University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

A Bestiary of Active Galaxies

Radio Galaxies

In the mid-1950s it was discovered that powerful radio sources are frequently associated with giant elliptical galaxies. This was a surprise because galaxies had been thought to be collections of billions of stars along with bits of hydrogen gas and dust. Stars like the sun are not strong radio emitters, nor is the Milky Way a powerful radio source, though there is a weak source, Sagittarius A, at the center. But these gE radio galaxies emit radio energy that outshines the combined light of the hundreds of billions of stars in the galaxy by as much as a factor of 100! The first radio galaxy discovered was Cygnus A whose radio structure, typical of giant radio galaxies, is shown below. The radio emission comes principally from giant radio lobes, well outside the visible portions of the galaxy, sometimes covering several million light-years. Frequently there is also a radio core, coincident with the galaxy nucleus.

The Radio Galaxy Cygnus A

The origin of the radio emission from Cygnus A and other radio galaxies was soon identified by UCSD's Geoffrey Burbidge as Synchrotron Emission, light produced by "relativistic" electrons, moving near the speed of light, spiralling around lines of magnetic force. But many puzzling questions remained:
  • What is the source of the tremendous radio power from radio galaxies?
  • What mechanism accelerates the electrons to such intense energies with speeds over 99.99% of the speed of light?
  • How are the relativistic particles and magnetic field transported from the galaxy itself, where they must originate, out to distances of millions of light years?
  • How do the electrons, which quickly radiate away their energy, maintain their high speeds over a journey which must take millions of years?

Electrons moving near the speed of light produce
Synchrotron Radiation when they are accelerated
around lines of magnetic force.

The Jet Structure of NGC 6251

As more detailed radio studies have become available, astronomers have traced the source of radio power back to the galaxy nucleus. These studies show that intense beams or jets, moving with highly-relativistic speeds are transporting the electrons and magnetic field out to the radio lobes. The radio lobes are believed to be produced when the jets ram into intergalactic gas clouds or possibly the interstellar medium of nearby dwarf galaxies. These jets are just like the jets that are believed to be responsible for quasars' super-luminal expansion, except that in this case we do not see them pointed at us. Time dilation, caused by moving at speeds almost equal to the speed of light allows the jets to carry the synchrotron electrons out to the radio lobes before they lose their energy. What is the ultimate source of energy? Most astronomers suspect that, like the quasars, powerful radio galacxies are powered by a massive black hole in the galaxy nucleus.

Seyfert Galaxies

In 1943, astronomer Carl Seyfert noticed that certain nearby spiral galaxies have very bright, pinpoint nuclei. Spectra of these galaxies, now named Seyfert galaxies, showed that they have unusual spectra with very strong, often broad, emission lines.
The Bright Nucleus of the Seyfert Galaxy NGC 7742

Seyfert galaxies are divided into two classes, based upon the widths of their spectral emission features. The Seyfert 1 galaxy above has hydrogen emission features with very large widths, indicating that the gas in the galaxy's central regions is moving with velocities of several thousand km/sec (Seyfert 1 galaxies show velocities up to almost 0.1c). Compare the Seyfert 1 spectrum with the spectrum of the quasar 3C273, on the previous page. In many respects Seyfert 1 galaxies are like "mini-quasars" and many astronomers believe that Seyfert 1 galaxies are in fact lower luminosity examples of the quasar phenomenon.

The Seyfert 2 galaxy shown above has much narrower emission features implying much lower velocities (note that the Seyfert 1 above shows narrow features as well).

A Quasar/AGN Model

  • Take a Quasar Tour at Mullard Space Sciences Lab, UK, and view their Beginner's Guide to AGN.

    Black Hole, Accretion Disk
    & "Relativistic Jet
  • The Monster in the middle - at the center of our model is a massive black hole -- 108 - 109 M. Stars which venture too near the central black hole are ripped apart by tidal forces, falling inward to form an accretion disk. Which slowly carries material inward to the Black Hole. Gravitational energy released by the inward spiral of the material heats the disk to temperatures of millions of Kelvin in the inner region, which is a strong source of x-rays. Black holes are "messy eaters" some of the material pulled inward is ejected at velocitiess near the speed of light along the rotation axis of the Black Hole, producing the jets seen at radio wavelengths.

  • Broad-Line Clouds
  • The Broad-Line Region: A light-year or so from the central source are clouds or filaments of dense gas swirling around at velocities of thousands of kilometers per second. These clouds are ionized and heated by ultraviolet and x-ray photons from the central source. Fluorescent emission features from these clouds produce the broad emission features seen in spectra of quasars and Seyfert 1 galaxies - a reflection of the strong radiation field and high velocities near the center.

  • HST Image of NGC 4261
  • Molecular Torus: Outside the broad-emission-line region, astronomers believe there exists a doughnut-shaped region of molecular gas and dust which is heated by the central source, emitting infrared light, but which obscures the central black hole, disk and broad-line region from observers viewing the quasar edge-on.

  • Narrow-Line Clouds
  • The Narrow-Line Region
  • Unification of Quasars and other types of AGN

    Many astronomers believe that the different types of Active Galaxies are really all the same type of phenomenon simply seen from different viewing angles with respect to the molecular torus described above.

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    Comments? Gene Smith
    Conducted by:
    Prof. H. E. (Gene) Smith
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    Last updated: 29 Sept. 2000