Properties of Molecular Gas in Galaxies in Early and Mid Stage of Interaction: II. Molecular Gas Fraction

We have investigated properties of the interstellar medium in interacting galaxies in early and mid-stage using mapping data of 12CO(1-0) and HI.Assuming the standard CO-H2 conversion factor, we found no difference in molecular gas mass, atomic gas mass, and total gas mass (a sum of atomic and molecular gas mass) between interacting galaxies and isolated galaxies. However, interacting galaxies have a higher global molecular gas fraction, global fmol (a ratio of molecular gas mass to total gas mass averaged over a whole galaxy), than isolated galaxies. The distribution of local molecular gas fraction, local fmol (a ratio of the surface density of molecular gas to that of total gas), is different from the distribution in typical isolated galaxies. By a pixel-to-pixel comparison, isolated spiral galaxies show a gradual increase in local fmol along the surface density of total gas until it is saturated at 1.0, while interacting galaxies show no clear relation. We performed pixel-to-pixel theoretical model fittings varying metallicity and external pressure. According to the model fitting, external pressure can explain the trend of local fmol in the interacting galaxies. Assuming a half of the standard CO-H2 conversion factor for interacting galaxies, the results of pixel-to-pixel theoretical model fitting get worse than adopting the standard conversion factor, although global fmol of interacting galaxies becomes the same as in isolated galaxies. We conclude that external pressure occurs due to the shock prevailing over a whole galaxy or due to collisions between giant molecular clouds. The external pressure accelerates an efficient transition from atomic gas to molecular gas. Regarding chemical time-scale, high local fmol can be achieved at the very early stage of interaction even if shock induced by the collision of galaxies ionises interstellar gas.