Normal galaxies are dynamically stable: they will change their large-scale structure only very gradually. Certain active galaxies exhibit structures that must be short-lived. The jets or tails seen attached to some galaxies can only last until the gravitational attraction of the parent galaxy pulls them back towards the galaxy. Indeed their very presence suggests that […]
Normal galaxies probably reach the peak of their luminosity soon after they are formed, and subsequently fade away relatively slowly. The most violently active galaxies show variations in their optical and radio brightness sometimes in only a few days, Although the radiating regions are extremely compact, measurements suggest that t ho disturbances responsible for the […]
The radio radiation emitted by normal galaxies is very weak indeed. However some of the most distant galaxies ever found have been discovered by radio astronomers because of the great intensity ‘of their radio emission. These are the strong extra-galactic radio sources described below. Many active galaxies radiate at radio wavelengths with intermediate strength. High-resolution […]
Most active galaxies have an optical spectrum rich in emission lines. These are superposed on a continuum spectrum which normally has essentially a black-body (thermal) form determined by the temperature of an object. In some active galaxies, however, the continuum spectrum appears to be of a non-thermal origin, in that it cannot be due to […]
The width of the lines in a spectrum of an object can indicate either the temperature of the emitting region or it can give the range of velocities found within the emitting region. A number of galaxies have particularly wide lines in their spectra. If the broad lines are attributed to high temperatures, these are […]
We know from studies of normal galaxies that regions of recent star formation are particularly luminous. It is possible to account for the high nuclear luminosity of certain galaxies by supposing the galaxy to be very young. This would mean that we were seeing its initial burst of star formation. Such an explanation undoubtedly accounts […]
The common feature of all the forms of energetic galactic activity is that all are relatively short-lived. We have a fairly good idea of the ages of stars and galaxies as well as of the way in which they evolve. ACTIVE GALAXIES are those that appear to be evolving in a manner that they could […]
The previous chapters described the general properties of normal galaxies in detail. The vast majority of galaxies are normal in the sense that they have general properties in common. In particular, their present rate of evolution, as measured for example by their rate of star formation, is consistent with their age as indicated by their […]
Clusters of galaxies are the largest entities known and studied in the Universe. They are also very luminous, with integrated brightnesses rivalling those of quasars. This property, along with their radio and X-ray emission, enables them to be discovered and investigated at large distances. It is generally hoped that this will lead to the use […]
Rich clusters of galaxies are certainly not due to chance associations or random encounters between galaxies. They are systems in which galaxies are dynamically interacting with each other and with any intracluster gas. In terms of the spatial distribution of member galaxies the richest clusters especially appear stable and gravitationally bound. Unlike the Solar System […]