15-GHz radio emission from nearby low-luminosity active galactic nuclei

TL;DR

This study uses high-resolution 15 GHz radio imaging to detect and analyze low-luminosity active galactic nuclei, exploring their emission properties, correlations with other wavelengths, and the shape of their radio luminosity function.

Contribution

It provides the first high-resolution radio detections for a large LLAGN sample and refines the fundamental plane relations and the radio luminosity function for these objects.

Findings

60% of LLAGN detected at 15 GHz with compact radio emission.

Established fundamental plane relations involving radio, X-ray, and optical emissions.

Identified a turnover in the radio luminosity function at low luminosities.

Abstract

We present a sub-arcsec resolution radio imaging survey of a sample of 76 low-luminosity active galactic nuclei (LLAGN) that were previously not detected with the Very Large Array at 15 GHz. Compact, parsec-scale radio emission has been detected above a flux density of 40 $\mu$Jy in 60% (45 of 76) of the LLAGN sample. We detect 20 out of 31 (64%) low-ionization nuclear emission-line region (LINER) nuclei, ten out of 14 (71%) low-luminosity Seyfert galaxies, and 15 out of 31 (48%) transition objects. We use this sample to explore correlations between different emission lines and the radio luminosity. We also populate the X-ray and the optical fundamental plane of black hole activity and further refine its parameters. We obtain a fundamental plane relation of log L$_\textrm{R}$ = $0.48\,(\pm0.04$) log L$_\textrm{X}$ + $0.79\,(\pm0.03$) log $\textrm{M}$ and an optical fundamental plane relation of log L$_\textrm{R}$ = $0.63\,(\pm 0.05)$ log L$_{[\rm O~III]}$ + $0.67\,(\pm 0.03)$ log $\textrm{M}$ after including all the LLAGN detected at high resolution at 15 GHz, and the best-studied hard-state X-ray binaries (luminosities are given in erg s$^{-1}$ while the masses are in units of solar mass). Finally, we find conclusive evidence that the nuclear 15 GHz radio luminosity function (RLF) of all the detected Palomar Sample LLAGN has a turnover at the low-luminosity end, and is best-fitted with a broken power law. The break in the power law occurs at a critical mass accretion rate of 1.2$\times$10$^{-3}$ M$_{\odot}$/yr, which translates to an Eddington ratio of $\rm \dot m_{Edd} \sim 5.1 \times 10^{-5}$, assuming a black hole mass of 10$^9 M_{\odot}$. The local group stands closer to the extrapolation of the higher-luminosity sources, and the classical Seyferts agree with the nuclear RLF of the LLAGN in the local universe.