Black hole masses, accretion rates and hot- and cold-mode accretion in radio galaxies at z ∼ 1

Abstract

Understanding the evolution of accretion activity is fundamental to our understanding of how galaxies form and evolve over the history of the Universe. We analyse a complete sample of 27 radio galaxies which includes both high-excitation (HEGs) and low excitation galaxies (LEGs), spanning a narrow redshift range of 0.9 < z < 1.1 and covering a factor of ∼ 1000 in radio luminosity. Using data from the Spitzer Space Telescope combined with ground-based optical and near-infrared imaging, we show that the host galaxies have masses in the range of 10.7 < log 10 (M/M ⊙ ) < 12.0 with HEGs and LEGs exhibiting no difference in their mass distributions. We also find that HEGs accrete at significantly higher rates than LEGs, with the HEG/LEG division lying at an Eddington ratio of λ ∼ 0.04, which is in excellent agreement with theoretical predictions of where the accretion rate becomes radiatively inefficient, thus supporting the idea of HEGs and LEGs being powered by different modes of accretion. Our study also shows that at least up to L 151MHz ∼ 3 × 10 27 W Hz −1 sr −1 , HEGs and LEGs are indistinguishable in terms of their radio properties. From this result we infer that, at least for the lower radio luminosity range, another factor besides accretion rate must play an important role in the process of triggering jet activity.

Observational evidence that positive and negative AGN feedback depends on galaxy mass and jet power

“…The Eddington ratio for radio galaxies is significantly higher in the first case, where λ = (L bol + Qjet)/L Edd , and this trend is dominated by high-excitation galaxies (HEGs; see Fernandes et al 2015; Fig. 7).…”

“…Antonucci 1993). Details of the selection of the quasars are presented by Falder et al (2010) while the RG selection is described by Fernandes et al (2015). The Herschel photometry is provided in Appendix A (Table A1) while a summary of the main properties of the sample objects is given in Appendix A (Table A2).…”

“…For this reason, we use the stellar mass of the galaxy, M gal , determined by the SED fitting of Fernandes et al (2015) for the same radio galaxy sample as used in this work. Fernandes et al (2015) used the same BH-bulge mass relation (Häring & Rix 2004) in order to calculate the BH mass of the radio galaxy sample. The implied MBH are given in Table A2.…”

“…For the radio galaxy sample, we adopted the values of L bol calculated by Fernandes et al (2015) from the rest-frame 12µm luminosity, using a bolometric correction of 8.5 (e.g. Richards et al 2006), L bol = 8.5λL12µm.…”

“…We estimate the jet power using the relation Qjet ≃ 3 × 10 38 f 3/2 (L151MHz/10 28 ) 6/7 W (4) (Willott et al 1999), where 1 f 20 represents the combination of several uncertainty terms when estimating Qjet from L151MHz. Following Fernandes et al (2015), we chose f = 10 as this is the expectation value of a flat prior in natural space. We note that the Qjet contributes significantly to the total power only in the radio galaxies of our sample, which is derived from the L12µm, and not in the RLQs (< 10 per cent).…”