Quote: "Galaxies grow by churning loose gas from their surroundings into new stars, or by swallowing neighbouring galaxies whole. However, they normally leave very few traces of their cannibalistic habits."
YES off course, however what create that gas? Herbig Haro systems between two paired Black Holes.
What created the merging dwarf elliptical (Ngc 1510): a secondary Supernova black hole created by Ngc1512 dwarf spiral stellar explosion!