We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations, and the availability of more mid-infrared data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type-2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss the recently discovered `fundamental plane' of black hole activity, showing that care must be taken when placing radio-loud AGN on such diagnostic diagrams.
Deep Dive into The active nuclei of z<1.0 3CRR radio sources.
We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our previous work, the reduction of bias in the sample as a result of new observations, and the availability of more mid-infrared data allow us to show conclusively that almost all objects classed as low-excitation radio galaxies in optical spectroscopic studies lack a radiatively efficient active nucleus. We show that the distribution of absorbing columns in the narrow-line radio galaxies differs from the population of X-ray-selected radio-quiet type-2 quasars and from that in local Seyfert 2s. We comment on the current evidence for the nature of the soft X-ray component in radio-galaxy nuclear spectra, concluding that a jet origin for this component is very hard to evade. Finally, we discuss th
The nature of the active nuclei of radio galaxies and radio-loud quasars has been a puzzle ever since large samples with optical identification and spectroscopy began to be constructed in the 1970s. Early work connected extragalactic radio sources with a bewildering variety of host objects, ranging in the local universe from recent merger remnants like the host galaxy of Centaurus A, NGC 5128, to quiescent ellipticals like M87, and in the more distant universe from undistinguished ellipticals with only stellar features in their optical spectra to the most powerful quasars known. Key steps in the understanding of this diversity included the discovery by Fanaroff & Riley (1974) that large-scale radio structure has a strong relationship to radio luminosity: their division of radio sources into centre-brightened (FRI) and edge-brightened (FRII) classes clearly encodes some important jet physics. Equally important, though far less widely cited, was the discovery by Hine & Longair (1979) that the optical emission lines from the nuclei of radio galaxy hosts could also be classified as 'weak' or 'strong', with a relationship to radio luminosity (weak-lined objects, hereafter low-excitation radio galaxies or LERGs tend to have low radio luminosity; stronglined objects, hereafter high-excitation radio galaxies or HERGs, tend to have high radio luminosities) but, crucially, no one-to-one correspondence with the Fanaroff-Riley morphological classes.
The development of unified models, beginning with the realization that both relativistic jets (e.g. Scheuer & Readhead 1979;Orr & Browne 1982) and active nuclei (e.g. Antonucci 1982) would have different appearances depending on the orientation of the observer, and culminating in quantitative constraints on source properties through population statistics (e.g. Barthel 1989; Urry, Padovani & Stickel 1991;Padovani & Urry 1992;Hardcastle et al. 2003) greatly simplified the picture (see Antonucci 1993 andUrry &Padovani 1995 for contemporary reviews). It became clear that quasars and high-power narrow-line radio galaxies (NLRGs) were likely to be the same population, seen at different orientations, and that low-power radio galaxies, which are generally LERGs (Hine & Longair 1979) could form the parent population of the mostly lineless BL Lac objects. A tendency to think of these models respectively as ‘FRII’ and ‘FRI’ unification, after the objects which dominate the HERG and LERG populations, has confused the literature ever since, and led to much work on physical differences in the nuclei of FRI and FRII radio galaxies. In fact as Hine & Longair showed, and as has been repeatedly pointed out over the ensuing decades (e.g. Barthel 1994;Laing et al. 1994;Jackson & Rawlings 1997;Chiaberge, Capetti & Celotti 2002;Hardcastle 2004;Whysong & Antonucci 2004) there is a population of FRII radio galaxies with low-excitation optical spectra, which modellers should not ignore; in fact, such low-excitation FRIIs are required to participate in low-luminosity unified models in order to explain the numerous BL Lac objects with FRII radio structure (e.g. Rector & Stocke 2001). There are clearly also radio sources with FRI structure but high-excitation optical characteristics (e.g. the broad-line FRI radio galaxy 3C 120, or the FRI quasar of Blundell & Rawlings 2001), although such objects are rarer in radio-selected samples.
The key unanswered question is therefore not ‘what causes the differences between FRI and FRII radio structures?’ -that can be adequately explained by the interactions between jets of different powers and their environments, see e.g. Bicknell (1995) -but ‘what is the nature and the cause of the differences between the low-excitation and high-excitation active nuclei?’. This question has taken on a new urgency with the advent of large surveys which provide populations of homogeneously selected AGN, both radioloud and radio-quiet, and show significant differences between their host galaxies and environments as a function of radio power (e.g. Best et al. 2006). Recently a consensus has started to emerge (Chiaberge et al. 2002;Whysong & Antonucci 2004;Hardcastle, Evans & Croston 2006, hereafter H06) that the LERGs in fact lack any of the conventional apparatus of an AGN -radiatively efficient accretion disc, X-ray emitting corona, and obscuring, mid-infra-red luminous torus -and that their radio through X-ray nuclear emission, and even such nuclear emission lines as are seen, can be explained purely as a result of the properties of the small-scale jet. In this picture some feature of the AGN and/or its fuel supply must account for the difference between the LERGs and the HERGs (i.e. NLRGs, broad-line radio galaxies (BLRGs) and quasars). We recently proposed (Hardcastle, Evans & Croston 2007, hereafter H07) that the low-excitation objects are those fuelled by accretion directly from the hot intergalactic medium, as is required in models in which cooling from the hot phase tri
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