The ionization cone, obscured nucleus, and gaseous outflow in NGC 3281 : a prototypical Seyfert 2 galaxy?

Abstract

We present narrow-band images and long-slit spectroscopy of the central region of the highly inclined Seyfert galaxy NGC 3281. The image of the continuum-subtracted [O III] λ4959 emission line shows a very clear conical morphology for the high-excitation gas, which seems to extend to a large distance (≈2 kpc) above the disk of the galaxy. A possible similar structure can also be seen on the other side of the nucleus, but is dimmed by patchy obscuration in the disk. Using the continuum images and long-slit spectroscopy, we derive and map the extinction in the inner regions of NGC 3281, and find heavy obscuration along our line of sight to the apex of the cone. This result suggests that the true nucleus is located at the apex and is obscured. Low-resolution long-slit spectra are used to study the stellar population, which is found to be old, uniform within 2.5 kpc of the nucleus and typical of the bulges of early-type galaxies. No dilution was observed in the absorption lines in any of the extracted spectra, confirming that the nuclear nonstellar source is obscured. The equivalent width of the Na r D absorption line is strongly correlated with the reddening of the starlight and, after subtraction of the contribution from stellar atmospheres, is used to map the interstellar obscuration. After subtraction of a "template" representing the stellar population, the fluxes of the emission lines are obtained, and maps showing the spatial distribution of emission-line ratios are constructed. These ratios are then compared with model calculations in order to investigate the nature of the ionizing source and characteristics of the emitting gas. According to these models, the abundances of nitrogen and sulfur in the gas are 3 times the solar value and the ionization parameter increases away from the apex along the axis of the cone. The density profile along the axis of the cone is obtained from the [S II] λ6717, 6731 doublet and compared with that predicted for a wind and for an hydrostatic atmosphere. Higher resolution long-slit spectra are used to study the kinematics of the emitting gas. A circular velocity model, intended to represent a rotational component of motion, is computed and subtracted from the observed gas velocities. The residuais indicate the presence of an outflow from the nucleus at 150 km sl within the cone. Observed double-peaked profiles are consistent with this interpretation. A comparison between the upper limit to the number of ionizing photons (Nion) inferred from the optical continuum and the observed number of Balmer recombination photons (Nrec) reveals a large discrepancy [Nrec/(Nion C)> 490, where C is the covering factor of optically thick gas as seen from the nucleus]. The nuclear ionizing source is, therefore, blocked from direct view (Av≥6 mag if the source radiates isotropically), apparently through shadowing by the dusty torus that hides the nucleus. The luminosity of the ionizing source is obtained from the emission line properties and used to predict the mid-and far-infrared luminosity produced through reprocessing by dust in the torus. This luminosity (5.3 ± 2.2 x 1010 Lʘ) is consistent with that inferred from the IRAS fluxes. All of the above characteristics are expected in "unified" models of AGNs, in which Seyfert 2's contain a Seyfert 1 nucleus which is hidden from direct view by an obscuring torus. We therefore suggest that NGC 3281 may be another example of a "hidden" Seyfert 1, even though there is no direct evidence for a broad­ line region in this particular galaxy.