The general principle for discovering AGNs is to make use of one or more of the ways in which they are not like stars or galaxies, for example, how they differ in the SEDs or emission-line spectra. The pointlike, i.e., spatially unresolved, nature of the nuclei is another distinguishing factor. It is also possible to make use of their great distances and correspondingly undetectable proper motions in quasar and AGN searches or their variability in brightness. In this article we will concentrate on techniques that make use of their SEDs and spectral-line properties.
The SEDs of AGNs are remarkable for their broad extent in frequency, from radio to -rays, which is much greater than for normal, thermal sources of astronomical radiation. The UV/optical emission-line spectra stand out for the strength and breadth of the principal emission lines and for the wide range of ionization. Typical line widths of permitted lines are 5000 km s-1 or more. The strongest individual lines are those of hydrogen (Ly, H, and H), C IV, C III], Mg II, and N V, while broad emission complexes of Fe II are visible. In addition, forbidden lines of [O I], [O II], [O III], and [S II] are prominent.
Some of the observable properties of AGNs provide diagnostic probes of the physical nature of the central engine. For example, the X-ray emission originates in regions as close as a few Schwarzschild radii of the central black hole and yields information about the inner part of the accretion disk and coronal region. The broad UV/optical emission lines are produced within a few light days of the central engine. One main goal of AGN research is to combine multiwavelength and spectral observations of AGNs with theoretical models of the accretion processes so that physical properties such as accretion rates and efficiencies can be inferred from observable data. When success is achieved in this subject, it will yield a significant advance in our understanding of AGN evolution.