The environments of luminous radio galaxies and type-2 quasars

C Ramos Almeida,P S Bessiere,J Holt,K J Inskip,C N Tadhunter,D Dicken,R Morganti,J I González-Serrano

doi:10.1093/mnras/stt1595

Abstract

We present the results of a comparison between the environments of 1) a complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7), 2) a complete sample of 20 radio-quiet type-2 quasars (0.3 < z < 0.41), and 3) a control sample of 107 quiescent early-type galaxies at 0.2 < z < 0.7 in the Extended Groth Strip (EGS). The environments have been quantified using angular clustering amplitudes (Bgq) derived from deep optical imaging data. Based on these comparisons, we discuss the role of the environment in the triggering of powerful radio-loud and radio-quiet quasars. When we compare the Bgq distributions of the type-2 quasars and quiescent early-type galaxies, we find no significant difference between them. This is consistent with the radio-quiet quasar phase being a short-lived but ubiquitous stage in the formation of all massive early-type galaxies. On the other hand, PRGs are in denser environments than the quiescent population, and this difference between distributions of Bgq is significant at the 3 sigma level. This result supports a physical origin of radio loudness, with high density gas environments favouring the transformation of AGN power into radio luminosity, or alternatively, affecting the properties of the supermassive black holes themselves. Finally, focussing on the radio-loud sources only, we find that the clustering of weak-line radio galaxies (WLRGs) is higher than the strong-line radio galaxies (SLRGs), constituting a 3 sigma result. 82% of the 2Jy WLRGs are in clusters, according to our definition (Bgq > 400) versus only 31% of the SLRGs.

Highlights

Quasars have long played an important role in the study of galaxy evolution
We found that weak-line radio galaxies (WLRGs) are in richer environments than strong-line radio galaxies (SLRGs), with mean clustering amplitudes of Bgaqv(SLRGs) = 303 ± 53 and Bgaqv(WLRGs) = 788 ± 140, and this difference is significant at the 3σ level
Summarizing, we find no clear correlations between the environments and the ratios between the radio powers and the emission line or mid-IR luminosities, we find that the objects with the largest clustering amplitudes – most of which are WLRGs/Fanaroff–Riley I (FRI) – tend to have larger ratios of radio luminosity to intrinsic active galactic nucleus (AGN) power

Summary

Introduction

Quasars have long played an important role in the study of galaxy evolution. We have seen widespread acceptance for the ubiquity of the supermassive black holes (SMBHs) that power their active nuclei, and the likelihood that feedback during the active galactic nucleus (AGN) phase may play an important role in moderating galaxy formation and evolution. We know surprisingly little about how and when quasars are triggered as part of the hierarchical growth of galaxies (see Alexander & Hickox 2012, for a recent review). The tidal torques associated with galaxy bars, disc instabilities, galaxy interactions and major mergers between galaxies are efficient mechanisms to transport the cold gas required to trigger and feed AGN and star formation to the centres of galaxies. The gas has to lose ∼99.9 per cent of its angular

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Conclusion