Active galactic nuclei (AGN) are powered by accretion onto supermassive black holes. The masses of these central engines of AGN can be estimated by means of the virial theorem, using the size of the nucleus of the galaxy and the velocity dispersion of the material in the vicinity of the nucleus. Estimates of the masses are around 109±1 M.
The Schwarzschild radius of a black hole, i.e. the radius of the sphere from within which light cannot escape, is given bywhere M is the mass of the black hole.
An indicator of the distance to an AGN is its cosmological redshift, z. The shift in its spectral lines Δλ reflects the recession speed of the AGN. According to Hubble's law, the larger the redshift, the more distant is the object.
When electrons travel through regions of space containing a magnetic field, they follow helical paths around the magnetic field lines and emit electromagnetic radiation. When the speeds of the electrons are relatively low, the emitted radiation is known as cyclotron radiation, and when the electron speeds are a significant fraction of the speed of light, the emitted radiation is known as synchrotron radiation.
The spectrum of synchrotron radiation produced by a single electron travelling with a speed has a broad shape with a maximum at a frequency max. This frequency is proportional to B 2 where B is the magnetic field component perpendicular to the velocity of the electron and is the electron's Lorentz factor, given byThe Lorentz factor is proportional to the electron's energy, since E = mec2, so max is also proportional to B E 2.
Given an ensemble of electrons whose energies are distributed according to a power law with particle exponent s, the flux density in the spectral energy distribution (SED) of the optically thin synchrotron radiation is given by where = (s −1)/2. At low frequencies synchrotron radiation is optically thick, and . This is known as synchrotron self-absorption (SSA).
Free electrons in an ionized gas radiate as they decelerate when they pass by protons or other positively charged ions. The continuous spectrum of electromagnetic radiation which arises is called thermal bremsstrahlung. Since the electrons are unbound throughout the process, the radiation is also known as free-free emission.
The Wien tail is the short-wavelength part of the blackbody spectrum (described by B(T) = (2 h 3 / c2) exp(−h / kT)), whilst the Rayleigh–Jeans tail is the long-wavelength part of the blackbody spectrum (described by B(T) = 2kT 2/c2).
Both blackbody radiation and bremsstrahlung are examples of thermal radiation, because the speeds of the electrons that are responsible for them follow a Maxwell speed distribution. Synchrotron radiation is an example of non-thermal radiation.
In the process of Compton scattering, high-energy photons scatter off relatively low-energy electrons and lose energy as a result. In the process of inverse Compton scattering, photons gain energy from higher energy electrons. Radiation which has been boosted to higher energies by this process is said to have been Compton upscattered.
In the synchrotron self-Compton (SSC) process, synchrotron radiation is upscattered by the same electrons which were responsible for the original emission.
The Balmer and Lyman series of hydrogen lines arise from transitions whose lowest energy levels are the n = 2 and n = 1 levels respectively. Hence the H line (in optical spectra) arises from transitions between n = 3 and n = 2, whilst the Ly line (in ultraviolet spectra) arises from transitions between n = 2 and n = 1. The Balmer limit at 3646 Å corresponds to transitions between n = 2 and the continuum whilst the Lyman limit at 912 Å corresponds to transitions between n = 1 and the continuum.
Most common spectral lines are ‘permitted’ by quantum mechanical selection rules. In low-density regions, so called ‘forbidden’ lines can also be produced. This is because collisions between atoms are rare if the density is low, so atoms in excited states with no permitted transitions execute low probability forbidden transitions. Forbidden lines are indicated by a square bracket notation, such as [OIII] λ5007 for the forbidden transition in doubly ionized oxygen, leading to emission of radiation of wavelength 5007 Å (5.007 × 10−7 m).
When linearly polarized radiation passes through a plasma, its direction of polarization is rotated in a phenomenon known as Faraday rotation. Consequently radiation travelling through an extended synchrotron source will have a rotation measure which depends upon the electron density and magnetic field along the propagation path. The different values of rotation for various emission locations within an extended source will lead to Faraday depolarization.
Quasars share some or all of the following properties: they are star-like objects identified with radio sources, they are luminous X-ray sources, they have time variable continuum flux, a large UV flux, broad emission lines and large redshifts. The radio SED of a quasar typically has F ν−0.7.
Radio galaxies and quasars typically show two components: a pair of spatially extended lobes extending for several kpc and a compact (spatially unresolved) core. Extended radio sources can be divided into Fanaroff–Riley class I (FR I) sources whose lobes are brightest in the centre and fainter towards their edges, and Fanaroff–Riley class II (FR II) sources whose lobes are limb-brightened and often show enhanced emission at the edges of the radio structure. FR II sources are more luminous than FR I sources.
The compact cores of radio galaxies are smaller than 0.01 pc in size and their spectra are usually flat (i.e. < 0.5) over several orders of magnitude in frequency.
Some radio galaxies have extended linear structures known as jets, extending from the core to the lobes. Jets often appear only on one side of the radio galaxy, and in cases where two are seen, one is usually much fainter than the other.
Quasars are variable in every waveband in which they have been studied, including both continuum and line emission, on timescales as short as a few days. This indicates that much of the radiation must come from regions of order light-days across.
Quasars often have unusually blue colours and an excess at ultraviolet fluxes compared to stars. Their UV–optical spectra have strong, broad emission lines from hydrogen, and other common chemical elements.
Since quasars are observed at high redshift, they provide a probe of the Universe when it was only a fraction of its current age. They can serve as luminous background sources against which other, closer structures may be observed.
Radio-quiet quasars are around 100 times fainter in radio emission and around 10–20 times more numerous than the radio-loud quasars.
Observational samples of galaxies and AGN are subject to the Malmquist bias. It is easier to detect more luminous objects at greater distances than it is to detect low-luminosity objects at these same distances. So, any flux-limited sample will contain a greater representation of high-luminosity objects and will contain relatively few low-luminosity objects at large distances, despite the fact that low-luminosity members of a class of objects are usually the most numerous. The 3C radio catalogue, for example, suffers from a strong Malmquist bias.