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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!

These monsters aren't any of the typical typical horror film terrors, though they might appear in one of your favorite science fiction movies. In reality they are supermassive black holes that spew forth tremendous amounts of energy from jets on their tops and bottoms. How can these incredible objects be explained?

Long ago when galaxies were young, the stars in their cores were very closely packed. Star collisions and mergers occurred, giving rise to a single massive black hole (MBH) with perhaps 106 - 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 you see 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 today (including our Galactic center??) may have a quiet MBH which happens not to have recently accreted gas. Seyfert galaxies have accretion onto a moderate-mass MBH, while the more luminous quasi-stellar objects (i.e. quasars) have accretion onto a high-mass MBH.

In ~10% of the AGN, the MBH + accretion disk somehow produce 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, which 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 artists conception of an AGN

Seyferts

Consider NGC 4151, a spiral galaxy 15 Mpc away. Photographs by Carl Seyfert in the 1940s showed a very bright point-like nucleus. Its spectrum is very unusual: in addition to continua + absorption lines from normal stars, Seyfert galaxy nuclei have very strong emission lines. Some are common lines (e.g. H-alpha, H-beta) but others are weird (e.g. twice-ionized oxygen lines), requiring hot gas far out of equilibrium. The lines are very 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. But 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 10-1000. 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

Active Galactic Nuclei observed at high (>100 MeV) energies form a subclass known as blazars; a blazar is believed to be an AGN which has one of its relativistic jets pointed toward the Earth so that the emission we observe is dominated by phenomena occurring in the jet region. Amongst all AGNs, blazars emit over the widest range of frequencies, being 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: (1) sources showing strong and broad emission lines, such as those of quasars (called Flat Spectrum Radio Quasars), and (2) sources showing a featureless optical spectrum (called BL Lac objects). There are additional important differences between these subclasses such as they show different luminosity and redshift distributions, and a different morphology of the extended radio emission.

Of the 80 blazars detected already by the EGRET experiment on board the Compton Gamma-Ray Observatory, most of them (about 80 %) are Flat Spectrum Radio Quasars (FSRQ), while the others have been identified as BL Lac objects.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

The Imagine Team
Project Leader: Dr. Jim Lochner
Curator:Meredith Bene
Responsible NASA Official:Phil Newman
All material on this site has been created and updated between 1997-2005.

http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html