Nuclear Activity and Host Galaxy Properties of Low-Luminosity AGN Identified from VLA Observations

TL;DR

This study analyzes 38 low-luminosity AGN from VLA observations, comparing multiwavelength detection methods and examining their host galaxy properties, revealing they host smaller black holes with lower accretion rates in diverse, often less active galaxies.

Contribution

It provides a comprehensive assessment of detection efficiencies across wavelengths and characterizes the host galaxy properties of low-luminosity AGN in the local universe.

Findings

Optical diagnostics recover 84.2% of LLAGN

X-ray detection rate is 63.2%

Infrared methods identify only 13.2% of LLAGN

Abstract

Low-luminosity active galactic nuclei (LLAGN; $L_{\rm bol} < 10^{42}$~erg~s$^{-1}$) may comprise a significant fraction of the local AGN population, yet their weak emission makes them difficult to detect. In this paper, we analyse 38 LLAGN identified from a 15~GHz sub-arcsecond Very Large Array survey and assess the effectiveness of X-ray, optical, and infrared wavelengths in identifying LLAGN. We found that optical emission-line diagnostics recovered $84.2^{+15.8}_{-22.9}$\% (32/38) of the sample, X-rays detected $63.2^{+25.7}_{-19.6}$\% (24/38), and infrared methods only identified $13.2^{+14.5}_{-8.0}$\% (5/38), reflecting limited X-ray sensitivity, weak or absent optical lines, and strong host galaxy contamination in the infrared. Compared to \textit{Swift}--BAT AGN, our LLAGN are $\sim$ 4.1~dex fainter in bolometric luminosity (log $L_{\mathrm{bol}} \approx$ 39.3 - 41.9 erg s$^{-1}$), host smaller black holes ($\sim$0.7~dex lower), and accrete at much lower rates (log $\lambda_{\text{Edd}} \approx$ -6.5 to -1.3, i.e., $\sim$ 4.2~dex lower). Host galaxies span a broad range of morphologies, from disk- to bulge-dominated, with a subset exhibiting prominent bulges, potentially representing systems where nuclear activity has faded while the bulge remains dominant. LLAGN also reside in galaxies with lower stellar masses ($\sim$0.3~dex) and suppressed star formation rates ($\sim$0.5~dex) relative to \textit{Swift}--BAT AGN. Overall, LLAGN in our sample systematically host smaller and weakly accreting black holes, residing in galaxies with diverse morphologies, but lower stellar masses and reduced star formation activity, demonstrating the connection between low-level black hole accretion and host galaxy properties in the local Universe.