The remarkable properties of radio galaxies spurred astronomers on in their efforts to find the optical counterparts to radio sources. This stimulus was partially responsible for the discovery, in the early 1960s of quasi-stellar objects or QUASARS. These are extra-galactic radio sources that match up optically with objects that cannot be resolved, i.e. they are […]
Until the discovery of radio galaxies in the early 1950s, radio astronomy was hardly more than an interesting sideline to the primary study of stars and galaxies by optical telescopes. How¬ever, in only a few years rich new vistas of research were opened up. A momentous breakthrough came in about 1954, when British and Australian […]
NGC 1275 is the brightest galaxy in the preseus cluster and it, is a strong radio source. It was by far the most distant of the twelve galaxies originally listed as Seyfert galaxies. Since the Perseus cluster is rich in elliptical galaxies it has been suggested that NGC 1275 might, be an active, elliptical galaxy […]
M87 is the biggest and brightest galaxy in the Virgo cluster. It is the nearest giant elliptical (at a distance of 15Mpc) in which there is clear evidence of activity, both within the nucleus and well out¬side it. The first direct evidence came with the discovery by H.D.Curtis in 1918 of a remarkable optical jet […]
This is the nearest galaxy in which we see evidence of violent activity . The first sign of activity came with the discovery of a small, week radio source in its nucleus. Optical studies showed that a large amount of gas had been ejected above and below the plane of M82 and the velocity of […]
These are galaxies first classified as a group by Carl Seyfert in 1943 as part of a survey of spiral galaxies. They have starlike nuclei that emit a rich emission-line spectrum with very broad lines. SEYFERT GALAXIES are relatively common. Approximately 10 per cent of the brightest galaxies are Seyfert-type galaxies although they are much […]
These are objects with most of their luminosity in a bright, star-like nucleus and surrounded by a faint, relatively compact nebulous envelope. The nuclei of N-TYPE GALAXIES have colours like quasars, whereas the envelopes have colours similar to those of giant elliptical galaxies. In many respects N-type galaxies are similar to relatively luminous, distant Seyfert […]
While the Californian astronomer Fritz Zwicky was compiling a catalogue of clusters of galaxies, he found a number of COMPACT GALAXIES of high surface brightness that are barely distinguishable from stars. The red compact galaxies found by Zwicky are prob¬ably normal galaxies with an unusually high surface brightness, while the blue galaxies in Zwicky’s list […]
These are objects discovered in the early 1970s by the Russian astronomer B.E.Markarian of the Byurakan Observatory in Armenia, He used a Schmidt telescope with an objective prism to search for galaxies with a strong ultraviolet continuum. The objects found fall mainly into two types: galaxies (often spirals) with a bright nucleus that is the […]
The many classes of active galaxies are primarily a consequence of the haphazard way in which they were discovered. Very few active galaxies fit neatly into one class; consequently there is con¬siderable overlap between the various classification schemes used. With this proviso we now review the main properties of several classes of active galaxies.