my colleague, R. J. L. Kingsford, that cosmogony might contain the potentialities of best-selling beyond the dreams of academic avarice.

At that time I frequently travelled by road to Worthing, where my parents lived. Jeans's home at Dorking was only a few yards off the main road and accordingly I proposed myself for lunch on a day when I was due to go to Worthing. It was the first time I had seen Jeans at home and he gave me a most friendly welcome. He produced an admirable claret and after lunch we retired to his study. After a few preliminary pourparlers, I approached my main topic and asked Jeans whether he would consider the writing of a popular book. His reply was characteristic. Looking at me with a kindly but slightly scornful expression, he said: 'Oh, yes, several publishers have approached me about that.’ ‘Well’, I replied, ‘what about us?’ ‘Oh’, he said ‘you're the finest mathematical printers in the world—but you couldn't sell a popular book.’ ‘Well, have you ever written one?’ I countered.

Looking for habitable conditions in the outer Solar System leads us to the natural satellites rather than the planets themselves. Although the theoretical conditions under which life might be sustained on natural satellites are similar to those of planets, there are key environmental differences which can make moons of particular interest in the search for extraterrestrial life. The gaseous giant planets cannot provide even the minimal conditions of a surface or interior with suitable pressures and temperatures to sustain life. But the moons around these planets offer a great range of possibilities for exploring habitability conditions and furthermore studying the question of the emergence and evolution of habitable worlds in our Solar System, in some cases more so than any other object closer to the Sun. Scientists generally consider the probability of life on natural satellites within the Solar System to be remote, though the possibility has not been ruled out.

Within the Solar System’s traditional habitable zone, the only candidate satellites are the Moon, Phobos and Deimos, and none of these has an atmosphere or water in liquid form. But, as discussed in Chapter 2, the habitable zone may be larger than originally conceived. The strong gravitational pull caused by the giant planets may produce enough energy to sufficiently heat the cores of orbiting icy moons. This could mean that some of the strongest candidates for harbouring extraterrestrial life are located outside the solar habitable zone, on satellites of Jupiter and Saturn. The outer Solar System satellites then provide a conceptual basis within which new theories for understanding habitability can be constructed. Measurements from the ground and also from the Voyager, Galileo and Cassini spacecraft have revealed the potential of these satellites in this context, and our understanding of habitability in the Solar System and beyond can be greatly enhanced by investigating several of these bodies together.