The nature of the IR emission in LLAGN at parsec scales

Abstract

The vast majority of AGN belong to the low-luminosity class (LLAGN): they exhibit a low radiation effciency ( L ≲ 10 42 erg s −1 ; L / L edd ≲ 10 −3 ) and the absence of the big blue bump in their spectrum, a signature of the accretion disk. The study of LLAGN is a complex task due to the contribution of the host galaxy, whose light outshines these faint nuclei. As a consequence, numerical models are usually compared with relatively poorly defined spectral energy distributions (SEDs). For a sample of six prototype nearby LLAGN, a multiwavelength dataset including radio, IR, optical/UV and X-ray measurements with a few tenths of arcsec resolution has been collected. These high-spatial resolution SEDs reveal that: i ) the mid-IR bump, indicative of thermal emission from the torus, is missing in LLAGN; ii ) the continuum emission of these nuclei is largely described by a self-absorbed synchrotron spectrum, suggesting that jet emission dominates the overall energy output in these objects. The optically thin radiation in the IR-to-UV range is produced in the jet launching region, very close to the central black hole. The very steep slope found in this component –with a spectral index in the 1-3 range– suggests that a large number of LLAGN are powered by young and compact jets with very high radiative losses.