STAR-FORMATION IN MERGING GALAXIES

During the merger of two spiral galaxies powerful starbursts can be in duced. By means of evolutionary spectral synthesis together with a 1-z one closed box model for the enrichment of individual chemical element s we investigate the evolution of the stellar population and the gas i n the progenitor galaxies through starburst, merging, and beyond. In a parameter study we discuss the influence of the 'Hubble types' (star formation histories) and ages of the progenitor spirals, and of the st rength and duration of an interaction-induced starburst, on the optica l to NIR colours, mass-to-light ratios, and chemical abundances of the merging system. The aim of this study is (i) to investigate under whi ch circumstances and how rapidly merger remnants may evolve into the c olour domain of E/S0 galaxies in different wavelength ranges, and (ii) to provide a grid of models for the interpretation of observed starbu rst systems and merger remnants. We find that strong as well as weak b ursts during a merger in Sa through Sc type spirals, after 8 to 12 Gyr of undisturbed evolution, can leave a remnant with elliptical like co lours at approximately 15 Gyr, provided the burst duration is approxim ately 10(7) to 10(8) yr. The late passive evolutionary reddening track s are very similar for a wide variety of parameters. Thus, the observe d narrow colour range of E/S0s does not prove a uniform single burst s cenario for their formation, as emphasized also by Schweizer & Seitzer . Chemical abundances and element ratios in burst stars and possibly i n part of the GC population might testify to a merger origin when many other signatures have already disappeared, and may, with accurate obs ervations and modelling, help revealing and timing past merger events. Late mergers of gas rich (Sc, Sd) galaxies produce by far the largest portion of their burst stars when the metallicity of the ISM is sligh tly subsolar. Any globular clusters formed during such mergers should stand out as having metallicities higher than about 1/3 Z. and abundan ce ratios typical of the ISM in the progenitor spirals. We discuss sta r formation efficiencies in massive spiral - spiral mergers and the co nditions for a starburst to develop an IR - ultraluminous phase.