Statistical Study of the Star Formation Efficiency in Bars: Is Star Formation Suppressed in Gas-rich Bars?

Abstract

The dependence of the star formation efficiency (SFE) on galactic structures—especially whether the SFE in the bar region is lower than those in other regions—has recently been debated. We report the SFEs of 18 nearby gas-rich massive star-forming barred galaxies with large apparent bar major axes (≧75″). We statistically measure the SFE by distinguishing the center, the bar end, and the bar regions for the first time. The molecular gas surface density is derived from archival CO(1–0) and/or CO(2–1) data by assuming a constant CO-to-H2 conversion factor (α CO), and the star formation rate surface density is derived from a linear combination of far-UV and mid-IR intensities. The angular resolution is 15″, which corresponds to 0.3–1.8 kpc. We find that the ratio of the SFE in the bar to that in the disk was systematically lower than unity (typically 0.6–0.8), which means that the star formation in the bar is systematically suppressed. Our results are inconsistent with similar recent statistical studies, which have reported that the SFE tends to be independent of galactic structures. This inconsistency can be attributed to the differences in the definitions of the bar region, the spatial resolutions, the α CO, and the sample galaxies. Furthermore, we find a negative correlation between the SFE and the velocity width of the CO spectrum, which is consistent with the idea that the large dynamical effects—such as strong shocks, large shears, and fast cloud–cloud collisions caused by the noncircular motion of the bar—result in a low SFE.