OBSCURATION, ORIENTATION, AND THE INFRARED PROPERTIES OF RADIO-LOUD ACTIVE GALAXIES

We report on a study of the mid- and far-infrared (MFIR) properties of several different classes of radio-loud active galactic nuclei (AGNs) using the IRAS database. Our goal is to try to improve the understand ing of the possible relationships between the diverse classes of AGNs. The MFIR and radio properties of radio-loud AGNs are especially usefu l in this regard, since (excluding the blazar class, which we do not s tudy here) the radio emission is thought to be emitted isotropically, and the radio and MFIR radiation should be much less affected by dust obscuration than radiation at shorter wavelengths. We have first compa red samples of 3CR broad-line radio galaxies (BLRGs) and narrow-line r adio galaxies (NLRGs) matched in radio flux and mean redshift. We find that the BLRGs are stronger than the NLRGs by a factor of 4-5 in thei r mid-IR emission but are similar to the NLRGs in the far-IR. This is qualitatively consistent with recent ''unification'' models for NLRGs and BLRGs which invoke thermal MFIR emission from dusty ''obscuring to ri,'' but there may be an additional source of far-IR emission present in the more luminous broad-line objects (the radio-loud quasars) stud ied previously by Heckman, Chambers, & Postman (1992). We have also co mpared samples of Fanaroff-Riley class I (FRI) and Fanaroff-Riley clas s II (FRII) radio galaxies matched in radio flux and redshift. The FRI I galaxies are stronger MFIR emitters than the FRI galaxies by a facto r of about 4. This is consistent with suggestions that the central eng ine in FRI galaxies produces relatively little radiant energy per unit jet power (especially since we find that the weak MFIR emission from the FRI galaxies may not be powered by the AGN). Comparing samples of gigahertz-peaked spectrum (GPS) and compact steep spectrum (CSS) sourc es versus non-GPS-CSS sources, we find that the GPS-CSS and non-GPS-CS S sources have similar MFIR strengths. This suggests that the efficien cy of the conversion of jet kinetic energy into radio emission is not much higher in the GPS-CSS sources, contrary to some theoretical predi ctions. Overall, we find that the MFIR and radio powers of all the cla sses of radio-loud AGNs we have studied correlate well with one anothe r over a range of about 10(3) in power. This is most naturally underst ood if the MFIR is primarily powered by the AGN in most highly luminou s radio-loud AGNs. However, other processes (starbursts or the intracl uster medium) may contribute significantly in the less radio-luminous radio galaxies.