This chapter is concerned with objects which are only rarely observed by amateur astronomers. This is due in part to the fact that observing them is difficult and requires almost professional equipment.

THE MORPHOLOGY OF ACTIVE GALAXIES, QUASARS, AND GRAVITATIONAL LENSES

The objects in this chapter have a typical angular size of a few arcseconds. Apart from a few exceptions, long exposure times and optics with long focal length are needed in order to recognize details in the objects. A few quasars are achievable goals for today's astrophotographers, but gravitational lenses are a challenge even for dedicated amateurs.

Quasars, whose name stems from the abbreviation of “quasi-stellar” objects, are a prominent class in the family of active galaxies. They are highly luminous in the radio part of the electromagnetic spectrum, and today it is known that they are the very bright, active nuclei of distant galaxies. The high central luminosity is characteristic of active galaxies and will be described later in this chapter from the astrophysical point of view. More exactly, it is thus only the quasi-stellar, bright core region of the galaxy which is visible, since due to the great distance the dim outer regions are very faint or not visible at all. The morphology of quasars is much simpler than that of the various types of elliptical or spiral galaxies.

Only the nearest quasars allow one to recognize structures in the galaxies which contain them. However, first the bright core region must be removed via special image processing. When this is done, about half of the host galaxies of quasars show fragment-like rudiments of spiral arms or indications of past interactions. Such a host galaxy is two to three magnitudes fainter than the quasar and is located in the wings of its point-spread function, which has a width of only 1–2”. Under good seeing conditions and with elaborate post-processing it is possible to separate the spectrum of the quasar from that of the host galaxy.