Necessary but not Sufficient Conditions to Trigger Starbursts

We present the analysis of a survey of atomic and molecular gas in interacting and merging galaxies (Horellou and Booth, 1997). The sample is optically selected and contains all interacting galaxies (≈ 60 systems) in a well-defined region of the Southern sky (Bergvall, 1981). In order to take into account the upper limits due to non-detections, we have carried out a survival analysis. The results are compared with the ones obtained when ignoring the non-detections. We have found evidence for a lower atomic gas content per unit area in interacting galaxies compared to isolated ones. Except for a few systems with high far-infrared luminosity and correspondingly high CO fluxes, the interacting galaxies are not unusually CO-bright. The observed HI deficiency is therefore not due to a conversion of HI into H2.Atomic gas from the outer part of the galaxies may have been swept by tidal interaction, or the optical extent of the galaxies may have increased due to the tidal interaction. The interacting galaxies seem to be more efficient at transforming their molecular gas into stars, as indicated by their higher LFIR/M(H2) ratio. We found no correlation between molecular gas fraction and degree of morphological distortion.