Clear detection of dusty torus signatures in a Weak-Line Radio Galaxy: the case of PKS 0043-42

TL;DR

This study presents the first clear detection of dusty torus signatures in a Weak-Line Radio Galaxy, PKS 0043-42, using infrared and X-ray spectra, challenging previous assumptions about WLRGs and suggesting diverse fueling mechanisms.

Contribution

It provides the first detailed infrared and X-ray analysis showing a dusty torus in a WLRG, indicating possible recent re-triggering of AGN activity and diversity within WLRG populations.

Findings

Detection of warm dust and silicate absorption in PKS 0043-42

Clumpy torus models fit the MIR spectrum with Type-2 AGN parameters

Evidence of cold gas accretion fueling the WLRG

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 wavelengths longer than ~20 micron suggestive of warm dust, as well as a 9.7 micron 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 deg) and relatively broad (torus angular width=60 deg), with an outer radius of 2 pc, hydrogen column density ~1.6x10^(23) cm^(-2), and AGN bolometric luminosity ~1.6x10^(44) 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.