Environmental Effects on Gaseous Disks of the Virgo Spiral Galaxies
We found high molecular fractions ($f_{\rm mol}$; 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 $^{12}$CO$(J=1-0)$ 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 H$_2$) and $f_{\rm mol}$ indicates that the three galaxies near the cluster center have larger $f_{\rm mol}$ values than expected for field galaxies, while the others show normal $f_{\rm mol}$. The large $f_{\rm mol}$ 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.