Ground state and excited galaxies

Excited galaxies (see Figure 26), i.e. objects that are not in the ‘ground state,’ fall into two broad classes: (1) diffuse galaxies with a high rate of star formation, such as starburst galaxies, Markarian galaxies, amorphous galaxies, Haro galaxies, intergalactic H II regions; and (2) objects with active nuclei, such as quasars, BL Lac objects (= blazars) and Seyfert galaxies. Most quasars and galaxies with active nuclei are located at large distances. This makes it difficult to study their morphology in detail. The images of luminous IRAS sources, including Seyfert 1, Seyfert 2, LINER (= low ionization nuclear emission-line region) and QSO parent galaxies by Hutchings & Neff (1991) suggest that many of them have a highly disturbed morphology including (what appear to be) tidal tails. Bahcall et al. (1996) show that the host galaxies of nearby quasars are frequently disturbed and interacting objects.

Active nuclei of galaxies can be fuelled by inward transport of gas (Morris & Serabyn 1996) via any one of the following processes:

• Gas may lose angular momentum to a stellar bar or oval disk by gravitational torques. Gas orbiting in a bar potential will have non-circular orbits resulting in collisions that produce shocks, which in turn result in loss of energy and angular momentum. The effect of spiral density waves is similar to that of bars, provided that there is no inner Lindblad resonance.

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