Galaxies and the giant black holes at their hubs fit together as if they were made for one another. Did the holes come first and guide the formation of their galaxies, did the galaxies come first and build up holes, or did some common factor sculpt both? At the American Astronomical Society meeting in January, Christopher Carilli of the National Radio Astronomy Observatory and his colleagues argued that the holes came first. They found that galaxies in the early universe were 30 times more massive than their black holes, whereas present-day galaxies are 1,000 times heavier. “Black holes came first and somehow—we don't know how—grew the galaxy around them,” Carilli said. Other astronomers were skeptical, wondering whether the ancient galaxies seem undersized merely because of a statistical selection effect. Even if true, the study does not explain how a black hole can nurture a galaxy; if anything, it should tear it apart.
This article was originally published with the title "Galactic Chicken and Egg" in Scientific American 300, 3, 23 (March 2009)