TRIGGERING OF REPETITIVE STARBURSTS IN MERGING GALAXIES

Merging galaxies are often observed to be the site of active star formation. In order to investigate the effect of mergers on the gas dynamics and the star formation process, we have carried out numerical simulations for the merger of two disc galaxies which contain gas clouds as well as stars. It has been assumed that each collision between two gas clouds leads to star formation. It is found that as the merger proceeds, the cloud-cloud collision rate is repetitively enhanced by up to approximately 10 times with a repetition period of the order of 10(8) yr. It is found that this repetitive increase is caused by the orbital motion of two dense gaseous cores (i.e. condensations of gas clouds) which have been formed at the centres of the original galaxies during the early phase of the merger. Once their mutual distance decreases to subgalactic scale (< 10 kpc), the two cores promptly sink toward the centre of the merging system, revolving around each other on elongated orbits. Therefore the separation between the two cores does not decrease monotonically but shows a damped oscillation. The cloud-cloud collision rate increases (decreases) as the separation decreases (increases). Thus the cloud-cloud collision rate (and hence star formation rate) is at a maximum (minimum), when the separation is at a minimum (maximum). Such repetitive starbursts are able to explain the observational result that merging galaxies show a wide range of star formation efficiency. The observed range is nearly the same as the numerically predicted enhancement of the cloud collision rate. It is often observed that the molecular gas distribution in a merging system has a double-core structure (or a bar-like shape with two cores not separated) in the centre, which is consistent with the numerical result. Our numerical study predicts that the separation between the cores (or the length of the molecular bar) should be anticorrelated with the star formation efficiency.