The EDGE-CALIFA survey: The role of spiral arms and bars in driving central molecular gas concentrations

Abstract

Shocks and torques produced by non-axisymmetric structures such as spiral arms and bars may transport gas to galaxy central regions. We test this hypothesis by studying the dependence of the concentration of CO luminosity (CCO) and molecular gas (Cmol) and the star formation rate (CSFR) in the central ∼2 kpc on the strength of non-axisymmetric disk structure using a sample of 57 disk galaxies selected from the EDGE-CALIFA survey. The Cmol is calculated using a CO-to-H2 conversion factor that decreases with higher metallicity and higher stellar surface density. We find that Cmol is systematically 0.22 dex lower than CCO. We confirm that high Cmol and strong non-axisymmetric disk structure are more common in barred galaxies than in unbarred galaxies. However, we find that spiral arms also increase Cmol. We show that there is a good correlation between Cmol and the strength of non-axisymmetric structure (which can be due to a bar, spiral arms, or both). This suggests that the stronger the bars and spirals, the more efficient the galaxy is at transporting cold gas to its center. Despite the small subsample size, the Cmol of the four Seyferts are not significantly reduced compared to inactive galaxies of similar disk structure, implying that the active galactic nucleus feedback in Seyferts may not notably affect the molecular gas distribution in the central ∼2 kpc. We find that CSFR tightly correlates with Cmol in both unbarred and barred galaxies. Likewise, elevated CSFR is found in galaxies with strong disk structure. Our results suggest that the disk structure, either spirals or bars, can transport gas to the central regions, with higher inflow rates corresponding to stronger structure, and consequently boost central star formation. Both spirals and bars play, therefore, an essential role in the secular evolution of disk galaxies.