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

Fumiya MaedaFumi EgusaKouji OhtaYusuke FujimotoAsao Habe
Nov 2022
The dependence of star formation efficiency (SFE) on galactic structures,especially whether the SFE in the bar region is lower than those in the otherregions, has recently been debated. We report the SFEs of 18 nearby gas-richmassive star-forming barred galaxies with a large apparent bar major axis($\geqq 75^{\prime\prime}$). We statistically measure the SFE by distinguishingthe center, bar-end, and bar regions for the first time. The molecular gassurface density is derived from archival CO(1-0) and/or CO(2-1) data byassuming a constant CO-to-H$_2$ conversion factor ($\alpha_{\rm CO}$), and thestar formation rate surface density is derived from a linear combination offar-ultraviolet and mid-infrared intensities. The angular resolution is$15^{\prime\prime}$, which corresponds to $0.3 - 1.8~\rm kpc$. We find that theratio of the SFE in the bar to that in the disk was systematically lower thanunity (typically $0.6-0.8$), which means that the star formation in the bar issystematically suppressed. Our results are inconsistent with similar recentstatistical studies that reported that SFE tends to be independent of galacticstructures. This inconsistency can be attributed to the differences in thedefinition of the bar region, spatial resolution, $\alpha_{\rm CO}$, and samplegalaxies. Furthermore, we find a negative correlation between SFE and velocitywidth of the CO spectrum, which is consistent with the idea that the largedynamical effects, such as strong shocks, large shear, and fast cloud-cloudcollisions caused by the noncircular motion of the bar, result in a low SFE.