Author submitted version of unrefereed Nature Astronomy comment. Version in journal format available at https://rdcu.be/KH6WRadiative efficiency in radio-loud active galactic nuclei is governed by the accretion rate onto the central black hole rather than directly by the type of accreted matter; while it correlates with real differences in host galaxies and environments, it does not provide unambiguous information about particular objects.Non peer reviewedFinal Accepted Versio

A Merloni

B Mingo

CM Urry

G Ghisellini

G Gürkan

M Gaspari

MJ Hardcastle

PN Best

R Narayan

RG Hine

RJ Turner

Nature Astronomy

English

arXiv

Martin Hardcastle

Crossref

Interpreting radiative efficiency in radio-loud AGNs

Radiative efficiency in radio-loud active galactic nuclei is governed by the accretion rate onto the central black hole rather than directly by the type of accreted matter; while it correlates with real differences in host galaxies and environments, it does not provide unambiguous information about particular objects

Hardcastle, M. J.

University of Hertfordshire Research Archive

     Citation for the published version:   Hardcastle, M. J. (2018). Interpreting radiative efficiency in radio-loud AGNs. Nature Astronomy, 2(4), 273-274. https://doi.org/10.1038/s41550-018-0424-1  Document Version:  Accepted Version    Link to the final published version available at the publisher:   https://doi.org/10.1038/s41550-018-0424-1       General rights Copyright© and Moral Rights for the publications made accessible on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (http://uhra.herts.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge. Take down policy If you believe that this document breaches copyright please contact us providing details, any such items will be temporarily removed from the repository pending investigation. Enquiries Please contact University of Hertfordshire Research & Scholarly Communications for any enquiries at rsc@herts.ac.uk Interpreting radiative efficiency in radio-loud AGNM.J. HardcastleApril 12, 2018AbstractRadiative efficiency in radio-loud AGN is governed by the scaled accretion rate on to the centralblack hole rather than directly by the type of accreted matter; while it correlates with real physicaldifferences, it does not give us unambiguous information about particular objects.Active galactic nuclei (AGN) selected by observations in the optical, infrared (IR) or X-ray are almostalways ‘radiatively efficient’: they show evidence of a luminous accretion disc, which produces opticaland UV emission directly, photoionizes ambient material to produce broad and narrow emission-lineregions, and drives strong X-ray emission from a ‘corona’ above the disc. Their radiation may beabsorbed and re-radiated in the mid-IR by a warm, dusty structure referred to as the ‘torus’. Thediversity of observed properties in these AGN is well explained on the whole by a combination ofintrinsic luminosity differences and orientation-dependent obscuration [1].AGN selected in radio observations can behave very differently. The evidence is overwhelming thatthese ‘radio-loud’ AGN (RLAGN) can operate in both radiatively efficient and inefficient modes [seee.g. 2, 3, 4, and refs therein]. The radiatively inefficient (RI) objects — known variously as low-excitation radio galaxies, weak-line radio galaxies or ‘jet-mode’ objects — have active jets, whichdrive the radio emission, but no evidence for a luminous accretion disc, torus, corona, or accretion-driven emission lines; by contrast, radiatively efficient (RE) radio-loud objects — high-excitation radiogalaxies, narrow-line radio galaxies, broad-line radio galaxies, radio-loud quasars, or ‘quasar-mode’objects — behave like conventional AGN with the addition of a jet. Nuclear emission in RI objectscomes from the jet only. In terms of their number, RI objects are the dominant population by a verylarge factor in the local universe [5] and so it is important to understand their nature and their effectson their environments.How does this RE/RI dichotomy come about? A long-standing proposal [6, 7] is that its basic driver isthe Eddington-scaled accretion rate: objects whose total radiative luminosity and jet power fall belowa few per cent of the estimated Eddington rate, LEdd =4πGMBHcmpσT, are RI, while objects above thislimit are capable of sustaining a conventional RE accretion disc. This is as expected from theory [8],though the exact threshold expected depends on details of the accretion disc model. This picture hasrecently received strong observational support from large-sample studies. [5] estimated total radiativeluminosity from the optical emission-line luminosity of a large sample of RLAGN drawn from SDSS,NVSS and FIRST, and