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Black Holes Behave

Swinburne University researchers have found a formula to predict the masses of black holes in galaxies of various sizes.

While the central black hole in large galaxies is related to the mass of the bulge of stars at the centre of the galaxy, some astronomers have claimed that this is not true for black holes at the centre of galaxies with small bulges.

Prof Alister Graham of Swinburne has previously calculated that the black hole in the bulge of the Milky Way is not set by chance but instead follows an astronomical rule. “The formula is quadratic, in that the black hole mass quadruples every time the bulge mass doubles,” he said. “Therefore, if the bulge mass increases 10 times, the black hole mass increases 100 times.”

Now, after studying more than 100 galaxies with black holes 4–40 times less massive than our Milky Way’s black hole, Graham found that they also follow this rule.

“It turns out that there is yet more order in our universe than previously appreciated,” Graham said. “This is exciting not just because it provides further insight into the mechanics of black hole formation, but because of the predictions it allows us to make.”

The gravitational collapse of massive stars can produce black holes that are only tens of times the mass of our Sun, and enormous black holes of between 100,000 and 10 billion solar masses have been identified at the centres of giant galaxies.

However, there is a missing population of intermediate-mass black holes. Astronomers don’t know if this is because of observational difficulties in finding them, or if the massive black holes at the centres of galaxies were initially intermediate-sized black holes that formed in the early universe.

This latest result, published in the Astrophysical Journal, extends the rule to much lower masses, opening the possibility that the smallest bulges might host these missing intermediate-mass black holes.

The researchers have identified a few dozen candidate galaxies in which they think intermediate-mass black holes may be hiding.