Complex sentence: This violent collision formed most of its (metal-rich) galactic halo and extended disk and during that epoch Andromeda's star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years.
Split into simpler sentences: This violent collision formed most of its (metal-rich) galactic halo and extended disk. During that epoch, Andromeda's star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years.