X-Ray versus Optical Observations of Active Galactic Nuclei: Evidence for Large Grains?

Recently, Maiolino et al. constructed a sample of active galactic nuclei for which both the reddening E(B-V) and the column density N_H to the nucleus could be determined. For most of the galaxies in their sample, they found that E(B-V)/N_H is substantially smaller than for the diffuse interstellar medium of our Galaxy. They asserted that either the dust-to-gas ratio is lower than in the Galaxy or the grains are so large that they do not extinct or redden efficiently in the optical. We show that there is no systematic increase in E(B-V) with N_H for the Maiolino et al. galaxies, which suggests that the X-ray absorption and optical extinction occur in distinct media. In a later paper, Maiolino et al. suggested that the observed lines of sight for the previous Maiolino et al. galaxies pass through the ``torus'' that obscures the broad-line region and nuclear continuum in Seyfert 2 galaxies and argued that the torus grains are larger than Galactic grains. There is no reason to believe that the lines of sight for these galaxies pass through the torus, since the observed column densities are lower than those typically observed in Seyfert 2 galaxies. We suggest instead that the X-ray absorption occurs in material located off the torus and/or accretion disk, while the optical extinction occurs in material located beyond the torus. The X-ray absorbing material could either be dust-free or contain large grains that do not extinct efficiently in the optical. There is no conclusive evidence that the grains in active galactic nuclei are systematically larger than those in the diffuse interstellar medium of our Galaxy. We discuss an alternative way to probe the properties of dust in Seyfert tori but find that observations of Seyfert 2 nuclei with higher resolution than currently available will be needed in order to place stringent limits on the dust.