Australasian Science: Australia's authority on science since 1938

Mid-sized Black Hole Found

By Stephen Luntz

The first intermediate-sized black hole has had its status confirmed in Science.

Supermassive black holes containing more than a million solar masses lie at the centre of galaxies, and stellar remnants 30 solar masses or smaller have been detected elsewhere. However, no object filling the vast gap in between had been authenticated until now.

For 3 years the object HLX-1 has been considered the most promising option (AS, Jan/Feb 2010, pp.17–20). Now Dr Sean Farrell of the University of Sydney’s School of Physics has established an upper bound for HLX-1’s mass at 90,000 solar masses.

“From studying other black holes we know that sucking in the gas creates X-rays, but there’s then a sort of reflux, with the region around the black hole shooting out jets of high-energy particles that hit gas around the black hole and generate radio waves,” Farrell says. The brightness of the radio waves and the delay after the X-ray emissions relate to the mass, enabling Farrell to establish the black hole’s upper limit.

On the other hand, HLX-1 was first noticed because of its bright X-ray emissions, and Farrell says it is 700 times as bright in this part of the spectrum as the theoretical limit for a 20 solar mass black hole.

“A number of other bright X-ray sources have been put forward as possibly being middleweight black holes. But all of those sources could be explained as resulting from lower mass black holes,” Farrell says. “Only this one can’t. It is ten times brighter than any of those other candidates. We are sure this is an intermediate-mass black hole – the very first.”

Farrell says HLX-1 is feeding either on a gas cloud or the winds from a companion star. In the latter case it is unclear whether the object is being torn to pieces by the hole’s gravitational fields or survives in a stable orbit.

HLX-1 lies in the halo of the galaxy ESO 243-49, 300 million light years away. Farrell says its location is typical of globular clusters and dwarf galaxies, and it may lie in the centre of one of these.

“One theory us that black holes grow by merging solar masses until they get to a supermassive size, and if this is the case you need something in between, and this is the sort of environment where you would expect to see it,” Farrell says.