What can active galaxies reveal about the history of the universe?
• Because quasars lie at great distances, they appear as they were long ago—over 10 billion years ago—when the universe was young and just forming galaxies. • The best images show that the host galaxies of quasars are distorted, and that suggests that they have erupted because they have been involved in mergers or collisions. Such interactions were more common in the distant past before the universe had expanded very far and galaxies were closer together. • It is also possible that at least some quasars are erupting while matter falls together to create a supermassive black hole as a galaxy begins to form. That is, some quasars may be caused by the formation of the first galaxies when the universe was young. • Quasars are most common with redshifts of about 2, which shows that there was an age when galaxy formation, interaction, and mergers were more common. The so-called dead quasars today are the dormant black holes at the centers of galaxies in which little matter is flowing into th
