Chapter 16: The Expanding Universe

Arp 220

In 1963, Caltech astronomer Maarten Schmidt made the amazing discovery of objects with large redshifts. The recession velocities can be so high that they are a significant fraction of the velocity of light. These objects were originally called quasi-stellar radio sources or quasi-stellar objects (QSO) denoting the fact that they appeared almost star-like on photographic plates and can have weak radio emission. The general term quasar refers to a distant galaxy in which the light from an active nucleus completely swamps the light from its surroundings. Astronomers think that the quasar phenomenon is caused by a supermassive black hole at the center of a galaxy. When the energy from close to the black hole outshines the surrounding galaxy, we see a quasar.

Artist's impression of quasar GB1508

There is a crucial distinction between galaxy redshifts and quasar redshifts. Galaxies follow the Hubble relation, in which estimated distance is correlated with radial velocity. This correlation has a natural interpretation in terms of the expansion of the universe. In the nearby universe, it is a relatively reliable procedure to measure a galaxy's redshift and assign it a distance. But quasars do not have any property that reliably indicates distance. For example, the absolute luminosity of quasars ranges over a factor of more than a thousand. This wide range means that the apparent brightness of any two quasars can differ by a factor of thousands at the same redshift. Quasar brightness is a very poor distance indicator. Also, quasar redshifts are far higher than the range over which the Hubble relation has been tested. The distance to a quasar is meaningful only in the context of a cosmological model. The enormous distance estimates are the result of the assumption of cosmological redshifts and a model that converts a redshift into a distance.

Quasar 1229+204 in the Core of a Colliding Galaxy

A large body of evidence now supports the idea that quasars are at the distances indicated by their cosmological redshifts. As their name implies, quasars are often not completely stellar in appearance. Images of quasars at redshifts from a range of z = 0.3 to 1 reveal fuzz, or nebulosity, surrounding the bright core. The nebulosity has the size and brightness expected of a luminous galaxy at that redshift. In addition, quasars often lie in clusters of galaxies at the same redshift; this association is strong circumstantial evidence that the quasars are at cosmological distances.

Quasar MC2 1635+119

Perhaps the most convincing evidence that quasars are extremely distant comes from observations of gravitational lensing. General relativity predicts that mass can bend light, which leads to the distorted image of a background object. Dark mass in galaxy clusters can distort the light of a background galaxy. About 1 in 300 quasars happens to lie directly behind an intervening galaxy. The lensing causes the magnification of the quasar light and the formation of multiple images. Gravity can deflect all forms of electromagnetic radiation, so we can observe the lensing of radio waves as well as visible light. If the alignment is nearly perfect, an Einstein ring is seen. If the lensing object is an elliptical galaxy, the quasar light usually splits into four images in a cross, with the lensing galaxy in between them. The probability of finding four unrelated quasars so close together, with identical redshifts and spectra, is infinitesimally small. This type of configuration can only be produced by the effect of gravitational lensing. The quasar thus must be more distant than the lensing galaxy.

Two quasars - the one on the right has a host galaxy while the one on the left does not

How are quasars related to other active galaxies? There is continuity in observed structure between normal galaxies, active galaxies at low redshift, and quasars at much higher redshift. Active nuclei are rare in normal galaxies, but a few nearby examples can be found. Searching larger volumes of space turns up even rarer but more luminous active nuclei, but at such large distances that the nebulosity from the surrounding galaxy is difficult to discern. At the greatest distances, only highly luminous objects can be seen, and the light from the surrounding galaxy is correspondingly weak. As redshift and distance increase, the brightness and angular size of a normal galaxy decrease. Also, the light from the active nucleus drowns out the starlight from the surrounding galaxy. By a redshift of z = 0.3 to 0.5, the host galaxy is barely visible. At this point, astronomers refer to the object as a quasar. The most distant quasars are too remote for the host galaxy to be resolved; they appear as point sources of intense light.