A triggering mechanism for enhanced star formation in colliding galaxies

Abstract

We propose a physical mechanism to explain the origin of the intense burst of massive-star formation seen in colliding/merging, gas-rich, field spiral galaxies. We explicitly take account of the different parameters for the two main mass components, H₂ and H 1, of the interstellar medium within a galaxy and follow their consequent different evolution during a collision between two galaxies. We also note that, in a typical spiral galaxy-like our galaxy, the Giant Molecular Clouds (GMCs) are in a near-virial equilibrium and form the current sites of massive-star formation, but have a low star formation rate. We show that this star formation rate is increased following a collision between galaxies. During a typical collision between two field spiral galaxies, the H 1 clouds from the two galaxies undergo collisions at a relative velocity of ~300 km s -1. However, the GMCs, with their smaller volume filling factor, do not collide. The collisions among the H I clouds from the two galaxies lead to the formation of a hot, ionized, high-pressure remnant gas. The overpressure due to this hot gas causes a radiative shock compression of the outer layers of a preexisting GMC in the overlapping wedge region. This makes these layers gravitationally unstable, thus triggering a burst of massive-star formation in the initially barely stable GMCs. The resulting value of the typical IR luminosity from the young, massive stars from a pair of colliding galaxies is estimated to be ~2 × 10¹¹ L, in agreement with the observed values. In our model, the massivestar formation occurs in situ in the overlapping regions of a pair of colliding galaxies. We can thus explain the origin of enhanced star formation over an extended, central area approximately several kiloparsecs in size, as seen in typical colliding galaxies, and also the origin of star bursts in extranuclear regions of disk overlap as seen in Arp 299 (NGC 3690/IC 694) and in Arp 244 (NGC 4038/39). Whether the IR emission from the central region or that from the surrounding extranuclear galactic disk dominates depends on the geometry and the epoch of the collision and on the initial radial gas distribution in the two galaxies. In general, the central star burst would be stronger than that in the disks, due to the higher preexisting gas densities in the central region. The burst of star formation is expected to last over a galactic gas disk crossing time ~4 × 10⁷ yr. We can also explain the simultaneous existence of nearly normal CO galaxy luminosities and shocked H² gas, as seen in colliding field galaxies. This is a minimal model, in that the only necessary condition for it to work is that there should be a sufficient overlap between the spatial gas distributions of the colliding galaxy pair.