Molecular Gas in Candidate Double‐barred Galaxies. I. The Diverse Morphology and Dynamics of NGC 2273 and NGC 5728

Double bars have been proposed as a means of transporting molecular gas pastinner Lindblad resonances into the nuclear regions, where it can fuel active orstarburst nuclei. Thus far, the existence of double bars has been determinedpredominantly through analysis of near infrared images, which can tell uslittle about the dynamics and inflow rates of these systems. We have observedtwo double bar galaxy candidates (NGC 2273 and NGC 5728) in CO J=1-0 with theOwens Valley Radio Observatory Millimeter Array. Despite the similarity in thenear infrared images of these galaxies, we see rather different nuclearmorphologies in the CO maps. NGC 2273 shows evidence of a nuclear gas bar,aligned with the nuclear stellar bar seen in the near infrared images. Both thenuclear gaseous and stellar bars are misaligned from the large scale bar byapproximately 90 degrees, which also allows the possibility that both are theresult of stars and gas populating the $x_2$ orbits of the primary bar.Estimates using dynamical friction arguments and star formation rates suggestsignificant gas inflow rates along the nuclear bar of NGC 2273. Conversely, NGC5728 does not show any evidence for a nuclear molecular bar, but shows an arcof CO clumps that peaks just to the south-west of the dynamical center andcurves to the south-east where it follows the dust lane to the south. Models ofdouble-barred galaxies suggest that these galaxies should contain large amountsof molecular gas in their nuclei. Our calculations suggest that both galaxiescontain sufficient amounts of gas in their nuclei, but only NGC 2273 showsevidence for a nuclear gas bar. This may be the result of past episodes of starformation exhausting and dispersing the nuclear gas of NGC 5728, but is morelikely evidence that NGC 5728 has undergone a minor merger event.Comment: 26 pages + 8 figures, to appear in the Aug 20, 2002 issue of Ap