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 less common among fainter galaxies. Seyfert galaxies differ from the other Markarian galaxies in that they show substantial non-thermal emission which is seen most easily at the ultraviolet end of their spectra. Indeed it is generally found that objects with broad emission lines in their spectra also show non-thermal nuclear continuum. The colours of Seyfert nuclei are rather similar to those of quasars.

The emission lines found in Seyfert spectra are both the permit¬ted hydrogen Balmer lines and the lines of ionized oxygen, nitro¬gen and neon. Seyfert galaxies may be divided into two sub-types on the basis of their spectra. These are Type 1 where the Balmer lines are broader than the lines of ionized metals and Type 2 where the Balmer lines and the metal lines are of similar width. The Balmer lines of Type 1 Seyferts are often several thousand kms-1 wide whereas the metal lines of Type 1 and all the lines of Type 2 Seyferts are usually 500-1000 kms-1 wide.

The complexity of Seyfert galaxy spectra has made it difficult to reach any firm conclusions about the origin of Seyfert-type activity. In certain respects, Seyfert galaxies are like quasars. Spectroscopically, quasars are similar to Type 1 Seyfert galaxies and the faintest quasars have a brightness in the range of the brightest Seyfert galaxies. A particularly interesting nearby Seyfert galaxy is IC4329A which has an exceptional HP flux which is only one-third of that of the well-known quasar 3C 48.

Seyfert galaxies are often optically variable sometimes over a few months. It is almost always the continuum which varies while the emission lines remain constant. This implies that the region that generates the non-thermal continuum is much more compact than the region that produces the emission line radiation. Detailed spectroscopic studies suggest that the ‘considerable line width in Seyfert galaxy spectra is a result of the emission lines coming from a number of distinct clouds of gas close to the centre of the galaxy. The velocities of these clouds can be as high as 700 kms-1 relative to the centre of the galaxy. The extended widths of the Balmer lines of Type 1 Seyferts imply much higher velocities, typically 3000kms-1 relative to the centre of the galaxy. These probably originate in turbulent regions closer to the centre of the galaxy where the gas density is high enough to stop any forbidden lines being produced. A correlation is observed between the width of the Balmer lines and the intensity of the non-thermal continuum. This might arise if the ultraviolet continuum radiation is respon¬sible for ionizing the gas that then produces the Balmer lines.

Most Seyfert galaxies have small radio sources in their nuclei; these are between 10 and 1000 times brighter than those found in the nuclei of normal spiral galaxies. None, however, has the characteristic double structure commonly found amongst the stronger extragalactic radio sources. Some Seyfert galaxies emit a considerable quantity of X-rays.

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