2018A&A...611A..46F

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 withVLTI/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 fromVLTI/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±20mJy at 10.5µm, measured with three different baselines inVLTI (UT1-UT2, UT3-UT4, and UT2-UT3; 46-58m), making this the faintest measurement so far achieved with mid-IR interferometry. The correlated flux is a factor of 3-4 times fainter than theVLT/VISIR total flux measurement. The observations suggest that most of the mid-IR emission has its origin on spatial scales between 10 and 80pc (40-340mas). 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. [NeII]_12.8µm) 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. [NeV]_14.3µm and [OIV]_25.9µm).