Active Galaxies and Quasars

The first half of this century saw a flurry of astronomical interest in galaxies, and they were studied in great detail. During this time, astronomers began to notice that, in terms of their radiation output, some galaxies were distinctly different from others.

In 1943, for instance, astronomer Carl Seyfert noted a number of spiral and barred spiral galaxies which appeared to have nuclei much brighter than normal. These became known as Seyfert galaxies and were the first of the active galaxies to be discovered. The term active is used to describe a galaxy which is producing very much more radiation that that produced by its stars.

The following year, in 1944, amateur astronomer Grote Reber was observing the heavens with a home-made radio telescope when he detected an incredibly powerful radio source emanating from something in the constellation of Cygnus. This was the first recognition of the radio galaxies which, as the name implies, radiate the majority of their energy output at radio wavelengths.

In the 1950s, the position of Cygnus A was gradually refined until it was finally identified with a fairly dim elliptical galaxy. Unlike a Seyfert galaxy, the radiation from a radio galaxy does not come directly from the nucleus. Rather, it is released from gigantic ‘lobes’ which exist on either side of the host galaxy. They appear to be fed by a pair of high-energy jets being squirted in opposite directions from the radio galaxy’s active galactic centre.

In 1963, a third class of active galaxy was finally identified by a team of radio astronomers led by Cyril I lazard. These peculiar celestial objects, known as quasars, appear superficially as stars but, under analysis, reveal that they are extremely distant and incredibly luminous galactic objects (see Fig. 1. 6).

Fig. 1.6. On ordinary photographs of the sky, quasars, such as the one identified here by the two vertical bars, cannot be distinguished from stars in the same field Only after more careful analysis (e.g. examination of their spectra) is it revealed that quasars arc, in fact, extremely distant and incredibly luminous galactic objects. (Photograph reproduced courtesy of Mount Palomar Observatory.)

They share many of the same features as Seyfert and radio galaxies but emit even more radiation. Curiously, quasars seem to be much more prevalent the further into space astronomers look and it is this fact which has led to the belief that they are of cosmological importance. The fact that they once existed in great numbers but now no longer do, may be of crucial importance to our understanding of how galaxies came into being and evolved with time.

On average, one in every ten known galaxies is active, i.e. the majority of its radiation output is not being produced by stars. Instead, some other physical mechanism is releasing copious amounts of radiation, up to 1,000 times greater than that emitted by entire galaxies, often from n region with approximately the same diameter as our own Solar System!

There are many different ways of classifying active galaxies, over and above the three types which have been mentioned here. The crucial question is: were all galaxies once active? Perhaps all of the so-called normal galaxies - our own Milky Way included - once passed through an active phase. Now, however, the active core has fallen dormant and the galaxies' emissions are dominated by starlight. If this is the case, then it would seem as if all young galaxies pass through an active stage because of the distribution of quasars throughout space.

If, on the other hand, active galaxies are intrinsically different from normal galaxies then this conclusion too would give us valuable insight into the early Universe. Why, for example, would the Universe promote the evolution of quasars billions of years ago but not now?

So, active galaxies are fascinating objects to study because the answers to some intensely perplexing cosmological puzzles seem wrapped up in their mysterious cores.