The nature of the IR emission in LLAGN at parsec scales

Does the jet dominate at low-luminosities?

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, Bonn, D–53121, Germany
² Instituto de Astrofísica de Canarias (IAC), Vía Láctea s/n, La Laguna, E–38200, Spain
³ Departamento de Astrofísica, Facultad de Física, Universidad de La Laguna, Astrofísico Fco. Sánchez s/n, La Laguna, E–38207, Spain
⁴ Astronomical Institute “Anton Pannekoek”, 1098 XH Amsterdam, Netherlands

^a e-mail: jafo@mpifr.de

Published online: 9 December 2013

Abstract

The vast majority of AGN belong to the low-luminosity class (LLAGN): they exhibit a low radiation effciency (L ≲ 10⁴² 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.