UCLA

We have compared the morphological characteristics of the 891 galaxies in the Extended 12 μm Galaxy Sample (E12GS) and assessed the effect of the 12 μm 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 H II/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 12 μm Seyfert galaxies show a small (10%) deficiency of edge-on disks. This is caused by extinction but is a much less severe effect than in optically selected samples. There is a similar modest deficit of highly inclined H II/starburst galaxies in the 12 μm sample. The galaxies with active nuclei (Seyfert galaxies and LINERs) have the same incidence of bars as normal spirals but show rings significantly more often than ormal galaxies or starbursts. The LINERs have elevated rates of inner rings, while the Seyfert galaxies 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., ≲ 108 yr) burst of star formation. Outer rings may result from similar disturbances but require much more time to form. They would then be associated with more intense nuclear activity if it takes 109 yr or more for the mass transfer to reach the center and raise the black hole accretion rate, by which time the bar has dissolved or has begun to do so. Inner rings form before outer ones, with a formation 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 it is likely to be a Seyfert galaxy. The trends we find with morphology and nuclear activity are not biased either by the distances of the galaxies or by the slightly elevated recent star formation rates shown by the 12 μm galaxies in general.