Astronomers have their own version of the classic chicken-or-egg question. Over the past decade, they have found that supermassive black holes lurk in the centers of practically every large galaxy. Even more surprising, the black holes somehow “know” about their host galaxies. Whatever the black hole's mass, the galaxy's surrounding bulge of old stars always contains about 700 times that mass.
Ever since this relation was discovered in 2000, astronomers have wondered about how the relationship is established, and about which came first: the black hole or the galaxy?
New results announced Wednesday at the American Astronomical Society meeting suggest that black holes were what formed first. An international team led by Chris Carilli (National Radio Astronomy Observatory) used radio telescopes in the U.S., Spain, and France to study four extremely distant quasars — galaxies with brilliant, black-hole-powered activity in their cores. We see these quasars as they existed when the universe was still a toddler, a billion years or less after the Big Bang.
By observing at radio wavelengths, the team could measure orbital motions (radial velocities) of gas within each galaxy. The speeds reveal the total mass that's within a gas cloud's distance from the galaxy's center. Red- and blueshifts of spectral lines from fast-moving gas close to the central black hole itself tell the black hole's mass alone.
To their surprise, Carilli and his colleagues found that in each of the four galaxies, the bulge was only 30 times more massive than the hole. “In all four cases, the black hole masses are 20 to 30 times larger than what one would have predicted from this relationship in the nearby universe,” says Carilli. “The simplest conclusion is that black holes come first, and somehow grow galaxies around them.”
Carilli is quick to point out that these results need to be confirmed by further observations, including better studies with big radio instruments currently under construction. He adds, “Our study doesn’t really address the origin of the relationship. Eventually, these galaxies have to migrate to the 700-to-1 relationship over cosmic time, but we don’t know how they do that.”
For more information: press release from the National Radio Astronomy Observatory.