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

TL;DR

This study investigates how spiral arms and bars influence the central concentration of molecular gas and star formation in disk galaxies, confirming that stronger non-axisymmetric structures facilitate gas inflow and enhance star formation.

Contribution

It demonstrates that both spiral arms and bars significantly contribute to transporting gas to galaxy centers, with stronger structures correlating with higher central molecular gas and star formation rates.

Findings

Higher $C_{mol}$ in barred galaxies compared to unbarred ones

Strong non-axisymmetric structures correlate with increased central gas concentration

Central star formation rate tightly linked to molecular gas content

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 concentration of CO luminosity ($C_{CO}$), molecular gas ($C_{mol}$), and star formation rate ($C_{SFR}$) in central $\sim$ 2 kpc on the $strength$ of non-axisymmetric disk structure using a sample of 57 disk galaxies selected from the EDGE-CALIFA survey. $C_{mol}$ is calculated using a CO-to-H$_2$ conversion factor that decreases with higher metallicity and higher stellar surface density. We find that $C_{mol}$ is systematically 0.22 dex lower than $C_{CO}$. We confirm that high $C_{mol}$ and strong non-axisymmetric disk structure are more common in barred galaxies than in unbarred galaxies. However, we find that spiral arms also increase $C_{mol}$. We show that there is a good correlation between $C_{mol}$ 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, $C_{mol}$ of the four Seyferts are not significantly reduced compared to inactive galaxies of similar disk structure, implying that the AGN feedback in Seyferts may not notably affect the molecular gas distribution in the central $\sim$2kpc. We find that $C_{SFR}$ tightly correlates with $C_{mol}$ in both unbarred and barred galaxies. Likewise, elevated $C_{SFR}$ 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.