Infrared spectroscopy around 4 μm of Seyfert 2 galaxies:: Obscured broad line regions and coronal lines

The state of the matter that is obscuring a small circumnuclear region in active galactic nuclei can be probed by observations of its broad emission lines. Infrared lines are particularly useful since they penetrate significant columns of obscuring matter, the properties of which can be constrained by comparing infrared and X-ray obscuration. We report on new 4 mum spectroscopy with ISAAC at the ESO VLT of a sample of 12 Seyfert 2 galaxies, probing for broad components to the Brackett alpha 4.05 mum hydrogen recombination line. Broad components are observed in 3 to 4 objects. All objects with a broad component exhibit relatively low X-ray obscuring columns, and our results are consistent with a Galactic ratio of 4 mum obscuration to the BLR and X-ray column. In combination with observations of a non-Galactic ratio of visual obscuration of BLRs and X-ray obscuring column in Seyferts, and interpreted in a unified AGN scheme, this result can be reconciled with two interpretations. Either the properties of dust near the AGN are modified towards larger grains, for example through coagulation, in a way that significantly flattens the optical/IR extinction curve, or the ratio of dust obscuration to X-ray column varies for different viewing angles with respect to the axis of symmetry of the putative torus. Our spectra also provide a survey of emission in the [Si IX] 3.94 mum coronal line, finding variation by an order of magnitude in its ratio to Bralpha. The first extragalactic detection of the [Ca VII] 4.09 mum and [CaV] 4.16 mum coronal lines is reported in the spectrum of the Circinus galaxy.