[Paper Review] Radio Sources in Low-Luminosity Active Galactic Nuclei. III. "AGNs" in a Distance-Limited Sample of "LLAGNs"

Neil M. Nagar, H. Falcke+2 more|ArXiv.org|Jul 8, 2002

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy121 references111 citations

TL;DR

This study conducts a high-resolution radio survey of 96 low-luminosity active galactic nuclei (LLAGNs) within 19 Mpc, finding that nearly half of LINERs and low-luminosity Seyferts host compact, flat-spectrum radio cores with brightness temperatures ≥10⁸ K, strongly indicating accreting supermassive black holes. The core radio power correlates with black hole mass, bulge luminosity, and optical emission-line luminosity, supporting the presence of low-accretion-rate AGNs across LLAGNs.

ABSTRACT

(abbreviated): This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D<19Mpc. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150mas resolution. Higher (2mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures > 10^8 K. The five LLAGNs with the highest core radio fluxes also have pc-scale `jets.' Compact radio cores are almost exclusively found in massive ellipticals and in type1 nuclei. The core radio power is correlated with the nuclear optical `broad' Halpha luminosity, the nuclear optical `narrow' emission line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ~150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (<150mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ~150 galaxies give: log P(2cm) = 1.31 log M_blackhole + 8.77 and log P(2cm) = 1.89 log L_B(bulge) - 0.17. Low accretion rates are implied in both advection- and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs.

Motivation & Objective

• To determine the incidence of compact radio cores in a complete, distance-limited sample of low-luminosity active galactic nuclei (LLAGNs).
• To investigate the nature of radio emission in LLAGNs and distinguish between AGN-driven and star formation-related origins.
• To test whether LLAGNs host accreting supermassive black holes by examining correlations between radio core power, black hole mass, and nuclear emission-line properties.
• To assess the role of galaxy morphology and bulge luminosity in shaping radio and optical emission-line characteristics in LLAGNs.

Proposed method

• Conducted high-resolution 2 cm (150 mas) and 6 cm (2 mas) radio observations using the Very Large Array (VLA) and Very Long Baseline Array (VLBA) on all 96 LLAGNs within 19 Mpc.
• Measured core flux densities, brightness temperatures, and spectral indices to identify flat-spectrum radio cores indicative of synchrotron emission from accretion inflows or jets.
• Performed partial correlation analysis to isolate the independent effects of black hole mass and bulge luminosity on core radio power.
• Analyzed inter-year variability in radio flux at 2 cm and 3.6 cm to assess the dynamic nature of the compact radio sources.
• Used emission-line diagnostic ratios and Hα line widths to classify nuclei as LINER, Seyfert, or transition types and correlate these with radio properties.
• Fitted linear regression models to the full sample of ~150 nearby bright galaxies to quantify the scaling of core radio power with black hole mass (log P₂cm = 1.31 log Mₘₚₒ + 8.77) and bulge luminosity (log P₂cm = 1.89 log Lʙ - 0.17).

Experimental results

Research questions

• RQ1 Do compact, flat-spectrum radio cores in LLAGNs indicate the presence of accreting supermassive black holes, as predicted by AGN unification models?
• RQ2 What is the incidence of pc-scale radio cores in a complete, distance-limited sample of LINERs and low-luminosity Seyferts?
• RQ3 How do core radio power, black hole mass, and bulge luminosity correlate in LLAGNs, and what do these correlations imply about accretion physics?
• RQ4 To what extent do radio properties in LLAGNs correlate with optical emission-line luminosities and widths, and how do these relate to the unified scheme?
• RQ5 Are the radio cores in LLAGNs variable over time, and what does this imply about their origin and stability?

Key findings

• 100% of the 16 LLAGNs observed at 2 mas resolution with the VLBA showed compact, flat-spectrum radio cores with brightness temperatures ≥10⁸ K, indicating non-thermal, AGN-like emission.
• The core radio power correlates strongly with both black hole mass (log P₂cm = 1.31 log Mₘₚₒ + 8.77) and bulge luminosity (log P₂cm = 1.89 log Lʙ - 0.17), with partial correlations confirming independent significance.
• Half of all LLAGNs with multiple-epoch data showed significant inter-year radio variability at 2 cm and 3.6 cm, supporting a dynamic, compact emission region.
• The five LLAGNs with the highest core fluxes all exhibited resolved pc-scale jets, further supporting a jet or accretion-powered origin.
• The ratio of core radio power to nuclear emission-line luminosity increases with bulge luminosity, suggesting a stronger radio contribution in massive, bulge-dominated galaxies.
• Transition nuclei with compact radio cores have emission-line diagnostic ratios close to those of LINERs/Seyferts, indicating they are composite systems with a dominant AGN component.