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

Recently, Maiolino et al. (2001a, A&A, 365, 28) constructed a sample ofactive galactic nuclei for which both the reddening E(B-V) and the columndensity N_H to the nucleus could be determined. For most of the galaxies intheir sample, they found that E(B-V)/N_H is substantially smaller than for thediffuse ISM of our Galaxy. They asserted that either the dust-to-gas ratio islower than in the Galaxy or that the grains are so large that they do notextinct or redden efficiently in the optical. We show that there is nosystematic increase in E(B-V) with N_H for the Maiolino et al. (2001a)galaxies, which suggests that the X-ray absorption and optical extinction occurin distinct media. Maiolino et al. (2001b, A&A, 365, 37) suggested that the observed lines ofsight for the Maiolino et al. (2001a) galaxies pass through the ``torus'' thatobscures the broad line region and nuclear continuum in Seyfert 2 galaxies andargued that the torus grains are larger than Galactic grains. There is noreason to believe that the lines of sight for these galaxies pass through thetorus, since the observed column densities are lower than those typicallyobserved in Seyfert 2 galaxies. We suggest instead that the X-ray absorptionoccurs in material located off the torus and/or accretion disk while theoptical extinction occurs in material located beyond the torus. The X-rayabsorbing material could either be dust-free or could contain large grains thatdo not extinct efficently in the optical. There is no conclusive evidence thatthe grains in active galactic nuclei are systematically larger than those inthe diffuse ISM of our Galaxy.Comment: 20 pages, 4 figures, accepted by Ap