Rotating Supermassive Black Holes (6) ...

Let us now pull together the various threads of this discussion and summarize the basic unified model and active galactic nuclei and quasars. The following cartoon illustrates.

Unification Hypothesis for AGNs
We hypothesize that AGNs are powered by rotating, supermassive black holes having all or most of the following features:

  • Accretion disks
  • Possibly a dusty torus surrounding the accretion disk
  • Clouds near the black hole that move at high velocity and produce broad emission lines
  • Lower-density and slower clouds further from the black hole that produce narrower emission lines
  • Possible jets perpendicular to the plane of the accretion disk

    • If the orientation of the system relative to the observer is as in the top row of the above figure, we see a Seyfert 2 galaxy if the AGN is in a spiral and a lobed radio galaxy if the AGN is in an elliptical galaxy and produces strong jets. On the other hand, if the orientation of the AGN central engine is as in the bottom row of the above figure, we see a Seyfert 1 galaxy if the host is a spiral, a blazar (BL Lac) if the host is elliptical and the jet is strong, and perhaps a core-halo radio galaxy if the jets are weaker in an elliptical system.

    ... and Quasars
    We then adopt the point of view that quasars are just particularly energetic forms of these active galaxies and are described by the same model. The primary difference is that quasars are very luminous because their central black hole engines are being fed matter at a higher rate than for moderately luminous AGNs. We surmise that quasars are more abundant at larger redshift because large redshift corresponds to earlier in the Universe's history, when matter was more dense and collisions between galaxies more frequent. Therefore, quasars may be just "better fed" active galactic nuclei from a period when more fuel was available to power the black hole engines. As a corallary to this, we may conjecture that many nearby normal galaxies once sported brilliant quasars in their cores, but have since used up the available fuel. Their massive black holes lie dormant, ready to blaze back to life should circumstances such as tidal interactions with another galaxy divert matter into the black hole. This may, in fact, be true of our own galaxy (see the box below).

    An Amusing Possibility

    Since quasars have large look-back times, we are seeing what they looked like billions of years ago. They may well be dormant now, and reside in normal galaxies. But an astronomer living today in the host galaxy of a quasar that we presently see at a look-back time of say 7 billion years will see the Milky Way also at a look-back time of 7 billion years (it takes light the same length of time to travel one direction as the other). Since the Milky Way may have a dormant black hole at its center, it may have had a quasar at its center 7 billion years ago that has since turned off. In that case, the distant astronomer will observe his own galaxy to be "normal" (because the quasar at its center will have long ago turned off) but will observe the Milky Way to be a quasar because he/she/it is observing the Milky Way as it was 7 billion years ago. We may even imagine that astronomy books on that distant world might contain images of the powerful Milky Way quasar (or whatever they call our galaxy in their language), and might address the question of why it seems to be so different from their "normal" home galaxy and what could account for its enormous energy output!