The huge black holes that lie at the centre of galaxies grow by devouring gas and stars that come too close, but their gravitational attraction can also encourage the birth of stars and the growth of galaxies. This dual role as creator and destroyer has left astronomers with a puzzle: which came first, the black hole or the galaxy? Research from radio astronomers now suggests that black holes got off to a faster start, at least in four galaxies that existed in the early Universe. “The significant implication is that the black holes formed first and then somehow they formed a stellar galaxy around them,” says Chris Carilli, of the National Radio Astronomy Observatory in Socorro, New Mexico, who presented the research at a meeting of the American Astronomical Society in Long Beach, California, on 6 January.
Astronomers had already identified what seemed to be a predictable relationship between the mass of the black hole at a galaxy's heart, and the mass of the galaxy's central bulge of stars and gas. The galactic bulge tends to be 700 times the mass of the black hole, a proportion that holds true for galaxies throughout billions of years of the Universe's history. But the ratio for the oldest galaxies — formed within the first billion years after the Universe's birth some 13.7 billion years ago — remained a mystery. Quasar black holes at the centre of these galaxies glow so brightly that they prevent optical telescopes from seeing the surrounding galaxies in detail, so that galactic mass estimates were mostly impossible.