Molecular Gas in Candidate Double-Barred Galaxies. III. A Lack of Molecular Gas?

Most models of double-barred galaxies suggest that a molecular gas component is crucial for maintaining long-lived nuclear bars. We have undertaken a CO survey in an attempt to determine the gas content of these systems and to locate double-barred galaxies with strong CO emission that could be candidates for high-resolution mapping. We observed 10 galaxies in CO J = 2-1 and J = 3-2 and did not detect any galaxies that had not already been detected in previous CO surveys. We preferentially detect emission from galaxies containing some form of nuclear activity. Simulations of these galaxies require that they contain 2%-10% gas by mass in order to maintain long-lived nuclear bars. The fluxes for the galaxies for which we have detections suggest that the gas mass fraction is in agreement with these models requirements. The lack of emission in the other galaxies suggests that they contain as little as 7 × 106 M☉ of molecular material, which corresponds to ≲0.1% gas by mass. This result combined with the wide variety of CO distributions observed in double-barred galaxies suggests the need for models of double-barred galaxies that do not require a large, well-ordered molecular gas component.