Mergers start out as collisions.
Two spiral galaxies sail into and through each other, pulling long trails of stars behind them, then come back and do it again. Along the way, they compress each other's gases to trigger the formation of stars, hot stars brighter than a welder's torch. Finally each is caught by the other and they merge. The two galaxies may have been the breathtaking pinwheels of astronomy picture books before the merger, but afterward all we see is a blob of light known as an elliptical galaxy. It could happen here.