Black Holes First, Galaxies Second

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.

Left: The tiny red speck on this piece of a Sloan Digital Sky Survey image is an extremely distant quasar at the core of a galaxy seen at a redshift of 6.42, when the universe was only 870 million years old. Right: A high-resolution radio image of it shows radio-emitting gas both close to the black hole and some 20,000 light-years out in the galaxy's body. Doppler shifts within each tell both the hole's mass and the galaxy's mass.
Alan MacRobert
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.

2 thoughts on “Black Holes First, Galaxies Second

  1. Rob Jones

    Has anyone ever compaired know matter/antimater(or dark mater) relationships to this galaxy mass/Blackhole mass ratio. Maybe black holes are simply the antimater of galaxies. So much mass in one area would surely punch a hole in the fabric of space and maybe spacetime itself.


  2. mark a. thomas

    Now, that it has been established that the black hole mass to galactic bulge mass is a constant which holds for the late time, where to next? This is an example of good science. The early time shows that this constant does not hold so what we have is a constant that will be a part of a theory which is approximate. On a speculative note what would be really weird is that the constant 1/700 is related to the tail end integer of Ramanujan’s constant e^{pi sqrt163} so that the constant is 1/744. Maybe, this would have implications for elliptic modular functions in galactic gravitational dynamics.

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