THE INFRARED PROPERTIES OF QUASARS AND RADIO GALAXIES - TESTING THE UNIFICATION SCHEMES

We have used the IRAS data base to compare the mid-far-infrared (MFIR) properties of two classes of radio-loud active galactic nuclei (AGNs) whose low-frequency radio emission arises in large-scale radio lobes. The two primary samples consist of 42 quasars and 75 narrow-line radio galaxies selected on the basis of their 178 MHz radio flux density from the 3 CR catalog and having redshifts greater-than-or-equal-to 0.3 and \Galactic latitudes\ greater-than-or-equal-to 20-degrees. Subsamples with more restricted redshift ranges have also been defined. Members of the rare blazar class have been excluded from all samples, and members of the gigahertz-peaked and compact steep spectrum radio classes have been excluded from some subsamples. All the pairs of radio galaxy and quasar samples and subsamples are well-matched in their distributions of 178 MHz radio flux, radio spectral shape. redshift, and low-frequency radio power. The most important new result is that the MFIR emission lambda = 12-100-mu-m in the Earth's frame and typically lambda almost-equal-to 6-50-mu-m in the AGN rest frame) is approximately 4 times stronger on-average in the quasars than in the radio galaxies. Even though the radio galaxies are systematically weaker MFIR sources than the quasars, they are still exceptionally luminous in the MFIR (L(MFIR) almost-equal-to 10(12) L.). The MFIR appears to be the energetically dominant wavelength regime for the radio galaxies and is comparable to the optical/near-UV regime for the quasars. Thus these powerful radio galaxies contain potent sources of radiation that are either hidden or energetically unimportant at shorter wavelengths. Since the 178 MHz radio emission is thought to be emitted essentially isotropically in these two types of AGNs, we conclude that at least one of the following two statements must be true: (1) The 3 CR quasars are intrinsically stronger sources of MFIR emission than 3 CR radio galaxies with similar radio powers. (2) The MFIR radiation from 3 CR quasars and radio galaxies is emitted anisotropically. If the first statement is true, then Barthel's simple "unification scheme" radio-loud quasars and radio galaxies are intrinsically identical objects selected from a common parent population solely on the basis of viewing angle must be incorrect or incomplete. If the second statement is true, then the popular model in which the far-IR is thermal emission from dust may be untenable at least for the 3 CR quasars. This is because in order for the MFIR to be emitted anisotropically, the emitting region must be optically thick in the far-IR. This requires very large column densities for the emitting material. Together with the large dimensions of the far-IR thermally emitting region, this may lead to implausibly large masses for the emitting structure.