2.2. The unified model

2.2.1. The Seyfert 1.8 and 1.9 galaxies

Osterbrock (1981) defined two new subclasses of Seyferts: Seyfert 1.8s show weak, but readily visible broad H and H emission, while Seyfert 1.9s show only broad H. In these objects, the broad H / H emission line ratios are unusually large compared with those of typical Seyfert 1s, suggesting that dust may be particularly important in the BLRs ([156]). The broad H / H ratios for these objects range from about 5 to 15; if the intrinsic values are typical of those of Seyfert 1s, that is ~ 3.1 ([16]), and are modified by reddening, extinction values in the range A_V = 1.2-4 mag. are implied ([156]). A broad Pa line has been observed in some of these objects confirming these results ([45]; [354]).

The presence of an unresolved central continuum source in Hubble Space Telescope (HST) images is a virtually perfect indicator of a Seyfert 1 nucleus; the converse is not true; over one-third of Seyferts with direct spectroscopic evidence for broad Balmer wings show no nuclear point source; but a number of them are classified as Seyfert 1.8 or 1.9; they appear more extinguished by dust absorption than those which display a compact central nucleus ([279]).

A broad Pa line ([159]; [362]; Veilleux et al. 1997a) has been observed in a small number of Seyfert 2s, showing that they contain a heavily reddened BLR (A_V 4 mag.) similar to that found in Seyfert 1s. A broad Pa has been found in a few NLRGs suggesting that the visual extinction in the broad line region in these objects is in the range A_V = 1.5-4.5 mag. ([193]). AGNs with narrow optical emission lines and broad IR hydrogen lines are here called S1is.

A hidden BLR has been found by spectropolarimetry of the nucleus of a number of Seyfert 2s; the broad line emission lines are scattered into our line of sight by free electrons. But hidden BLRs are observed only in galaxies with warmer far infrared colours, indicating that the scattering particles must lie very close to the plane of the torus and therefore that, to see these lines, we must have a direct view of the inner wall of the torus ([189]). The objects in which broad Balmer lines are detected by spectropolarimetry are called S1hs.

Seyfert 1 nuclei have been found to reside in earlier type galaxies than Seyfert 2 nuclei; this could be explained if the absorption in a fraction of all Seyfert 2 nuclei were due to galactic dust rather than to a dusty torus surrounding the nucleus, the galactic dust having a higher covering fraction than the torus; and indeed the Seyfert 2s are significantly more likely to show nuclear dust absorption in the form of irregular lanes and patches ([279]). [321] have found that old and powerful starbursts are relatively common in Seyfert 2s while absent in genuine Seyfert 1s; this would be naturally explained if a fraction of all Seyfert 2 nuclei, located in spiral galaxies were indeed obscured by galactic dust. These objects should not show hidden polarized broad line regions.

The optical spectrum of the radio galaxy 3C234.0 shows broad components to the H and H lines with a Balmer decrement H / H=7.3, implying an extinction A_V = 2.2 mag.; this object could then be classified as a Seyfert 1.8, i.e. a Seyfert 1 whose nucleus is seen directly, although through a relatively large extinction. However, spectropolarimetry reveals strongly polarized (~ 20-25%) broad Balmer lines due to scattering of the nuclear lines; the broad-line photons seen in total flux are also light scattered from the hidden nucleus; 3C234.0 should be considered as a Seyfert 2 galaxy as no broad emission lines are seen directly from the nucleus ([417]; [488]). IRAS11058-1131 and IRAS23060+0505 ([486]) are similar examples.