Morphology of the 12 micron Seyfert galaxies. I. Hubble types, axial ratios, bars, and rings

We have compared the morphological characteristics of the 891 galaxies in t he Extended 12 mu m Galaxy Sample (E12GS) and assessed the effect of the 12 mu m selection criterion on galaxy properties. The normal spirals in the E 12GS have the same axial. ratios, morphological types, and bar and ring fra ctions as other normal spirals. The H II/starburst galaxies have a higher i ncidence of bars and more than twice the normal rate of "peculiar" morpholo gies, both of which are attributable to relatively recent disturbances. The 12 mu m Seyfert galaxies show a small (10%) deficiency of edge-on disks. T his is caused by extinction but is a much less severe effect than in optica lly selected samples. There is a similar modest deficit of highly inclined H II/starburst galaxies in the 12 mu m sample. The galaxies with active nuc lei (Seyfert galaxies and LINERs) have the same incidence of bars as normal spirals but show rings significantly more often than normal galaxies or st arbursts. 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 o f these differences. Bars form relatively quickly and indicate that materia l 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., less than or similar to 10(8) yr) burst of star formation. Outer rings may resu lt from similar disturbances but require much more time to form. They would then be associated with more intense nuclear activity if it takes 10(9) 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 enhan ced star formation in its center, while later it is more likely to show LIN ER activity, and still later it is likely to be a Seyfert galaxy. The trend s we find with morphology and nuclear activity are not biased either by the distances of the galaxies or by the slightly elevated recent star formatio n rates shown by the 12 mu m galaxies in general.