Molecular gas in the poststarburst galaxy NGC 7331

We present the result of the high-resolution CO observations of the nearby ''poststarburst'' galaxy NGC 7331 obtained with the Nobeyama Millimeter Array (NMA). The molecular gas is distributed in two bowlike structures, and these seem to be parts of the ringlike structure, which was also seen in the radio continuum. The ring has a radius of similar to 3 kpc, corresponding to the turnover radius of the rotation curves obtained both by radio and optical observations. We could not detect the CO emission in the central region, although the central concentrations of molecular gas were found in other ''poststarburst'' galaxies, NGC 4736 and NGC 5055. The distribution of mass surface density in NGC 7331 is different from that of the critical surface density for disk stability (where the disk is unstable and breaks up), although the molecular gas in the active star-forming ring is unstable and that in the central poststarburst region is stable. This suggests that the global molecular gas distribution in NGC 7331 is not controlled by the disk stability. The central hole of molecular gas in NGC 7331 might be formed by gas consumption due to the long evolution after the past starburst event.