2014MNRAS.439.1270R

We present an analysis of infrared Hubble Space Telescope (HST) and Spitzer data for a sample of 13 Fanaroff-Riley II (FRII) radio galaxies at 0.03 < z < 0.11 that are classified as narrow-line radio galaxies (NLRGs). In the context of unified schemes for active galactic nuclei (AGNs), our direct view of AGNs in NLRGs is impeded by a parsec-scale dusty torus structure. Our high-resolution infrared observations provide new information about the degree of extinction resulting from the torus, and about the incidence of obscured AGNs in NLRGs.

We find that the point-like nucleus detection rate increases from 25 per cent at 1.025 µm, to 80 per cent at 2.05 µm, and to 100 per cent at 8.0 µm. This supports the idea that most NLRG host an obscured AGN in their centre. We estimate the extinction from the obscuring structures using X-ray, near-IR and mid-IR data. We find that the optical extinction derived from the 9.7 µm silicate absorption feature is consistently lower than the extinction derived using other techniques. This discrepancy challenges the assumption that all the mid-infrared emission of NLRG is extinguished by a simple screen of dust at larger radii. This disagreement can be explained in terms of either weakening of the silicate absorption feature by (i) thermal mid-IR emission from the narrow-line region, (ii) non-thermal emission from the base of the radio jets, or (iii) by direct warm dust emission that leaks through a clumpy torus without suffering major attenuation.