found a clear division between RI and RE objects in Eddington-scaled accretion1arXiv:1804.04043v1  [astro-ph.GA]  11 Apr 2018rate, while studies using X-ray and mid-IR emission as proxies of radiative power instead [9, 10] havecome to very similar conclusions, albeit with smaller samples. The basic picture is that all RLAGNhave accretion flows that drive jets of varying powers, but only a small minority have accretion rateshigh enough (relative to Eddington) to also power a RE disc. In RI objects, the estimated kinetic jetpower may exceed upper limits on the radiative luminosity from the accretion flow by many orders ofmagnitude.Ten years ago my collaborators and I [11] proposed that there was a one-to-one relationship betweenthe two accretion modes and the two obvious sources of fuel available to AGN in general: RE sourceswould be fuelled by cold gas (which could be carried into the mostly elliptical hosts of RLAGN bymergers, or produced by cooling) and RI sources by hot gas (directly from the hot phase of the inter-galactic or intracluster medium). A consequence would be that RI objects are capable of participatingin the feedback loop between AGN and hot gas in cluster centres, while RE sources, which get theirfuel from other sources, may input far more energy into their hot-gas environments than is required tooffset cooling.This ‘hot-mode/cold-mode’ picture, though widely adopted, has partly been superseded by our im-proved understanding over the past decade. If the Eddington-scaled accretion rate controls the radiativeefficiency, there is no reason why a RI source cannot be produced by accreting at a low level from acold gas reservoir, or why rapid accretion from the hot phase (now widely thought [12] to be mediatedby cooling-instability driven ‘chaotic accretion’ rather than Bondi accretion) should not be capable ofsustaining high accretion rates. Indeed, we can point to individual AGN, such as NGC 1275 in thePerseus cluster, that clearly show signatures of RE accretion from cold material while also being (pre-sumably) fundamentally regulated by cooling from the hot phase. Thus at first sight one might imaginethat the ‘hot-mode/cold-mode’ model has no further role to play. But it is surprising, given these facts,that it has proved so successful in predicting or incorporating the known observational differences be-tween RE and RI objects at low z. Observationally we see that RI objects in general tend to lie in richenvironments, prefer massive quiescent galaxies, and have little star formation, all as expected if theyare fuelled by hot gas; RE objects tend to lie in poorer environments, prefer smaller galaxies often withmajor mergers, and can be strongly star-forming, as would be expected if there were a copious cold gassupply.Why does the model make such good predictions if its basic premise is wrong? I suggest that the bestexplanation is that there is a strong association between the conditions needed to make a RI systemand accretion from the hot phase. The RI/RE threshold can be recast as a ratio Ṁ/MBH between theaccretion rate and the black hole mass; for the elliptical hosts of RLAGN MBH scales well with galaxymass. Galaxies that are the dominant (most massive) systems in a group or cluster environment, for agiven jet power, will be more likely to be RI than galaxies in the field both because they will have highMBH and because the main source of fuel will be cooling at relatively low Ṁ from the hot phase; merger-driven accretion of large quantities of cold gas in these systems is disfavoured because of ram-pressurestripping or strangulation of satellite galaxies in the hot-gas environment. By contrast, powerful REAGN, where high accretion rates, close to Eddington, are required, can most easily be produced insystems with high Ṁ and relatively low MBH and are expected to favour lower-mass galaxies in fieldenvironments. All that is required for this preference to give rise to significant differences between thetwo populations in large samples (as illustrated in Figure 1) is for there to be a wide range in Ṁ/MBHand in RLAGN environment, both of which we know to be true for powerful sources. Thus, although2the fundamental basis of the model of [11] has changed, the predictions on a sample basis can remaincorrect.There are two important consequences of this revised picture. Firstly, unlike in the ‘hot-mode/cold-mode’ model, the RI/RE observational classification of any given RLAGN does not necessarily tell usanything very fundamental about the source. If a source is close (in terms of Ṁ/MBH) to the boundarybetween the two classes, it may change type repeatedly over its lifetime as a result of comparativelysmall changes in the accretion rate; there is a great deal of evidence