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.

J0107a: A Barred Spiral Dusty Star-forming Galaxy at z = 2.467

“…In addition to the stellar bar, the 888 μm dust continuum map suggests the presence of a dust/gas bar (Figure 1 shows three peaks and is reminiscent of the enhancement of SFR at the galactic center and the bar ends in local barred galaxies (Maeda et al 2023). Elevated central star formation is common in local barred galaxies (e.g., Chown et al 2019;Lin et al 2020) and explained by theoretical studies as a consequence of gas being funneled by the bar toward the center and feeds the star formation and supermassive black hole (e.g., Carles et al 2016).…”

Metallicities of Classical Cepheids in the Inner Galactic Disk

“…It is tempting to combine these Cepheids with our sample at 3 < R GC < 5.6 kpc to discuss whether the metallicity gradient is flat or slightly positive (i.e., the [Fe/H] being lower at R GC < 300 pc). However, Matsunaga et al (2016) found that there is a little population of Cepheids in the intermediate range (0.3 < R GC < 3 kpc), where star formation may be suppressed by the bar, as seen in other barred galaxies (Maeda et al 2023). It is totally unknown what connection, if any, we should expect between the two groups of Cepheids in terms of chemical evolution.…”

Star Formation Efficiency in Nearby Galaxies Revealed with a New CO-to-H2 Conversion Factor Prescription

“…Previous studies have found that SFR and molecular gas surface densities are highly correlated (i.e., the molecular Kennicutt-Schmidt, or mKS, relation; Kennicutt 1998) and that t dep is usually at 1-4 Gyr across nearby star-forming galaxies (e.g., Bigiel et al 2008;Leroy et al 2008;Saintonge et al 2011;Schruba et al 2011;Utomo et al 2017;Sun et al 2023). Despite the minor variation in general, t dep is also found to vary systematically with local and global host galaxy properties, which could be driven by environmental and/or dynamical effects from, e.g., metallicity, molecular cloud structure, bar instabilities, active galactic nuclei, or galaxy interactions (Saintonge et al 2011(Saintonge et al , 2012Schruba et al 2019;Ellison et al 2021aEllison et al , 2021bQuerejeta et al 2021;Lu et al 2022;Villanueva et al 2022;Jiménez-Donaire et al 2023;Maeda et al 2023).…”

“…Due to the lack of a widely agreed prescription that can accurately predict α CO , many studies could only assume a constant α CO referencing the Milky Way (MW) disk average (e.g., B13) to convert CO observations to molecular gas mass. This has made α CO variation one of the dominant sources of uncertainty in current molecular gas and SFE studies (see the discussions in Ellison et al 2020b;Maeda et al 2023;Sun et al 2023).…”

“…The impacts of α CO variations on both SFE and cloud evolutionary timescale estimates are particularly critical in galaxy centers (Leroy et al 2013;Utomo et al 2017;Muraoka et al 2019;Ellison et al 2020b;Pessa et al 2021;Maeda et al 2023;Sun et al 2023). In those environments, α CO can be 5-15 times lower than the Galactic disk value (Ackermann et al 2012;Sandstrom et al 2013;Israel 2020;Teng et al 2022Teng et al , 2023den Brok et al 2023).…”