Far-infrared excess in radio quasars

I.M. van Bemmel ¹ P.D. Barthel ²

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching

² Kapteyn Institute for Astronomy, PO Box 800, NL-9700 AV Groningen

With IRAS it was already observed that radio-loud quasars (QSRs) are in general brighter at 60 and 100  than radio galaxies (RGs) (Heckman et al., 1994). This is in contradiction with the expectation within the unified model for radio-loud AGN (Barthel, 1989; Hes et al., 1995), postulating a dusty torus shielding the central engine from direct view in the RGs. Models for thermal far-infrared (FIR) torus emission suggest that it is isotropic at wavelengths longward of 60  (Pier & Krolik, 1992; Granato & Danese, 1994). It was suggested that beamed non-thermal emission could be responsible for the IR excess in QSRs (Hoekstra et al., 1997), but an investigation of three IRAS detected double-lobed QSRs showed that beaming alone could not cause such an excess (van Bemmel et al., 1998).

To address the anisotropy in more detail, an ISO project was conceived, studying six matched pairs of intermediate redshift double-lobed QSRs and RGs, taken from the 3C-catalog. Four of these pairs were actually observed with ISO at 60, 90 (C100) and 160 um (C200) in raster mode. Nearly simultaneous observations at four cm-mm bands with the VLA were conducted to permit an assessment of possible non-thermal FIR emission in the QSRs. These measurements were supplemented with JCMT-SCUBA observations at 850 and 450 .

All four QSRs are detected at all three ISO wavelengths, while the RGs only have a 25% detection rate. The classes differ markedly, even at 160 . After subtraction of non-thermal flux, the QSRs are brighter at 160  in three cases, while in all cases the QSRs are brighter at 60 and 90 . Current torus models predict isotropy at these wavelengths. The origin of this FIR-excess in the QSR class is difficult to understand within the framework of the unification theory. One viable explanation may be the occurrence of beamed non-thermal flares, as observed in blazars (Brown et al., 1989).