Embedded AGN and star formation in the central 80 pc of IC 3639^★

¹ Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: j.a.fernandez.ontiveros@gmail.com, jafo@iac.es
² Universidad de La Laguna (ULL), Dpto. Astrofísica, Avd. Astrofísico Fco. Sánchez s/n, 38206 La Laguna, Tenerife, Spain
³ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁵ School of Physics & Astronomy, University of Southampton, Southampton SO17 1BJ, UK

Received: 6 September 2017
Accepted: 3 November 2017

Abstract

Aims. Our goal is to probe the inner structure and the nature of the mid-IR emission in the active galaxy IC 3639, which hosts a Seyfert 2 nucleus and shows signatures of strong star-forming activity.

Methods. We used interferometric observations in the N-band with VLTI/MIDI to resolve the mid-IR emission of this nucleus. The origin of the nuclear infrared emission is determined from: (1) the comparison of the correlated fluxes from VLTI/MIDI with the fluxes measured at subarcsecond resolution (VLT/VISIR, VLT/ISAAC); (2) diagnostics based on IR fine-structure line ratios, the IR continuum emission, IR bands produced by polycyclic aromatic hydrocarbons (PAH) and silicates; and (3) the high-angular resolution spectral energy distribution.

Results. A large fraction of the total mid-IR emission of IC 3639 is produced in the innermost ≲80 pc with only ~1% of the total luminosity released in the UV/optical range. The unresolved flux of IC 3639 is 90 ± 20 mJy at 10.5 μm, measured with three different baselines in VLTI (UT1–UT2, UT3–UT4, and UT2–UT3; 46–58 m), making this the faintest measurement so far achieved with mid-IR interferometry. The correlated flux is a factor of 3–4 times fainter than the VLT/VISIR total flux measurement. The observations suggest that most of the mid-IR emission has its origin on spatial scales between 10 and 80 pc (40–340 mas). The emission confined within the inner 80 pc is either dominated by a starburst component or by the AGN core. The brightness distribution could be reproduced by a single component associated with the AGN, although this scenario would imply a very extended dust distribution when compared to other nearby Seyfert galaxies detected with MIDI. The extended component could also be associated with polar dust emission, that is, with a dusty wind blown by the AGN. However, a mixed contribution dominated by the star formation component over the AGN is favoured by the diagnostics based on ratios of IR fine-structure emission lines, the shape of the IR continuum, and the PAH and silicate bands.

Conclusions. A composite AGN-starburst scenario is able to explain both the mid-IR brightness distribution and the IR spectral properties observed in the nucleus of IC 3639. The nuclear starburst would dominate the mid-IR emission and the ionisation of low-excitation lines (e.g. [Ne II]_{12.8 μm}) with a net contribution of ~70%. The AGN accounts for the remaining ~30% of the mid-IR flux, ascribed to the unresolved component in the MIDI observations, and the ionisation of high-excitation lines (e.g. [Ne V]_{14.3 μm} and [O Iv]_{25.9 μm}).