The Nuclear Infrared Emission of Low-Luminosity Active Galactic Nuclei

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.