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

摘要

The state of the matter that is obscuring a small circumnuclear region in AGNcan be probed by observations of its broad emission lines. Infrared lines areparticularly useful since they penetrate significant columns of obscuringmatter, the properties of which can be constrained by comparing infrared andX-ray obscuration. We report on new 4um spectroscopy of a sample of 12 Seyfert2 galaxies, probing for broad components to the Brackett alpha 4.05um line.Broad components are observed in 3 to 4 objects. All objects with a broadcomponent exhibit relatively low X-ray obscuring columns, and our results areconsistent with a Galactic ratio of 4um obscuration to the BLR and X-raycolumn. In combination with observations of a NON-Galactic ratio of VISUALobscuration of BLRs and X-ray obscuring column in Seyferts, and interpreted ina 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 theoptical/IR extinction curve, or the ratio of dust obscuration to X-ray columnvaries for different viewing angles with respect to the axis of symmetry of theputative torus. Our spectra also provide a survey of emission in the [SiIX]3.94um coronal line, finding variation by an order of magnitude in its ratio toBr alpha. The first extragalactic detection of the [CaVII] 4.09um and [CaV]4.16um coronal lines is reported in the spectrum of the Circinus galaxy.