Active and dormant black holes in galactic nuclei

BHs of all sizes are very common in the universe. There are numerous known stellar-sized BHs in our galactic neighborhood, and in several nearby galaxies, with masses in the range 3–30 M⊙. These are the remnants of core collapse in type-II supernovae (SNs) with very massive progenitors of at least 20 or perhaps even 30 M⊙. Such objects are found in binary systems that are also strong X-ray sources, probably the result of accretion onto the BH. A direct causal connection between the SN explosion and the remnant BH has been established in several cases. A well-known example is the strong X-ray source in M100 with a location that coincides with that of a 1979 SN explosion (SN 1979C). As of 2011, this is the youngest known BH.

Active supermassive BHs, in galactic centers, have been known since the early discovery of QSOs in the 1960s. However, the idea that most galaxies, especially those with dynamically relaxed bulges, contain dormant supermassive BHs in their centers took much longer to develop. Detailed studies of the stellar velocity field and gas motion in about 60 nearby galaxies suggest the existence of such objects. Moreover, there seems to be a strong correlation between the mass of the bulge, its luminosity, and the mass of the BH. Because the physical sizes of the two masses are very different, and the ratio of masses is very large (about 100—4000; see Chapter 8), it is difficult to find a mechanism that will link the two. Understanding these relationships has become an area of intensive research in astronomy and is discussed later in this book.

The field of active galactic nuclei (AGNs) is exploding. From a narrow discipline dealing with massive active black holes (BHs) and their immediate surroundings, it now includes the host galaxies of such BHs, the correlated evolution of BHs and galaxies, and the physics of extremely energetic phenomena like γ-ray jets. More than 1000 articles are being published in refereed journals every year about this topic, and the numbers are still growing. The equivalent number in the mid-1970s was about 200.

This book, The Physics and Evolution of Active Galactic Nuclei, is an attempt to cover most of the central topics in this large field in a way that emphasizes the basic physics and the complex connections between AGNs and other astronomical objects. It grew from a graduate-level course taught at Tel Aviv University over many years and from numerous international schools on this topic where I participate as a lecturer, and it contains three main themes. The first is a comprehensive description of the more important physical processes associated with AGNs: the physics of photoionized gas; dust in AGNs; nonthermal processes; and various modes of accretion onto BHs, including accretion disks and accretion flows. The second is a detailed description of various subgroups of AGNs and the main components in individual sources. These include radio-loud and radio-quiet AGNs, type-I and type-II sources, LINERs, blazars, broad and narrow emission line regions, broad and narrow absorption lines, megamasers, dusty tori, and X-ray-emitting gas near the BH. The third part deals with the various connections, evolutionary and others, between BHs and their host galaxies, including star-forming galaxies.

How to classify AGNs

General and historical classification schemes

The classification of AGNs into subgroups is based on the history of research in this area. In particular, the discovery and general understanding of quasars, in the early 1960s, preceded the detailed study of the local members of this group, the Seyfert galaxies. This statement can be challenged by historians because of the seminal paper by Seyfert in 1943, in which he described the spectra of seven members of this group. However, the unusual features of these spectra were neglected for a long time, including the enormous velocities inferred from the widths of the emission lines and the extreme nuclear luminosities. These objects were rediscovered in the mid-1960s and studied in greater detail following the great interest in quasar spectra and redshifts.

The earlier detailed observations of local, low-redshift AGNs provided enough data to define several of the subgroups that are still used today: Seyfert 1 galaxies, Seyfert 2 galaxies, radio galaxies, quasars, blazars, LINERs, and so on. Additional observations of higher-luminosity, higher-redshift objects helped to refine the classification. Many observed properties of such sources have been described in Chapter 1.

Current AGN classification is based on higher-quality observations of a much larger number of sources, on better understanding of the physics of accretion and the line-emitting processes, and on the realization that many of the observed characteristics depend on the luminosity and inclination of the central source. The main subgroups of today are introduced in this book as type-I radio-quiet AGNs (in earlier years, Seyfert 1 galaxies and radio-quiet QSOs), type-I radio-loud AGNs (in earlier years, BLRGs, radio-loud QSOs, or QSROs), type-II radio-quiet AGNs (Seyfert 2s in earlier years), type-II radio-loud AGNs (in earlier years, NLRGs), LINERs, and blazars (BL-Lac objects and OVVs of earlier days). The following are three sets of somewhat different questions that can be posed to subclassify AGNs.