Lecture 24 - Quasars and Active Galaxies (4/15/99)

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ASTR012 Reading:
Chapter 23, 24 (ZG4)

pages 96-101

The radio galaxy Cygnus-A (M87) as seen by the VLA at 20cm. (Courtesy Phil Blanco/UCSD)
? Key Question:   What are quasars?
! Key Principle:   Accretion and Eddington Luminosity
# Key Problem:   How do you generate 10^13 Lsun with a galaxy nucleus?


  1. Redshift
  2. The Universe at High Redshift (z > 0.1)
  3. The Radio Sky
  4. Radio Galaxies
  5. Quasars
  6. Central Engine: Supermassive Black Holes
  7. Gravitational Lenses
  8. Large Scale Distribution of Galaxies
  9. Clusters of Galaxies

Large Scale Structure of the Universe in Outline

  1. Active Galactic Nuclei
  2. Radio Galaxies
  3. Quasars
  4. Gravitational Lenses
  5. Luminosity Function of Galaxies
  6. Large Scale Distribution of Galaxies
  7. Clusters of Galaxies

Interacting Galaxies

Computer simulations of the interactions between galaxies have been an important development in our understanding of this process. In essence, model galaxies are set up with "stars" (particles) and "gas" (fluid elements), and then moved along a grid interacting via Newtons laws and hydrodynamics. Early simulations demonstrated the production of "tidal tails". Current state-of-the art simulations are showing the extent of the influence of interactions on the evolution of galaxies in our universe.

The Cartwheel Galaxy, courtesy HST/STSCI

The above image is from the Hubble Space Telescope, and shows a galaxy that has been disturbed by what was probably a direct hit by the (also disturbed) small blue galaxy on the right. The ring and spokes are what is left of the disks of the galaxies, with shock-induced star formation going on in the ring, and some gas accumulating in the center of the remnant.

Some galaxy interaction resources:

Supermassive Black Holes at the Centers of Galaxies

Radio galaxy 3C353 multi-band image from VLA, courtesy Alan Bridle (NRAO).

Some internet links to quasar and radio galaxy sites:

Gravitational Lenses

One reason we believe that quasars really are very distant is because sometimes we see them lensed by a distant galaxy between us and the quasar!

A galaxy in front of a background galaxy or quasar will gravitationally lens the background source by the bending of the light rays. Since galaxies are 10^12 times more massive than stars, the image splittings are arcseconds instead of 10^-6 arcseconds (angle proportional to the square-root of the mass).

I am currently involved in the Cosmic Lens All-Sky Survey (CLASS) which is currently mapping around 10000 radio sources looking for indicences of multiple images indicative of lensing. So far, we have found 5 new lenses and a large number of candidates that we are checking whether they are lenses. For a description of CLASS and the lenses we have found, see my CLASS Home Page.

Large clusters of galaxies, which contain thousands of galaxies and total masses of 10^14 to 10^15 Msun within a radius of several Mpc, also act as powerful gravitational lenses. Background galaxies and QSOs are magnified and distorted into arcs by the lensing action of the cluster. The Hubble Space Telescope has taken some spectacular pictures of gravitational lensing by the cluster Abell 2218

Galaxy Cluster A2218

For more on the gravitational lenses I found in my VLA-based radio survey, see the Penn CLASS Page.

Large Scale Surveys of Galaxies

There are a number of large-scale surveys aimed at measuring redshifts of a large number of galaxies and thus delineating the structure that we see in the Universe around us.

The Hubble Deep Field. Courtesy HST/STSCI

Though not a redshift survey, the Hubble Deep Field (HDF) is also revolutionising our understanding of the distant universe, as we can see galaxies as they were during the early phases of life.

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smyers@nrao.edu Steven T. Myers