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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: syu@mpifr-bonn.mpg.de
² Department of Astronomy, University of Maryland, College Park, MD 20742, USA
³ Department of Astronomy, University of Illinois, Urbana, IL 62801, USA
⁴ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, 04510 México, D.F., Mexico
⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁷ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 20 June 2022
Accepted: 29 August 2022

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 (C_CO) and molecular gas (C_mol) and the star formation rate (C_SFR) 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 C_mol is calculated using a CO-to-H₂ 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, the C_mol 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 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.