[Paper Review] Radio Sources in Low-Luminosity Active Galactic Nuclei.IV. Radio Luminosity Function, Importance of Jet Power, and Radio Properties of the Complete Palomar Sample

Neil M. Nagar, H. Falcke+1 more|University of Groningen research database (University of Groningen / Centre for Information Technology)|Feb 25, 2005

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy125 references169 citations

TL;DR

This study presents a complete high-resolution radio survey of 200 low-luminosity active galactic nuclei (LLAGNs) from the Palomar Sample, finding that over half host compact radio cores and sub-parsec jets, indicating accreting black holes. The key result is that jet kinetic power, not radiated luminosity, dominates energy output in LLAGNs, with jets potentially regulating gas supply to the accretion disk.

ABSTRACT

We present the completed results of a high resolution radio imaging survey of all (~200) low-luminosity active galactic nuclei (LLAGNs) and AGNs in the Palomar Spectroscopic Sample. The high incidences of pc-scale radio nuclei, with T(brightness) >=10^7 K, and sub-parsec jets argue for accreting black holes in >=50% of all LINERs and low-luminosity Seyferts; there is no evidence against all LLAGNs being mini-AGNs. The radio luminosity function (RLF) of Palomar Sample LLAGNs and AGNs extends three orders of magnitude below, and is continuous with, that of `classical' AGNs. We find marginal evidence for a low-power turnover in the RLF; nevertheless LLAGNs are responsible for a significant fraction of present day mass accretion. Adopting the jet model of Falcke & Biermann, we show that the accretion energy output in LLAGNs is dominated by the energy in the observed jets rather than the radiated bolometric luminosity. The Palomar LLAGNs and AGNs follow the same scaling between jet power and narrow line region (NLR) luminosity as the pc to kpc jets in powerful radio galaxies. Eddington ratios of <= 10^{-1} - 10^{-5} are implied in jet models of the radio emission. We find evidence that, in analogy to Galactic black hole candidates, LINERs are in a `low/hard' state (gas poor nuclei, low Eddington ratio, ability to launch collimated jets) while low-luminosity Seyferts are in a `high' state (gas rich nuclei, higher Eddington ratio, less likely to launch collimated jets). The jets are energetically more significant than supernovae in the LLAGN host galaxies, and are potentially able to deposit sufficient energy into the innermost parsecs to significantly slow the gas supply to the accretion disk.

Motivation & Objective

• To determine the ubiquity of compact radio cores and jets in low-luminosity AGNs (LLAGNs) and assess their implications for accretion physics.
• To construct the radio luminosity function (RLF) of LLAGNs and compare it with classical AGNs to assess their role in cosmic accretion.
• To evaluate whether jet kinetic power or radiated luminosity dominates energy output in LLAGNs using a relativistic jet model.
• To investigate the connection between Eddington ratio, nuclear state (e.g. 'low/hard' vs. 'high' state), and jet production efficiency in LLAGNs.

Proposed method

• Conducted high-resolution radio imaging (VLBI and aperture synthesis) at 15 GHz and 8.4 GHz on all 200 LLAGNs in the Palomar Spectroscopic Sample.
• Measured nuclear radio luminosities and brightness temperatures to identify compact, incoherent radio cores with T_b ≥ 10^7 K, indicating non-thermal emission from relativistic jets.
• Constructed the radio luminosity function (RLF) for LLAGNs using space density and luminosity data, fitting log ρ ∝ -0.78 × log L_15 GHz with evidence for a low-luminosity turnover.
• Applied the relativistic jet model of Falcke & Biermann to estimate jet kinetic power and compare it with radiated bolometric luminosity.
• Correlated jet kinetic power with narrow-line region (NLR) luminosity to test scaling relations with powerful radio galaxies.
• Classified LLAGNs by host morphology and emission-line type (LINER vs. low-luminosity Seyfert) to assess differences in accretion state and jet efficiency.

Experimental results

Research questions

• RQ1 What fraction of low-luminosity AGNs host compact radio cores and sub-parsec jets, and what does this imply for the presence of accreting black holes?
• RQ2 How does the radio luminosity function (RLF) of LLAGNs compare with that of classical AGNs, and is there evidence for a turnover at low luminosities?
• RQ3 Is jet kinetic power the dominant form of accretion energy output in LLAGNs, and how does it compare to radiated luminosity?
• RQ4 Do LINERs and low-luminosity Seyferts differ in Eddington ratio, nuclear gas density, and jet production efficiency, suggesting distinct accretion states?
• RQ5 Can radio jets in LLAGNs deposit sufficient energy into the interstellar medium to regulate gas inflow to the accretion disk?

Key findings

• Over 50% of LINERs and low-luminosity Seyferts host compact radio cores with brightness temperatures ≥ 10^7 K, indicating accreting black holes; no evidence against all LLAGNs being mini-AGNs.
• The radio luminosity function (RLF) of Palomar LLAGNs extends three orders of magnitude below classical AGNs and shows marginal evidence for a low-luminosity turnover near log L_15 GHz ≈ 19.
• Jet kinetic power dominates over radiated bolometric luminosity by factors of ∼2 to >100 in LLAGNs, making jets the primary energy output mechanism.
• LLAGNs follow the same scaling between jet kinetic power and NLR luminosity as powerful radio galaxies, indicating a universal jet-accretion connection.
• Eddington ratios in jet models range from 10^{-1} to 10^{-5}, with LINERs in a 'low/hard' state (low Eddington ratio, gas-poor, efficient jet launching), while low-luminosity Seyferts are in a 'high' state (higher Eddington ratio, gas-rich, less efficient jet production).
• Radio jets in LLAGNs are energetically more significant than supernovae in their host galaxies and can deposit sufficient energy in the inner parsecs to slow gas inflow to the accretion disk.