The luminous infrared composite Seyfert 2 galaxy NGC 7679 through the [O III] λ 5007 emission line

• Published in: Astronomy and astrophysics (Berlin) 2007-07, Vol.469 (3), p.891-898
• Main Authors: Yankulova, I. M., Golev, V. K., Jockers, K.
• Format: Article
• Language: English
• Subjects: galaxies: individual: NGC 7679 (Mrk 534); galaxies: ISM; galaxies: Seyfert; galaxies: starburst
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20077440

Context. NGC 7679 (Mrk 534) is a nearby (z = 0.0177) nearly face-on SB0 luminous infrared Sy2 galaxy in which starburst and AGN activities co-exist. The ionization structure is maintained by both the AGN power-law continuum and starburst. The galaxy is a bright X-ray source possessing a low X-ray column density NH < 4 $\times$ 1020 cm-2. Aims. The Compton-thin nature of such unabsorbed objects infers that the simple formulation of the Unified model for SyGs is not applicable in their case. The absorption is likely to originate at larger scales instead of the pc-scale molecular torus. The main goal of this article is to investigate both gas distribution and ionization structure in the circumnuclear region of NGC 7679 in search for the presence of a hidden Sy1-type nucleus, using the [O III]λ5007 luminosity as a tracer of AGN activity. Methods. NGC 7679 was observed with the 2m RCC reflector of the Ukraine National Astronomical Observatory at peak Terskol, Caucasus, Russia. The observations were carried out in October 1996 with the Focal Reducer of the Max-Planck-Institut für Sonnensystemforschung, Germany. All observations were taken with tunable Fabry-Perot narrow-band imaging with spectral FWHM of the Airy profile $\delta\lambda$ between 3 and 4 Å depending on the used wavelength. Results. The [O III]λ5007 emission-line image of the circumnuclear region of NGC 7679 shows elliptical isophotes extended along the PA ≈ 80° in the direction of the counterpart galaxy NGC 7682. There is a maximum of this emission which is shifted ~4 arcsec from the center as defined by the continuum emission. The maximum of ionization by the AGN power-law continuum traced by [O III]λ5007/Hα ratio is displaced by ~13 arcsec eastward from the nucleus. The direction where high ionization is observed at PA ≈ 80° ± 10° coincides with the direction to the companion galaxy NGC 7682 (PA ≈ 72°). On the contrary, at PA ~ 0° the ionization in the circumnuclear region is entirely due to hot stars. Conclusions. Both the ratio $(N_{\rm ph}/N_{\rm ion})_{h\nu\,>\,55\,{\rm eV}} \approx$ 0.2-20 of the number Nph of photons traced by [O III]  to the number Nion of high-energy ionizing photons and the presence of weak and elusive Hα  broad wings indicate a hidden AGN. We conclude that the dust and gas in the high ionization direction PA ≈ 80° has a direct view to the central AGN engine. This possibly results in dust/star-formation decay. A large fraction of the unabsorbed Compton-thin Sy2s with [O