Morphology of the 12-micron Seyfert Galaxies: I. Hubble Types, Axial Ratios, Bars, and Rings

Abstract

We have compared the morphological characteristics of the 891 galaxies in the Extended 12-micron Sample (E12GS), and assessed the effect of the 12-micron selection criterion on galaxy properties. The normal spirals in the E12GS have the same axial ratios, morphological types, and bar and ring fractions as other normal spirals. The HII/starburst galaxies have a higher incidence of bars, and more than twice the normal rate of "peculiar" morphologies, both of which are attributable to relatively recent disturbances. The galaxies with active nuclei (AGNs: Seyferts and LINERs) have the same incidence of bars as normal spirals, but show rings significantly more often than normal galaxies or starbursts. The LINERs have elevated rates of inner rings, while the Seyferts have outer ring fractions several times those in normal galaxies. The different formation times of bars and rings suggest an interpretation of these differences. Bars form relatively quickly, and indicate that material is recently being transported (by redistribution of angular momentum) to the center of the galaxy, where it is likely to trigger a short (e.g., $\lesssim 10^8$ yrs) burst of star formation. Outer rings may result from similar disturbances, but require much more time to form, while inner rings form in a shorter time, more comparable to bars. Thus it may be that after an interaction or instability triggers an infall of gas, the galaxy in the earliest stage is likely to show enhanced star formation in its center, while later it is more likely to show LINER activity, and still later likely to be a Seyfert.