that RLAGN and AGN in gen-eral can undergo large-amplitude long-timescale variation of the accretion power. Treating the currentRI/RE status of a RLAGN as though it were an immutable property of the system over its lifetime isincorrect and may lead to other errors. More generally, it is never safe to conclude that a particular ob-ject’s fuel source is hot gas or cold gas based on its RI/RE classification; the model predicts a statisticalpreference that may be over-ridden by other factors. Secondly, the results on the cosmological evolu-tion of the two populations now becoming available [5] need to be interpreted with caution, becauseit is not clear that the associations between fuel source and accretion mode will operate in quantita-tively the same way in the earlier Universe; for example, the evolution of black-hole masses will meanthat sources are more likely to be classed as RE for the same physical accretion rate Ṁ in the earlierUniverse, so environments of similar richness might be expected to produce more RE sources.In the picture presented here, RE/RI classification is a proxy of a specific quantity (accretion rate perunit black hole mass) that may not be particularly relevant to the purpose of a given study. Dividingobservationally generated samples by their radiative efficiency certainly captures some real physicaldifferences, but other methods of classifying them might provide more understanding. The ideal situ-ation would be one in which we had direct estimates of the physically relevant quantities (jet kineticpower, radiative power, black hole mass...) and could divide up our samples according to these ratherthan simply according to the RI/RE class, but these parameters, particularly jet power which is funda-mental to all studies of RLAGN, are hard to estimate uniformly and reliably. Developing techniquesto extract jet power from radio observations using Bayesian inference based on detailed models of theevolution of radio sources [13, 14], and so moving away from the use of one well-studied but funda-mentally limited AGN property, will be one of the key challenges for RLAGN science in the era of theSKA and its precursors.Martin Hardcastle is at the Centre for Astrophysics Research, University of Hertfordshire, CollegeLane, Hatfield AL10 9AB, UK. e-mail: m.j.hardcastle@herts.ac.ukReferences[1] Urry, C. M. & Padovani, P. Publ. Astron. Soc. Pacific 107, 803 (1995).[2] Hine, R. G. & Longair, M. S. Mon. Not. R. Astron. Soc. 188, 111 (1979).[3] Laing, R. A., Jenkins, C. R., Wall, J. V. & Unger, S. W. In Bicknell G.V., Dopita M.A., Quinn P.J.(ed.) The First Stromlo Symposium: the Physics of Active Galaxies, 201 (ASP Conference Seriesvol. 54, San Francisco, 1994).[4] Hardcastle, M. J., Evans, D. A. & Croston, J. H. Mon. Not. R. Astron. Soc. 370, 1893 (2006).3108 109Black hole mass (M)10−410−310−210−1100101102Accretionrate(Myear−1)Switch lineRadiatively inefficientRadiatively efficientFigure 1: Black hole mass and accretion rate in a Monte Carlo simulation of a toy model in which blackhole mass is uniformly distributed in log space between 107.5 and 109.5M while intrinsic accretion rate,which is unrelated to MBH, has a lognormal distribution centred on 0.1M year−1 with a dispersionof 1 dex. The RE/RI division is a straight line in this plot corresponding to 3% of the Eddingtonrate (assuming 10% conversion efficiency of accreted matter). Even in this toy model, RE sourceson average have black hole masses a factor 3 less than RI sources and available (radiative/kinetic)luminosities, accounting for the Eddington limit, 1.2 decades higher, although there is essentially nodifference between RE and RI sources close to the green line. In reality, the accretion rate is likelynot uncorrelated with the black hole mass, but the details of this relationship need to be fleshed outobservationally.4[5] Best, P. N. & Heckman, T. M. Mon. Not. R. Astron. Soc. 421, 1569–1582 (2012).[6] Ghisellini, G. & Celotti, A. Astron. Astrophys. 379, L1 (2001).[7] Merloni, A. & Heinz, S. Mon. Not. R. Astron. Soc. 388, 1011–1030 (2008).[8] Narayan, R. & Yi, I. Astrophys. J. 444, 231 (1995).[9] Mingo, B. et al. Mon. Not. R. Astron. Soc. 440, 269–297 (2014).[10] Gürkan, G. et al. Mon. Not. R. Astron. Soc. 452, 3776–3794 (2015).[11] Hardcastle, M. J., Evans, D. A. & Croston, J. H. Mon. Not. R. Astron. Soc. 376, 1849 (2007).[12] Gaspari, M., Ruszkowski, M. & Oh, S. P. Mon. Not. R. Astron. Soc. 432, 3401–3422 (2013).[13] Turner, R. J. & Shabala, S. S. Astrophys. J. 806, 59 (2015).[14] Hardcastle, M. J. Mon. Not. R. Astron. Soc. 475, 2768–2786 (2018).5

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Interpreting radiative efficiency in radio-loud AGNs

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