A picture-perfect spiral galaxy may harbour two colossal black holes instead of the usual one, new observations suggest. But if it does, astronomers are mystified as to how the second black hole got there without ruining the galaxy's spiral structure.
Most galaxies are thought to contain a single large black hole with a mass proportional to that of its galaxy. But galaxies grow through collisions, and astronomers believe their black holes also merge over time.
Two galaxies appear to contain pairs of black holes. These galaxies emit telltale X-rays when superhot matter falls towards the massive objects.
But now a trio of astronomers, led by Damián Mast of Argentina's National University of Cordoba, says there may be two black holes inside the galaxy M83, which lies about 15 million light years away - even though it does not emit these X-rays.
Such a possibility was first raised in 2000 by another team in Germany. That team studied the motion of the galaxy's stars along the line of sight between Earth and M83, discovering two concentrations of mass, or "nuclei". One shone brightly while the other - which lay closer to the apparent centre of the galaxy - was dim. The team suggested that gas and dust were blocking light from the second object, which they thought might be the galaxy's true centre - around which gas and stars circle.
Millions of Suns
Now, Mast's team has studied the motion of ionised gas near M83's centre. They used the data to better locate the two knots of matter, which appear to lie about 200 light years from one another.
They also estimated the knots' masses, with the bright one containing the equivalent of five million Suns and the dark one 10 million Suns. By comparison, the Milky Way's central black hole has the mass of about four million Suns.
Those masses, along with a previous observation of an arc of star formation near the galaxy's centre, led the team to consider the possibility that the nuclei are supermassive black holes. The team thinks the high stellar birth rate may have been kick-started by the dark nucleus travelling through dense gas - an event that could arise in a galactic merger. The event may have begun about eight million years ago - the age of the oldest stars in the arc.
But the team is careful not to call these objects "black holes", acknowledging they could instead be massive clusters of stars.
David Merritt, an astrophysicist at the Rochester Institute of Technology in New York, US, doubts the galaxy contains two black holes. Black holes with their masses would be born in galaxies of roughly equal size, he says. And mergers between two such equals would probably result in a galaxy shaped more like a blob - not a "classic" spiral such as M83, he says.
"My guess is there is one black hole at the site of the obscured nucleus, and the other bright nucleus is just a clump of star-forming gas," he told New Scientist.
The research was posted on an online preprint server on 13 May.
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