An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

1 Meet your first active galactic nuclei

Figure 1 compares two nearby spiral galaxies of similar distance and type. NGC 5548, on the left, has a brighter nucleus than that of NGC 3277, on the right. This extra emission from the central regions of NGC 5548 is not generated by stars. Instead this light is thought to be ultimately powered by material falling in the gravitational field of a supermassive black hole at the centre of NGC 5548. Similar non-stellar emission is also seen at the centre of many other galaxies. These bright central regions are generally known as active galactic nuclei or AGN. In this course you will gain an overview of these enormously energetic objects and how they are observed and studied.

Figure 1
Figure 1 Optical images of the nearby spiral galaxies (a) NGC 5548, (b) NGC 3277. Note that the nucleus in NGC 5548 appears brighter than that in NGC 3277. It is the very concentrated bright central source in NGC 5548 which causes the diffraction pattern apparent as the diagonal spikes emanating from the centre of image (a)

The first AGN was discovered in 1908 when E. A. Fath took a spectrum of M77, also known as NGC 1068. This nearby spiral galaxy is shown in Figure 2. Carl Seyfert was the first to realise that there were other similar galaxies, when he discovered that NGC 1275, NGC 3516, NGC 4015, NGC 4151 and NGC 7469 all had spectra similar to that of NGC 1068. Consequently these objects, and others like them, are called Seyfert galaxies.

Figure 2
Figure 2 An optical image of Messier 77, also known as NGC 1068. This image was produced by combining exposures made with several different filters to give the colour, and has been processed to reduce the contrast between the brightest and faintest regions, so that more of the detailed structure is visible

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