Circumnuclear structure and black hole fueling: Hubble Space Telescope NICMOS imaging of 250 active and normal galaxies

Why are the nuclei of some galaxies more active than others? If most galaxies harbor a central massive black hole, the main difference is probably in how well it is fueled by its surroundings. We investigate the hypothesis that such a difference can be seen in the detailed circumnuclear morphologies of galaxies using several quantitatively defined features, including bars, isophotal twists, boxy and disky isophotes, and strong nonaxisymmetric features in unsharp-masked images. These diagnostics are applied to 250 high-resolution images of galaxy centers obtained in the near-infrared with NICMOS on the Hubble Space Telescope. To guard against the influence of possible biases and selection effects, we have carefully matched samples of Seyfert 1, Seyfert 2, LINER, starburst, and normal galaxies in their basic properties, taking particular care to ensure that each was observed with a similar average scale (10-15 pc pixel(-1)). Several morphological differences among our five different spectroscopic classifications emerge from the analysis. The H II/starburst galaxies show the strongest deviations from smooth elliptical isophotes, while the normal galaxies and LINERs have the least disturbed morphology. The Seyfert 2s have significantly more twisted isophotes than any other category, and the early-type Seyfert 2s are significantly more disturbed than the early-type Seyfert 1s. The morphological differences between Seyfert 1s and Seyfert 2s suggest that more is at work than simply the viewing angle of the central engine. They may correspond to different evolutionary stages.