Evidence for Higher Black Hole Spin in Radio-loud Quasars

Andreas Schulze,Chris Done,Youjun Lu,Fupeng Zhang,Yoshiyuki Inoue

doi:10.3847/1538-4357/aa9181

Abstract

Abstract One of the major unsolved questions concerning the understanding of the active galactic nucleus population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests that radio-loud quasars have a higher black hole spin. However, the measurement of black hole spin remains extremely challenging. We here aim at comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet Sloan Digital Sky Survey (SDSS) quasars at 0.3 < z < 0.8 . We use the [O iii] luminosity as an indirect average tracer of the ionizing continuum in the extreme-UV regime where the differences in the spectral energy distribution (SED) due to black hole spin are most pronounced. We find that the radio-loud sample shows an enhancement in [O iii] line strength by a factor of at least 1.5 compared to a radio-quiet sample matched in redshift, black hole mass, and optical continuum luminosity or accretion rate. We argue that this enhancement is caused by differences in the SED, suggesting higher average bolometric luminosities at fixed accretion rate in the radio-loud population. This suggests that the radio-loud quasar population has on average systematically higher radiative efficiencies and therefore higher black hole spin than the radio-quiet population, providing observational support for the black hole spin paradigm.

Highlights

Quasars constitute the most luminous types of Active Galactic Nuclei (AGN), where a supermassive black hole (SMBH) is powered by significant mass accretion through a thin accretion disc
RL and RQ quasar samples matched in optical continuum luminosity and black hole mass should have consistent mean L[OIII] if they have on average the same radiative efficiency, black hole spin
We briefly review the dependencies of the radio-loud fraction (RLF) on black hole mass, luminosity and accretion rate for our sample

Summary

Introduction

Quasars constitute the most luminous types of Active Galactic Nuclei (AGN), where a supermassive black hole (SMBH) is powered by significant mass accretion through a thin accretion disc. Only about 10% of all quasars are radio-loud (RL), i.e. have relativistic jets with high bulk Lorentz factor, Γ ∼ 10. The majority have much weaker core radio emission, so are termed radio–quiet (RQ). This distinction in radio to optical flux is not sharp, but there are clearly two populations (e.g. Kellermann et al 1989; Ivezic et al 2002; Balokovic et al 2012). Viewing angle with respect to the jet will change the observed intensity due to relativistic beaming (Scheuer & Readhead 1979), but it is clear that inclination unifies different classes of RL AGN rather than explaining the difference between RL and RQ Viewing angle with respect to the jet will change the observed intensity due to relativistic beaming (Scheuer & Readhead 1979), but it is clear that inclination unifies different classes of RL AGN rather than explaining the difference between RL and RQ (e.g. Urry & Padovani 1995)

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Conclusion