Lecture 37 - Active Galaxies and Gravitational Lenses (4/17/96)

Seeds: Chapter 14

  1. Active Galactic Nuclei
    • Some galaxies have extremely luminous nuclei, often outshining the rest of the galaxy.
    • These objects are called active galactic nuclei. The Milky Way appears to have such an active nucleus.
    • Some active galaxies, like our Milky Way, appear to be powered by supermassive black holes at the center.
    • Other types of active galaxies show evidence of widespread star formation in the inner hundreds of parsecs.
    • Some of these star-forming galaxies, called starbursts, can also be very luminous from large numbers of O and B stars and H II regions.
    • Supernovae in the starburst regions will also produce large amounts of energy.
  2. Radio Galaxies
    • Radio astronomy began in the 1940s using WWII radar technology.
    • Many of the brightest radio sources are associated with distant galaxies.
    • The first crude maps showed a "double" structure to these radio galaxies.
    • The radio emission comes from a region much larger than the optical galaxy - often greater than 1 Mpc in extent.
    • The emission is synchrotron radiation from relativistic electrons moving in weak magnetic fields.
    • This jets of emission extend from a central nucleus to hotspots at the far edges of the lobes.
    • The jets feeding the electrons outward to the lobes from the central black hole (or other energy source).
    • The total radio luminosity from synchrotron in powerful radio galaxies is 10^36 to 10^38 Watts (10^10 to 10^12 Lsun).
    • The total energy contained in the lobes is about 10^53 J, equivalent to 10^6 Msun c^2 of mass-energy! This could be obtained over the typical 10^6 year lifetime of a radio source by conversion of 1 Msun per year into energy.
    • The likely source for this energy is a supermassive black hole of mass 10^6 to 10^8 Msun. Such a monster could gravitationally extract 10% of the rest mass energy of 10 Msun/yr of fuel to supply the needed power.
    • Some of the largest radio galaxies extend out to several Mpc on either side of the nucleus.
    • Some radio sources show evidence of interaction with intergalactic gas, being swept backward as the galaxy moves forward at a few hundred km/s.
  3. Quasars
    • Some bright radio sources seemed to be not associated with galaxies but with stellar like objects.
    • These are quasars, or "quasi-stellar radio sources".
    • The optical spectra of quasars showed strange lines which did not seem to correspond to known elements.
    • Maarten Schmidt at Palomar Observatory realized in 1962 that these were highly redshifted Balmer lines of hydrogen!
    • This was controversial since these redshifts (z=0.16 for 3C273 and z=0.37 for 3C48, the two first discovered) indicated extreme distances and thus huge luminosities (10^15 Lsun)..
    • Quasars were soon found with redshifts z > 1. These were at that time the most distant objects known.
    • Currently, the most distant quasar known has a redshift z > 5!
    • Some quasars are were discovered that did not have any radio emission - these were dubbed QSOs, or "quasi-stellar objects".
    • Some of the quasar emission may be beamed, like a searchlight.
  4. Gravitational Lenses
    • For lensing by a galaxy (10^12 Msun) instead of a star (1 Msun), the splitting are an arcsecond or more - easily seen!
    • The first lens, a double quasar, was found in 1979.
    • Currently, there are over 30 gravitational lenses known.
    • There are a number of gravitational lens surveys being carried out that will greatly increase the number of known lenses in the next few years.
    • Gravitational lenses magnify the background galaxy or QSO, acting as a gravitational telescope.
    • The different light ray paths that correspond to the different images have different lenghts, so there are relative time delays that can be seen if the source is variable.
    • If the time delays can be measured in addition to the image separations and relative magnifications, then the distance to the lens and source can be separated out from the mass of the lens.
    • This will lead to a measurement of the Hubble constant independent of the standard distance ladder.
    • Clusters of galaxies (10^15 Msun) can also act as gravitational lenses, distorting and magnifiying background galaxies and QSOs. The image separations are larger (>10") and usually result in arcs as extended background galaxies are smeared out and highly magnified.
    • Cluster lenses are an important tool for measuring the mass in clusters.

Next Lecture - Large Scale Structure

Active Galactic Nuclei

Active galaxies can be classified by whether the derive their energy from stars and heat (thermal) or from some compact object like a black hole causing synchrotron emission (nonthermal). Starbursts are thermal, with their emission from hot young stars. Strictly speaking, X-ray emission from accretion disks can be thermal in origin, but accretion disks are usually lumped into the nonthermal category, as they also show synchrotron emission. We will concentrate on nonthermal energy sources in galaxies.

Radio Galaxies

Radio astronomy began after WWII as radar technology developed in the war made its way into peacetime science. By the 1950s the first radio astronomers were mapping the skies with their oftentimes homebuilt telescopes. Many of the brightest sources in the sky were not what was expected, not the Sun, planets, and galactic objects (although the center of the galaxy was a very strong radio source). These were later tracked down to being associated with distant galaxies.

The radio source Cygnus A is the second brightest radio source in the sky. It is associated with an somewhat distorted looking giant elliptical galaxy about 2000 Mpc away from us. Early maps of Cygnus A showed two radio lobes about 50 kpc in diameter each on either side of the galaxy. Better radio maps show thin jets of emission from a central radio nucleus (coincident with the center of the galaxy) extending out to the outermost parts of the lobes, where they terminate in bright areas of emission called hotspots. The picture we have of this archetypical radio galaxy is that a central black hole engine is generating twin jets of relativistic electrons, which stream along magnetic field lines outward from the nucleus for 50 kpc. At the hotspots, the jets hit the gas of the galactic corona (which has been swept outward in front of the advancing hotspots as the jets drill outward from the center) and form a shock, which causes the strong radio emission of the hotspot. The electrons then diffuse back into the lobe area which fills the cavity created by the advancing hotspot, emitting 10^38 Watts of synchrotron energy!

Quasars

For a long time, there was controversy whether QSOs and quasars were actually at the huge distances that the redshifts implied, or if they were actually much nearer and the redshifts were non-cosmological in origin. This, through the accumulation of large amounts of evidence, has been resolved in favor of the cosmological redshift - these objects really are very distant and superluminous!

Note that some of the large implied luminosity is probably due to beaming. Like a searchlight, the light can be concentrated in a narrow angle, so the flux we see does not fully represent a luminosity emitted in all directions. Relativistic effects in the synchrotron emission also increase the apparent luminosity through beaming effects.

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

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Steven T. Myers - Last revised 25Apr96