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Why are the nuclei of some galaxies more active than others? If most galaxiesharbor a central massive black hole, the main difference is probably in howwell it is fueled by its surroundings. We investigate the hypothesis that sucha difference can be seen in the detailed circumnuclear morphologies of galaxiesusing several quantitatively defined features, including bars, isophotaltwists, boxy and disky isophotes, and strong non-axisymmetric features inunsharp masked images. These diagnostics are applied to 250 high-resolutionimages of galaxy centers obtained in the near-infrared with NICMOS on HST. Toguard against the influence of possible biases and selection effects, we havecarefully matched samples of Seyfert 1, Seyfert 2, LINER, starburst and normalgalaxies in their basic properties, taking particular care to ensure that eachwas observed with a similar average scale (10-15 parsecs per pixel). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The HII/starburst galaxies show thestrongest deviations from smooth elliptical isophotes, while the normalgalaxies and LINERS have the least disturbed morphology. The Seyfert 2 galaxieshave significantly more twisted isophotes than any other category, and theearly-type Seyfert 2s are significantly more disturbed than the early-typeSeyfert 1s. The morphological differences between Seyfert 1s and 2s suggestthat more is at work than simply the viewing angle of the central engine. Theymay correspond to different evolutionary stages.Comment: 47 pages, 10 figures (4 postscript, 6 gif), to appear in ApJ, 1  December 200

Circumnuclear Structure and Black Hole Fueling:Hubble Space TelescopeNICMOS Imaging of 250 Active and Normal Galaxies