Clear detection of dusty torus signatures in a weak-line radio galaxy: the case of PKS 0043-42

Abstract

We report the clearest detection to date of dusty torus signatures in a weak-line radio galaxy (WLRG). The deep Spitzer InfraRed Spectrograph (IRS) rest-frame mid-infrared (MIR) spectrum of the WLRG PKS 0043-42 (z = 0.116) shows a clear spectral turnover at lambda greater than or similar to 20 mu s suggestive of warm dust, as well as a 9.7-mu m silicate absorption feature. In addition, the hard X-ray results, based on Chandra data, strongly support a picture in which PKS 0043-42 has a torus and accretion disc more typical of strong-line radio galaxies (SLRGs). The MIR and X-ray spectra are markedly different from those of other WLRGs at similar redshifts, and here we show that the former can be successfully fitted with clumpy torus models with parameters characteristic of Type-2 AGN tori: close to edge-on (i = 74 degrees) and relatively broad (Sigma = 60 degrees), with an outer radius of 2 pc, N-H = 1.6 +/- 0.2(0.1) x 1023 cm-2, and AGN bolometric luminosity LAGN(bol) = 1.6 +/- 0.2(0.1) x 1044 erg s-1. The presence of a compact torus in PKS 0043-42 provides evidence that this WLRG is fuelled by cold, rather than hot, gas accretion. We suggest that WLRGs are a diverse population, and PKS 0043-42 may represent a type of radio galaxy in which the AGN activity has been recently re-triggered as a consequence of intermittent gas supply, or in which the covering factor of the narrow-line region (NLR) clouds is relatively low.