The Nuclear Infrared Emission of Low-Luminosity Active Galactic Nuclei

TL;DR

This study investigates the infrared emission properties of low-luminosity active galactic nuclei (LLAGN) using high-resolution imaging and spectral data, revealing diverse IR characteristics linked to accretion rates and radio activity.

Contribution

It provides a detailed analysis of IR properties of LLAGN, highlighting differences based on Eddington ratio and radio emission, and suggests the absence of a Seyfert-like torus in these objects.

Findings

Low-Eddington LLAGN show host-dominated IR with star formation indicators.

Radio-loud LLAGN exhibit synchrotron-dominated IR emission.

Higher Eddington ratio LLAGN display diverse MIR SEDs with silicate features.

Abstract

We present high-resolution mid-infrared (MIR) imaging, nuclear spectral energy distributions (SEDs) and archival Spitzer spectra for 22 low-luminosity active galactic nuclei (LLAGN; Lbol \lesssim 10^42 erg/sec). Infrared (IR) observations may advance our understanding of the accretion flows in LLAGN, the fate of the obscuring torus at low accretion rates, and, perhaps, the star formation histories of these objects. However, while comprehensively studied in higher-luminosity Seyferts and quasars, the nuclear IR properties of LLAGN have not yet been well-determined. We separate the present LLAGN sample into three categories depending on their Eddington ratio and radio emission, finding different IR characteristics for each class. (I) At the low-luminosity, low-Eddington ratio (log Lbol/LEdd < -4.6) end of the sample, we identify "host-dominated" galaxies with strong polycyclic aromatic hydrocarbon bands that may indicate active (circum-)nuclear star formation. (II) Some very radio-loud objects are also present at these low Eddington ratios. The IR emission in these nuclei is dominated by synchrotron radiation, and some are likely to be unobscured type 2 AGN that genuinely lack a broad line region. (III) At higher Eddington ratios, strong, compact nuclear sources are visible in the MIR images. The nuclear SEDs of these galaxies are diverse; some resemble typical Seyfert nuclei, while others lack a well-defined MIR "dust bump". Strong silicate emission is present in many of these objects. We speculate that this, together with high ratios of silicate strength to hydrogen column density, could suggest optically thin dust and low dust-to-gas ratios, in accordance with model predictions that LLAGN do not host a Seyfert-like obscuring torus.