Comparison of Bar Strengths and Fractions of Bars in Active and Nonactive Galaxies

Abstract

Gravitational perturbation strengths and bar fractions in active and nonactive galaxies are compared using the Ohio State University Bright Galaxy Survey, which forms a statistically well defined sample of 180 disk galaxies. Bar fractions are studied using (1) the optical and near-IR classification of bars made by Eskridge and coworkers in 2002 and (2) our own bar classification based on Fourier decomposition of near-IR images (Fourier bars). The gravitational perturbation strengths are calculated using the bar torque method, taking the maximum ratio Q₉ of the tangential force to the mean background radial force as a measure of the nonaxisymmetric perturbation. In addition, two-dimensional bulge-disk-bar decomposition is used to study the properties of bulges of the sample galaxies. In the near-IR, Seyfert galaxies, LINERs, and H II/starburst galaxies were found to have a similar fraction, 72%, of Fourier bars (or SB-type bars), compared to 55% in the nonactive galaxies. However, if SAB-type bars are also included, practically all (95%) H II/starburst galaxies have bars. In addition, a large fraction (34%) of bars in LINERs are obscured by dust in the optical region. We find that bars in early-type galaxies are at the same time long and massive and have weak perturbation strengths. Weak perturbation strengths can be explained by dilution of the nonaxisymmetric forces by the massive bulges: for a bulge-to-disk mass ratio B/D ranging from 0 to 1, the dilution may reduce Q₉ from as high as 0.6 to as low as 0.1. On the other hand, bar length (relative to disk scale length) is not correlated with B/D, contrary to expectation. Seyfert- or LINER-type nuclear activity is present in most galaxies that have thin and thick planar bar components, whereas nuclear activity does not appear in those late-type galaxies that have extremely massive bars and strong perturbation strengths.