University of Groningen

Building Hunt, Leslie Kipp

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Publication date: 2004

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Citation for published version (APA): Hunt, L. K. (2004). Building galaxies: from low-metallicity compact dwarfs to active galactic nuclei. s.n.

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Download date: 29-09-2021 CHAPTER 12

Circumnuclear Structure and Fueling: HST/NICMOS Imaging of 250 Active and Normal Galaxies

Abstract1

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 sur- roundings. We investigate the hypothesis that such a difference can be seen in the detailed circumnuclear morphologies of galaxies using several quantitatively defined features, includ- ing bars, isophotal twists, boxy and disky isophotes, and strong non-axisymmetric features in unsharp masked images. These diagnostics are applied to 250 high-resolution images of centers obtained in the near-infrared with NICMOS on HST. 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 − per pixel). Several morphological differences among our five different spectroscopic classifi- cations 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 dis- turbed morphology. The Seyfert 2 galaxies 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 2s suggest that more is at work than simply the viewing angle of the central engine. They may correspond to different evolutionary stages.

1. Introduction The established theoretical explanation for Seyfert activity requires that all active galactic nuclei (AGNs) must have formed central massive black holes (BHs) and are now fueling them, presumably with gas from the host galaxy that has lost most of its orbital angular momentum. However, BHs are not exclusive to AGNs, since quiescent ones are now found in most, if not all, galaxies with massive spheroids. The BH mass seems to depend on certain properties of the bulge

1This chapter is taken from Hunt, L.K. & Malkan, M.A. 2004, ApJ, in press.

247 Chapter 12 248 in which it resides (Ferrarese & Merrit