Active Galactic Nuclei and Quasars

Active Galactic Nuclei (AGN) were known and studied before the concept of SMBHs became firmly established. Observationally, an AGN is defined as a galactic nucleus which displays energetic phenomena such as large electromagnetic luminosities and/or powerful jets. The first known AGN was the ``radio quasar'' 3C 273. This object was first identified by radio observations, but radio imaging alone was insufficient to localize its position on the sky well enough to allow follow-up investigation with optical telescopes. A major breakthrough was the use of lunar occultation techniques (i.e., precision measurement of the time at which the radio emissions from 3C 273 were blocked as the Moon passed in front of it) by Cyril Hazard and collaborators [98] which localized 3C 273 to within 1arcsec. This allowed an identification and subsequent spectroscopy of the corresponding optical object. The optical spectrum was initially confusing, displaying emission lines at wavelengths that did not correspond to any known atomic transition, until it was realized by M.Schmidt that the spectrum was redshifted by a seemingly enormous factor [99]

$$z\equiv\frac{(\lambda_{\rm obs}-\lambda_{\rm emit})}{\lambda_{\rm emit}}=0.158.$$ 3

Interpreting such a redshift as cosmological (i.e., due to the expansion of the Universe), suggests the object to be extremely distant and, therefore, extremely luminous. These enormous luminosities were the clue that led some researchers, most notably Salpeter, Zeldovich and Lynden-Bell, to suggest the existence of accreting SMBHs [100,101,102]. Accretion of matter into a relativistically deep gravitational potential well is one of the few processes in nature efficient enough to produce such luminosities without having to process unreasonably large amounts of mass. The supermassive nature of the black holes follows from the requirement for the AGN to remain gravitationally bound (and hence long-lived) despite the enormous outward pressure exerted by the electromagnetic radiation.

Figure 4: Two possible geometries relevant for active galactic nuclei (AGN). The dark shading represents dusty molecular gas that is in either a toroidal geometry (top) or warped disk geometry (bottom). The lighter shading represents the jets that are known to exist in radio-loud AGN and may well exist (albeit in a weaker form) in radio-quiet AGN as well.

After four decades of study, there is a large and complex phenomenology associated with AGN. It has proven useful to classify AGN according to their luminosity, electromagnetic spectrum, and spatial (radio) morphology⁶. A basic dichotomy seems to exist between radio-loud AGN and radio-quiet AGN. Radio-loud AGN (of which 3C 273 and M87 are examples) possess back-to-back twin jets of plasma that are produced in the vicinity of the SMBH and propagate outwards at relativistic velocities (with Lorentz factors of 5-10 or more) for, in some cases, distances of $10^5-10^6{\rm\thinspace pc}$. These jets tend to be copious sources of radio and X-ray emission due to, respectively, synchrotron and inverse Compton emission by high-energy electrons within the magnetized plasma. The primary physical mechanisms initiating, accelerating and collimating these jets are still far from clear, although it seems likely that they are launched from the inner accretion disk or ergosphere of the SMBH and are accelerated/collimated by hydromagnetic processes. In the rare cases where one of the jets is directed straight at us, special relativistic beaming strongly enhances the observed jet emission, often diluting and thus making it difficult to observe any other emissions from the AGN. Such objects are known as blasars (of which the well-studied BL-Lac objects are a sub-class) and can be sources of extremely energetic emissions (with photon energies up to $\sim 10$TeV having been detected from the BL-Lac objects Mrk501 and Mrk 421 [104,105]).

Radio-quiet objects, on the other hand, seem not to possess these powerful jets. Radio-quiet AGN with moderate electromagnetic luminosities ($L\sim 10^{43}-10^{45}\hbox{${\rm\thinspace erg}{\rm\thinspace s}^{-1}\,$}$), commonly referred to as Seyfert galaxies, are a particularly important and well studied sub-class. This is due to the fact that they are reasonably common (constituting 1-10% of all major galaxies), leading to some relatively nearby and easily studied examples. Seyfert galaxies themselves have been classified into two broad categories. Those objects in which we can directly view the energetic regions immediately around the SMBH are called type-1 Seyfert galaxies (often referred to as Seyfert-1 galaxies). On the other hand, if the SMBH region is obscured by large amounts of dust and gas (situated near to the AGN and/or within the host galaxy), it is called a type-2 Seyfert galaxy. Spectropolarimetry of some nearby Seyfert galaxies (in particular NGC 1068; [106]) provides evidence that the same AGN may appear as a type-1 or type-2 depending upon the orientation at which we view the system. Two possible geometries that are often discussed are the dusty torus geometry or the warped disk geometry (see Fig. 4). In these geometries, an observer viewing the AGN along a line of sight that intercepts the dusty torus or the warped disk will be obscured, leading to a type-2 classification. An observer who views the SMBH region unobscured will assign a type-1 classification. Since our discussion focuses on observational constraints on the region very close to the black hole, we shall focus on type-1 AGN.

In the absence of strongly beamed jet emissions, the overall electromagnetic spectrum of type-1 radio-quiet and radio-loud AGN are qualitatively similar. There is often a distinct bump in the spectrum at optical/UV wavelengths, generically referred to as the ``big blue bump'', that is thought to correspond to thermal radiation from the optically-thick portions of the accretion flow. Furthermore, a power-law spectral component extends from the big blue bump up to hard X-ray energies. As we will discuss in §3.3, we believe that this component arises from inverse Compton scattering of optical/UV photons in a very hot corona associated with the accretion flow. A high-energy exponential cut-off at $kT\sim 100-200{\rm\thinspace keV}$, corresponding to the temperature of the coronal plasma, has been observed directly in some Seyfert-1 galaxies.