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 star-like or quasi-stellar . But how do we know that these are not galactic stars that emit radio waves ? The answer is contained in the optical spectrum, which invariably has a large redshift, the principal hallmark of a quasar. The first two quasars that were investigated, 3C273 and 3C48. have redshifts of 0.158 and 0.367 respectively, which, in the mid-1960s, were considered large. They imply distances of approximately 480 and 1100Mpc if we assume that the redshift is due to the systematic recession of distant objects in the. expanding Universe.

Gradually, optical and radio astronomers have found quasars of larger and larger redshifts. This has required considerable effort because the higher redshifts results in ultraviolet emission lines being shifted into the visible region, so in the early days at least astrophysicists were on unfamiliar ground. By the end of 1976 several redshifts of nearly 4 had been encountered, with some dozens of objects displaying redshifts exceeding 2.

At higher redshifts we have to take into account the theory of special relativity in order to convert the redshift, z, to a velocity of recession, v. The light is shifted from its rest wavelength ? to an observed wavelength (1 + z ) ? at velocity v in accordance with a relation which is graphed . Here c denotes the speed of light. For velocities below about 0.4c an approximate formula z = v/c is sufficiently precise. For a quasar with a redshift of 2 the recession speed is 0.8 c, i.e. 80 per cent the speed of light.

Quasars quite frequently have a rich spectrum of emission lines and absorption lines..The emission lines can generally be arranged neatly into a sequence of spectral lines that display one redshift. For the absorption lines astronomers have noted that a different arrangement is not uncommon. In a given quasar the absorption lines may belong to several different systems, each with a different redshift that is less than the emission-line redshift. An example, PHL957, will clarify this finding: its emission-line redshift is 2.69, but the array of absorption lines seems to slot into five spectral sequences with redshifts of 2.67, 2.55, 2.54, 2.31, and 2.23. An obvious, and likely, explanation is that the emission-line redshift is the intrinsic redshift, but the quasar has shed several clouds of matter at speeds approaching that of light. Radiation from the quasar that passes through such an expanding shell will have a lower redshift absorption spectrum impressed upon it.

A further optical property of quasars, and one that they share with the nuclei of active galaxies such as Seyferts, is variability. The brightest quasars are present on patrol photographs dating from the nineteenth century. Archival material and modern photometric data show that quasars vary on timescales from a few days and upwards to decades, and with amplitudes from 0.1 to 3 mag Variations on a timescale of weeks would indicate that the region generating the bulk of the luminosity is less than 0.1 pc in size Now these tiny volumes are pouring out to the Universe up to 100 times as much energy as a galaxy, so we see once more the scale of the energy problem faced by theorists.

Quasars are remarkably similar to radio galaxies when their radio properties are compared. Many quasars show the double structure and have the spectrum that is characteristic of synchrotron radiation. Faced with a spectrum and radio map of a newly-found double radio source, it would not be possible without optical observations to tell whether it was a radio galaxy or quasar. However, some quasars are highly-compact radio sources, unlike the radio galaxies. Interferometers with an intercontinental baseline have demonstrated that the radio components in some quasars are less than 0.001 arc sec in size. Furthermore, there is ample evidence that these structures are short-lived because they alter significantly in only a few months; this is additional evidence that the major part of the energy is generated in remarkably small regions embedded inside the quasars.

Skilled optical astronomers have gone to extraordinary lengths, using the world’s finest telescopes, to detect structure around quasars. Generally this has been a disappointing endeavour, although a few wisps of gas seem to be present round 3C48. It appears to be fairly certain that some quasars at least are in clusters of galaxies This is because a few quasars have the same redshift as very faint galaxies that are only a small angular distance away.

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