Molecular gas in barred galaxy nuclei

Abstract

<p>Double bars have been proposed as a means of transporting molecular gas past inner Lindblad resonances into the nuclear regions, where it can fuel active or starburst nuclei. Thus far, the existence of double bars has been determined by isophote twists seen in the near infrared, which could probe the bulge properties of these galaxies rather than the disk properties. We have observed two double bar galaxy candidates (NGC 2273 and NGC 5728) in 12 CO J = 1-0 with the Caltech Millimeter Array. Despite the similar near infrared images of the two galaxies, we see rather different nuclear morphologies in the CO maps. NGC 2273 shows evidence of a nuclear bar misaligned from the main stellar bar by ∼90°, and aligned with the near infrared isophote twists. NGC 5728 shows an arc of CO clumps that peaks just to the south-west of the dynamical center and curves to the south-east where it follows the dust lane to the south. The lack of a nuclear bar in the CO maps of NGC 5728 may be evidence that it is in a later stage of evolution. Bar dissolution may have just begun, and the gas has responded first, which may explain why we see a nuclear bar in the near infrared images of NGC 5728, but not in the CO maps. Models of these double barred galaxies show that the nuclear molecular morphology depends on the assumed gas properties. We have performed a multi-transition CO study of the nuclei of seven double barred galaxies that exhibit a variety of molecular gas morphologies in order to determine if the molecular gas properties are correlated with the nuclear structure and activity. We find that the 12 CO J = 3-2/J = 2-1 line ratio is lower in galaxies with molecular bars in the nucleus and higher in galaxies with CO emission dispersed around the galactic center. The galaxies without central concentration show signs of past star formation which may have exhausted, heated, and dispersed the molecular gas in the nucleus, resulting in the observed distributions and high CO line ratios. The multiline CO data corroborates our hypothesis that the galaxies with nuclear molecular bars are at an earlier stage of double barred galaxy evolution. In an attempt to detect other double barred galaxies that are bright enough in CO emission to be mapped at high resolution, we have undertaken a CO survey with the JCMT and NRAO 12-m of all double barred galaxies that are observable from the northern hemisphere. (Abstract shortened by UMI.)</p>