High Resolution Mid-Infrared Observations of Low-Luminosity AGN with the Keck Telescope and LWS

Abstract

We present high-resolution Keck LWS SiC filter (10.5-12.9 micron) mid-IR (MIR) images and photometry of a sample of nearby LLAGN. All MIR images show extended emission on small scales (e.g. 90% maximum widths 18 - 106 pc) deep within the nucleus. Because the profile of this small-scale emission is inconsistent with the near-IR stellar emission profile, we conclude the MIR emission is not from the stellar population. Because strong starburst emission is not reported for our objects, we eliminate this origin as well, and conclude that the extended component of the MIR emission is most likely due to a warm dust structure deep in the nucleus, consistent with the gross details of a Unified Model Torus. Because MIR and X-ray flux measurements are very insensitive to the effects of pure dust extinction, the ratio of MIR and X emission can be used as a discriminator of emission processes that is insensitive to dust extinction. X and MIR emission are known to be correlated in quasars and Seyfert galaxies. Our LLAGN objects follow this correlation with small scatter down to extremely low luminosity, suggesting that the same continuum emission mechanisms are present in LLAGN as in Seyferts and quasars. This result suggests that weak UV emission (relative to other bands) in LLAGN compared to Seyferts and quasars could be due to dust extinction; it then follows that modifications to standard AGN continuum emission models for LLAGN (e.g. ADAF accretion) are not required.