Beyond Simple AGN Unification with Chandra-observed 3CRR Sources at 0.5 < z < 1

Abstract

Low-frequency radio selection finds radio-bright galaxies regardless of the amount of obscuration by gas and dust. We report Chandra observations of a complete 178 MHz–selected, and so orientation-unbiased, sample of 44 0.5 < z < 1 3CRR sources. The sample is comprised of quasars and narrow-line radio galaxies (NLRGs) with similar radio luminosities, and the radio structure serves as both an age and an orientation indicator. Consistent with unification, intrinsic obscuration (measured by N _H, X-ray hardness ratio, and X-ray luminosity) generally increases with inclination. However, the sample includes a population not seen in high- z 3CRR sources: NLRGs viewed at intermediate inclination angles with N _H < 10 ²² cm ⁻². Multiwavelength analysis suggests that these objects have lower L/ L _Edd than typical NLRGs at similar orientation. Thus, both orientation and L/ L _Edd are important, and a “radiation-regulated unification” provides a better explanation of the sample’s observed properties. In comparison with the 3CRR sample at 1 < z < 2, our lower-redshift sample shows a higher fraction of Compton-thin NLRGs (45% versus 29%) but a similar Compton-thick fraction (20%), implying a larger covering factor of Compton-thin material at intermediate viewing angles and thus a more “puffed-up” torus atmosphere. We posit that this is due to a range of L/ L _Edd extending to lower values in this sample. In contrast, at high redshifts, the narrower range and high L/ L _Edd values allowed orientation (and so simple unification) to dominate the sample’s observed properties.