2006ApJ...651..804N

We found high molecular fractions (f_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 ¹²CO J=1-0 observations with the Nobeyama 45 m telescope equipped with a multibeam receiver, BEARS. We interpret this as a result of environmental effects. Combining the CO data with H I data, the relationship between the surface density of the total gas (H I plus H₂) and f_molindicates that the three galaxies near the cluster center have larger f_molvalues than expected for field galaxies, while the others show normal f_mol. The large f_molis interpreted as being due either to effective H I 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.