The Distribution of Bar and Spiral Arm Strengths in Disk Galaxies

The distribution of bar strengths in disk galaxies is a fundamental property of the galaxy population that has only begun to be explored. We have applied the bar-spiral separation method of Buta and coworkers to derive the distribution of maximum relative gravitational bar torques, Q_b, for 147 spiral galaxies in the statistically well-defined Ohio State University Bright Galaxy Survey (OSUBGS) sample. Our goal is to examine the properties of bars as independently as possible of their associated spirals. We find that the distribution of bar strength declines smoothly with increasing Q_b, with more than 40% of the sample having Q_b ≤ 0.1. In the context of recurrent bar formation, this suggests that strongly barred states are relatively short-lived compared to weakly barred or nonbarred states. We do not find compelling evidence for a bimodal distribution of bar strengths. Instead, the distribution is fairly smooth in the range 0.0 ≤ Q_b < 0.8. Our analysis also provides a first look at spiral strengths Q_s in the OSUBGS sample, based on the same torque indicator. We are able to verify a possible weak correlation between Q_s and Q_b, in the sense that galaxies with the strongest bars tend to also have strong spirals.