The Antennae Galaxies are two interacting galaxies, likely in the phase of joining to become a single larger galaxy.   Their two nuclei will eventually merge in about 400 million years to form a single much more massive galaxy and therefore a supermassive black hole at its center.  These galaxies are currently in an extreme starburst phase where the collision of clouds of gas and dust are causing significant and rapid amount of star formation.  This galaxy pair is also a relatively strong source of X-rays which also shows large amounts of neon, magnesium, and silicon.   The galaxies started to merge about 1.2 billion yars ago and passed through each other 600 million years ago.   Note that when two galaxies of hundreds of billions of stars pass through each other they rarely ever cause stars to collide direclty due to the vast emptiness of space.