Environmental Effects on Gaseous Disks of the Virgo Spiral Galaxies1

We found high molecular fractions ( fmol; ratio of the molecular to total gas surface densities) in three of five Virgo spiral galaxies in spite of their low total gas column density, based on 12CO(J = 1−0) observations with the Nobeyama 45 m telescope equipped with a multi-beam receiver, BEARS. We interpret this as a result of environmental effects. Combining the CO data with HI data, the relationship between the surface density of the total gas (HI plus H2) and fmol indicates that the three galaxies near the cluster center have larger fmol values than expected for field galaxies, while the others show normal fmol. The large fmol is interpreted as being due either to effective HI gas stripping, even in the inner disks, or to large ISM pressure induced by the high ICM pressure and/or ram pressure, although the possibility of an unusually high metallicity cannot be ruled out. Subject headings: galaxies: clusters: individual (Virgo), galaxies: ISM, ISM: molecules, radio lines: ISM