Classification of radio-quiet nuclei

Within the sub-class of radio-quiet AGN, classification is performed primarily on the basis of optical/UV spectral properties. The phenomenology of the main classes will now be briefly described.

• Seyfert 1 galaxies / radio-quiet quasars : The spectrum of these objects is complex and multi-component. They are often called broad-line (or type-1) AGN due to the presence of broad (with full-width half maximum, FWHM - ) permitted optical/UV emission lines in the nuclear spectrum (see Netzer 1991 for a review of the emission line properties of AGN). Both low-ionization broad lines (e.g. MgII and hydrogen lines) and high-ionization broad lines (e.g. CIV , OVI and NeVIII ) are common. The region from which these emission lines arise is known as the broad line region (BLR). Although the basic nature of the BLR is a topic of ongoing research, most workers agree that the optical/UV emission lines arise from relatively dense (electron density ) photoionized clouds with a small volume filling factor. The presence of low-ionization lines, coupled with photoionization modelling, suggests that at least some of the material responsible for the lines is optically-thick to Lyman continuum radiation (i.e. radiation with wavelength Å). There is a general consensus that the line widths are due to cloud motions within the gravitational potential well of the central black hole. It is thought that collisional de-excitation in this relatively dense plasma dominates over forbidden line emission, thereby explaining the lack of broad forbidden emission lines.

  As well as broad lines, the optical/UV spectra of such objects also display narrow (FWHM ) permitted and forbidden emission lines. Commonly observed narrow lines are the hydrogen lines, [OII] , [OIII] , [NII] and [SII] . The material responsible for these lines, which resides in the narrow line region (NLR), is more tenuous than that in the BLR and lies at greater distances from the putative black hole. The NLR can be spatially resolved in nearby AGN and is found to have a typical size of (e.g. Pogge 1988; Schmitt & Kinney 1996).

  The distinction between a Seyfert 1 nucleus and a radio-quiet quasar (RQQ; often referred to as a quasi-stellar object, QSO) was traditionally made on the basis of whether the galaxy or AGN, respectively, was discovered first. Thus, Seyfert 1 nuclei are generally of lower-luminosity than radio-quiet quasars: the dividing bolometric luminosity is difficult to precisely define but is of order . In practice, the properties of both Seyfert 1 nuclei and radio-quiet quasars seem to form a continuous sequence in luminosity. Such nuclei typically display strong continuum radiation from IR through to -ray wavebands. The higher-energy wavebands are often observed to be rapidly variable: at X-ray wavebands, the variability of the continuum radiation can approach the theoretical maximum (see Section 2.3 for a quantitative discussion).

• Seyfert 2 galaxies : The spectra of Seyfert 2 galaxies display narrow permitted and forbidden emission lines that are very similar to those of Seyfert 1 galaxies. However, they differ from Seyfert 1 galaxies in as much as no broad lines are readily observable and the optical/UV continuum radiation is much weaker. The fact that broad optical/UV lines can often be observed at sufficiently long wavelengths (e.g. H at red-optical wavelengths or Pa in the infra-red region) suggests that at least some Seyfert 2 nuclei are simply reddened Seyfert 1 nuclei. In this hypothesis, the BLR would have to lie behind the material responsible for the reddening. Supporting evidence for this comes from the fact that X-ray observations of many Seyfert 2 nuclei reveals the photoelectric absorption from the cold gas that is presumably associated with the dust.
• Narrow Line Seyfert 1 galaxies (NLS1) : This sub-class of AGN differ from `normal' Seyfert 1 galaxies in several ways. Firstly, the permitted optical/UV lines are significantly narrower than in normal Seyfert 1 galaxies. Note that these are still broader than the narrow lines from the NLR. Also, the ratio of the [OIII] line flux to that in H is <3, a criterion which seems to differentiate Seyfert 1 galaxies from Seyfert 2 galaxies (Shuder & Osterbrock 1981). Secondly, these objects often display stronger than typical optical line complexes due to FeII emission. Lastly, the X-ray continuum is found to be much softer and more variable than in normal Seyfert 1 galaxies (Puchnarewicz et al. 1992; Boller, Brandt & Fink 1996).
• Narrow Emission-Line Galaxies (NELGs) : NELGs are galaxies possessing narrow (FWHM ) emission lines, typically of H , H and [OIII] . Deep X-ray imaging with the ROSAT satellite (see Section 1.7.1) show that these galaxies often have associated X-ray sources with X-ray luminosities (Roche et al. 1995; Boyle et al. 1995). As such, these galaxies are typically two orders of magnitude more luminous than `normal' galaxies. Although NELGs are often included as a category of AGN, it is possible that some or all of the activity may be the result of starburst activity. As many as 10 per cent of all (low-redshift) galaxies may be NELGs: the large number of these sources, together with the fact that they possess hard X-ray spectra (Almaini et al. 1996; Romero-Colmenero et al. 1996) makes them the best candidate objects for producing the hard X-ray background.