5.3. Type 2 QSOs

There have been several claims about the discovery of X-ray selected ``narrow line'' QSOs which have been called type 2 QSOs. All of them turned out to be normal broad line objects or, more often, NLS1s (which are difficult to distinguish from Seyfert 2s on low dispersion spectra) ([180]; [182]).

Radio quiet type 2 QSOs should not look like QSOs at all, as both the non-thermal continuum and the broad emission lines should be completely suppressed by the obscuring torus, but rather as galaxies with high excitation narrow emission lines; such objects would not be optically conspicuous.

Radio loud type 2 QSOs however are relatively easy to find, since in low frequency surveys, the radio emission is dominated by the large radio lobes that are unaffected by any obscuration or beaming effects. The 3CR survey ([35]) is the best studied of all low frequency radio surveys; it covers the northern sky, including all point sources with flux density greater than 9.0 Jy at 178 MHz. From an analysis of the 3CR sources in the redshift range 0.5 < z < 1.0, [26] concluded that type 2 (i.e. NLRGs with a high radio luminosity) are twice as numerous as type 1 QSOs; however four of the narrow line objects in his sample (3C22.0, 41.0, 265.0 and 325.0) are now known to have broad Balmer lines; the relative number of type 2 to type 1 QSOs then become 1.4:1. [253] carried out a similar analysis, and found that at high radio luminosities, the numbers of narrow and broad line objects are about equal.

Véron (1977) has built a new catalogue based on the 3CR at | b| > 10°, correcting the fluxes for confusion and resolution; it contains 205 extragalactic sources. The redshift of all of them except three (3C89.0, 3C249.0 and 4C-01.04) has been measured. This sample contains 17 objects having broad emission lines and a radio luminosity larger than 10³⁶ erg s^-1 Hz^-1; 16 are QSOs (M_B < - 24); it is reasonable to assume that all the 17 narrow line objects with such a high radio luminosity are type 2 QSOs, which leads to a ratio of type 2 to type 1 QSOs near one in the absolute magnitude range -28 < M_B < - 24. This result is in good agreement with [253].

Stickel et al. (1996) have found 14 flat-spectrum radio sources with very weak optical-infrared counterparts and with very steep infrared-to-optical continua; many of them have narrow emission line spectra. Redshifted H I 21 cm absorption has been detected in four out of five of them, with neutral hydrogen column densities ~ 4 and 80 10¹⁸ cm^-2 suggesting that these objects are reddened by dust; the steep optical-infrared slopes suggest lower limits to rest-frame values of A_V between two and seven magnitudes; the location of the absorber, within or outside the host galaxy, is not known ([75]). The corrected absolute magnitude of these objects could be brighter than M_B = -24 and therefore they could be type 2 QSOs.

From thermal-infrared (3.8 µm) imaging of 19 z ~ 1 3C NLRGs, [389] determined that in ~ 70% of these objects the nuclear extinction is A_V >15 mag.

In the so-called receding torus model, the obscuring region has a constant physical thickness, but its inner edge is determined by the radius at which dust evaporates; this radius should be larger in more luminous objects, increasing as L^0.5; the general increase in UV luminosity with radio power will cause the opening angle of the obscuring torus to increase on average with radio luminosity and therefore the relative number of type 2 QSOs to decrease ([193]).

The obscuring torus present in AGNs is hiding the nuclear continuum and broad line region in type 2 objects; the waste heat from the torus appears in the IR, and while it may not scale exactly with luminosity if the torus opening angle is changing, it should still be conspicuous in type 2 QSOs. Dust emission in QSOs accounts for ~ 30% of the luminosity in UV-selected QSOs ([368]); if the torus is opaque and if it produces most of the FIR luminosity, then the torus covering factor is typically 30% and there should be twice as many type 1 as type 2 QSOs at a given value of an isotropic parameter such as FIR luminosity ([9]); this is significantly larger than the observed ratio.

Three non-BALQSOs are known which show strong polarization of both the optical continuum and the broad emission lines: OI287 ([158]), WNJ0717+4611 ([100]) and IRASF10214+4724 ([160]). They have been interpreted as QSOs in which both the broad-line region and the continuum are completely obscured from direct view, being seen only by scattered radiation, the scattering producing a high degree of polarization of both the continuum and the broad emission lines. They could be considered as type 2 QSOs. 3C68.1 is an intermediate case with both reddened scattered (polarized) QSO light diluted by even more dust-reddened QSO light reaching us directly from the nucleus ([67]).