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Active Galaxies and Quasars

A Monster in the Middle

Most large galaxies have ~1011 Mo of stars, ~109-10 Mo of interstellar gas, and ~1012 Mo of dark matter. But at least 5% of galaxies — though it may be all of them — also have something else lurking inside: a monster in the middle. This monster is a supermassive black hole, which ejects tremendous amounts of energy from jets at its top and bottom. How can this incredible objects be explained?

Long ago, when galaxies were young, the stars in the central regions of the galaxies were very closely packed. Star collisions and mergers occurred, giving rise to a single massive black hole (MBH) with perhaps 106 to 109 Mo. Gas from the galaxy's interstellar medium, from a cannibalized galaxy, or from a star that strays too close, falls onto the MBH. As in X-ray binary star systems, an accretion disk forms, emitting huge amounts of light across the electromagnetic spectrum (infrared to gamma rays). The MBH plus accretion disk produces the phenomena seen in active galactic nuclei (AGN). Below are optical and radio images of the active galaxy NGC 4261. The central object, accretion disk and lobes are all visible.

HST image of NGC 4261
Ground-based and Hubble Space Telescope images
of the Active Galaxy NGC 4261

The different types of AGN are variations on this theme. Many galaxies may have a quiet MBH that has not recently accreted gas. Seyfert galaxies exhibit accretion onto a moderate-mass MBH, while the more luminous quasi-stellar objects, like quasars, exhibit accretion onto a high-mass MBH.

In approximately 10% of the AGN, the MBH + accretion disk somehow produces narrow beams of energetic particles and magnetic fields, and eject them outward in opposite directions away from the disk. These are the radio jets, which emerge at nearly the speed of light. Radio galaxies, quasars, and blazars are AGN with strong jets that can travel outward into large regions of intergalactic space. Many of the apparent differences between types of AGN are due to our having different orientations with respect to the disk. With blazars and quasars, we are looking down the jet. For Seyferts, we are viewing the jet broadside.

Considerable uncertainties remain. Exactly how are jets produced and accelerated? Where do the clouds producing the broad emission lines come from? Can we empirically confirm that a MBH is actually present?

An AGN (Artist's conception)
An artist's conception of an AGN

Seyferts

Consider NGC 4151, a spiral galaxy 15 Mpc away. Photographs by Carl Seyfert in the 1940s showed a bright point-like nucleus. Its spectrum is unusual: in addition to a continuum and absorption lines from normal stars, Seyfert galaxy nuclei have strong emission lines. Some are commonly found lines, such as hydrogen (e.g. the Balmer series H-alpha, H-beta lines). Others are not as common, or even rare, like the lines for twice-ionized oxygen, in which the oxygen atom has lost two of its electrons, and whose formation requires extremely hot gas. The lines are broad, requiring that the gas be Doppler shifted in all directions up to ~20,000 km/s. The nuclei vary in brightness on timescales of months, requiring them to be < 1 parsec in size. The total luminosity can be equivalent to 1010 Lo

What is this bizarre object in the center of Seyfert's spiral galaxies?

Later in the 1940s, astronomers began scanning the skies with radio telescopes. They found strange radio structures on opposite sides of radio galaxies, plus a tiny source of radio emission at the nucleus. The nuclei of these radio galaxies shoot out narrow beams of extremely energetic electrons and magnetic fields, producing radio synchrotron radiation. The radio components include: the compact core at the galaxy nucleus, jets, lobes, and a hot spot where the jet slams into the interstellar medium.

Quasars

In the 1960s, some radio sources seemed to be associated with "stars," and were called quasi-stellar radio sources or quasars. However, they had spectra similar to Seyfert galaxy nuclei. It became clear that they are Seyferts, and radio galaxies where the nucleus out shines all of the stars by factors of 10 to 1,000. The luminosity of quasars can reach 1012 Lo. They also tend to be farther away than either Seyfert galaxies or blazars.

In the 1970-80s, findings include:

  1. X-ray satellite telescopes found strong and very rapidly variable X-ray emission from Seyferts and quasars. Timescales for these variations were as short as days, hours, or even minutes.
  2. Rare BL Lac objects and blazars were discovered. These are radio galaxies with jets pointing directly at us, ejected by the active nucleus at velocities near the speed of light.
  3. Optical astronomers find thousands of faint distant quasars which are not radio-loud. Strangely, there were many more quasars early in the Universe than there are today.
  4. In 1993, the Compton Gamma-Ray Observatory discovers incredibly intense gamma-rays from the jets of some blazars: Stronger than X-ray, optical, radio emission combined.

Blazars

AGNs observed at high (>100 MeV) energies form a subclass known as blazars, which is thought to be an AGN that has one of its relativistic jets pointed toward Earth so the emission we observe is dominated by phenomena occurring in the jet region. Among all AGNs, blazars emit over the widest range of frequencies, and have been detected from radio to gamma-ray.

Specifically, to be classified as a blazar an AGN must be seen with one of the following properties:

  • high radio-brightness accompanied by flatness of the radio spectrum
  • high optical polarization,
  • strong optical variability on very short timescales (less than few days).

In the class of objects selected according to these criteria, there appear to be two subgroups:

  • Sources showing strong and broad emission lines, such as those of quasars (called Flat Spectrum Radio Quasars)
  • Sources showing a featureless optical spectrum (called BL Lac objects). 

There are additional important differences between these subclasses. For example, blazars show different luminosity and redshift distributions, and a different morphology of the extended radio emission. 

In its first year of operation, the Fermi Gamma-Ray Space Telescope detected 709 active galaxies, most of which are blazars. Of these, 300 are BL Lac objects, nearly 300 are Flat Spectrum Radio Quasars (FSRQ), 41 are other types of AGN, and 72 are of unknown types.

Last Updated: January 2011

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