Astronomers know that at the heart of every supermassive galaxy is a giant black hole. But new data suggests that the two did not necessarily form in tandem. Instead, black holes may have formed earlier, or at least much more quickly, than their surrounding galaxies. Previous studies had revealed a striking link between black holes and the amount of gas and stars contained in [their] galaxies’ bulges — the regions that lie within a few thousand light-years of the galaxies’ cores. Regardless of their size, the bulges always turned out to be 700 times as massive as the giant black holes at the galaxies’ hubs [Science News]. New measurements of much more distant galaxies, which appear much younger, defy the expected mass ratio. In these younger pairings, the relative mass of the black holes is much greater, hinting that the black holes came first.
Researchers used the Very Large Array radio telescope in New Mexico and the Plateau de Bure Interferometer in France to measure the mass of four distant galaxies as they appeared less than two billions years after the Big Bang. From the motions of the molecular gas, which concentrates in the central part of the galaxies, the team calculated the total amount of mass in the bulges and compared that number to the mass of the central black holes [Science News]. The galactic bulges were only about 30 times more massive than their central black holes. At the American Astronomical Society‘s meeting, where the work was presented, astronomer Chris Carilli said, “The simplest conclusion is that the black holes come first and they somehow grow the galaxy around them” [Wired News].
Though the new observations suggests an answer to the chicken-and-egg style conundrum about galaxies and black holes, they also raise plenty of new riddles. The fixed mass ratio observed in older galaxies suggests a correlation, if not a symbiotic relationship, between the growth of galaxies and their black holes. If the new data hold, astronomers will have to explain how the black holes came to be so big, so quickly, without a surrounding galaxy for nourishment. They would have to devour gas at maximum rates starting almost immediately after the Big Bang, says Carilli. “It’s a total puzzle to me” [Nature News].
Some theorize that the strong winds and jets surrounding black holes could help feed star formation and induce galaxies to grow. But the violent environments of black holes have also been thought too chaotic to harbor stable star formation [Wired News], although yesterday’s conference sessions brought a report of stars forming on the edge of our Milky Way’s central black hole. Many in the field, although impressed by the new data, remain skeptical of the black-holes-came-first conclusion drawn from them. Astronomer Tod Lauer says finding four galaxies with extra-large black holes might be expected by chance alone…”I’m not challenging the observation; I’m challenging the inference from it,” he says [Nature News].
Image: Chandra X-Ray Observatory (a view of the Milky Way and its central black hole, Sagitarrius A*)
