ASP: Beacons in the Gloom

Beacons in the Gloom

A tussle in the distant Universe. This Hubble Space Telescope image shows evidence for a merger between a quasar and a companion galaxy. The quasar is the bright object just beneath the image center, and the wisps of material around it are remains of a galaxy disrupted by the interaction between it and the quasar. Image courtesy of J. Bahcall (Institute for Advanced Study) and NASA.

by Erik Stengler, University of La Laguna and Science Museum of San Sebastian, Spain

Quasars. What lurks behind such a strange name? It took a long time for astronomers to answer this question after these remote objects were actually discovered and named. How the name came about, however, is a much simpler story. Let us begin with this story, as it gives us an insight into the process of discovery in astronomy.

The name quasar is made up with parts of the words "quasi-stellar radio." The origin of the name is closely related to their discovery: Astronomers named this class of objects -- first discovered by their radio emission -- by the way they looked through the optical telescope, that is, like a star.

Later, when it was found that most objects of the kind do not show radio emission and are mainly detected in the visible part of the spectrum, they started to be called simply "quasi stellar objects," and the acronym QSO began to be used more frequently. The name quasar remained in use only for those that show radio emission, although this distinction is not followed too strictly, and I will stick with the moniker quasar.

Why were they discovered as radio sources if they can be seen through an optical telescope? The name quasar contains the answer: Since they look like stars in the "visible" sky, and since we see so many stars in the foreground of the Milky Way Galaxy, no matter the direction in which we look, it was unlikely that anybody would check closely what seemed to be just another very faint star. Yet in the radio sky quasars looked quite different from any other object; stars are generally not expected to emit large amounts of radio emission.

A History in the Making

Since the 1920s the idea of an expanding Universe has been supported by Edwin Hubble's discovery that most galaxies or groups of galaxies move away from each other. This is the basis of the Big Bang model of the Universe because it implies that they all may have been very close to each other in the past, if we assume that they moved the same way in the past and in every part of the Universe. This assumption, called the "cosmological principle," is the starting point of our whole knowledge of the Universe as a whole, so it had better be true!

To determine the speed at which distant galaxies are receding from us, astronomers use something called "redshift." As you wait to cross a street, you notice the frequency of the sounds of approaching cars increases, while it decreases quickly as they drive past and away. This effect -- a change in frequency when the source has a component of motion toward or away from you -- was described mathematically by Hans Doppler years ago. What is true for sound is also true for light: The frequency of light received by an observer from a source is lower, and hence "redder," if the source is moving away from the observer. And the greater the source's speed, the larger the "redshift." A "blueshift" would be observed for a source moving toward the observer. Taken together with the notion of an expanding Universe, in which we observe that the most distant objects move the fastest, redshifts provide us with a means to estimate distance: the higher the redshift, the more distant the object.

Lensing on a grand scale. Abell 2218 is the rich galaxy cluster shown in this Hubble Space Telescope image. Because of its great mass, the cluster bends light from distant objects behind it, much as an optical lens bends light, and produces the arc-like structures in the image. Called gravitational lensing, this process brightens, magnifies, and distorts images of those distant objects, which in this case are a population of galaxies that extends from five to ten times the distance to Abell 2218. Image courtesy of W. Couch (University of New South Wales), R. Ellis (Cambridge University), and NASA.

In the 1960s galaxies were known to have redshifts of up to 0.2. Imagine the surprise when in 1963 Maarten Schmidt, an astronomer at Cambridge University, found that what was thought to be an anomalous star with radio emission showed a redshift of 0.158. Such a redshift implied that the "star" was at a great distance, comparable to that of the most distant galaxies known then -- but how could a single star shine with so high a brightness that it could be detected at such a distance? That it was a star had to be ruled out, yet it was still too bright to be identified even as a galaxy.

Other "anomalous star[s] with radio emission" were subsequently discovered to have large redshifts, and although their nature remained unknown for a long time, these powerful objects were recognized immediately by astronomers as the most distant objects in the Universe. Since Schmidt's discovery nearly four decades ago, the race for finding objects at ever higher redshifts has intensified; at present, the record is close to a redshift of 5.

But the eagerness to find distant quasars is not only due to the wish to hold a record. The fact that such bright objects can be observed, and that because of their brightness they can be detected at great distances, has given rise to quasars' use as probes of the furthest regions of the observable Universe: Not only the regions where they actually are, but also those through which their light has traveled before reaching us.

There is a large gap between the most distant galaxies that can be observed and the region where quasars are most abundant. And while there are probably many galaxies in that gap, their light is too faint to reach us, and we can only know of their existence and about their properties through their effects on the light coming from the background quasars. The same applies to galaxies and gas clouds which, although being closer, are too faint to be seen. There are two main ways such galaxies may interact with the light from quasars: They can absorb part of it if they lie directly in the line of sight, or their gravitational force can bend the light rays passing by them.

No. 46 - Spring 1999

Beacons in the Gloom