Are black holes at the center of galaxies?

Bubbles of dark matter could be masquerading as supermassive black holes at the centres of galaxies. If so, they could explain the puzzling pattern of X-ray emissions from the heart of the Milky Way.

Cosmologists know that most galaxies host a compact, supermassive object at their centre and they believe these must be black holes. Such a black hole is thought to be responsible for the X-ray flares coming from the middle of our galaxy, which would be caused by the black hole devouring surrounding matter. But recent observations show that these flares fire roughly every 20 minutes – a regularity that is hard to explain in terms of the behaviour of a black hole.

Now Anatoly Svidzinsky, a physicist at Texas A&M University in College Station, Texas, thinks that hypothetical particles called axions could solve the mystery. Axions have very little mass and no electric charge, and they barely interact with other particles. They were originally proposed to fix a problem with the strong force in particle physics, but have more recently been considered as possible candidates for dark matter, the unseen stuff thought to make up nearly 90 per cent of a galaxy's mass.

In the 1990s, computer simulations of clouds of dark matter made of axions showed that giant bubbles of these particles would burst out from the clouds. Svidzinsky thinks that such bubbles exist at the centre of galaxies. His model shows that the axion bubbles would expand and contract with a period of 20 minutes – matching the period of infrared and X-ray flares from Sagittarius A*, the location of the supermassive compact object at the centre of our galaxy. The model predicts that stable axion bubbles would weigh between about 1 million and 2.5 billion times the mass of the sun – exactly the mass range observed for compact objects at the centres of galaxies (www.arxiv.org/ astro-ph/0607179).

"The proposal looks quite intriguing," says Tim Sumner, who is leading the search for galactic dark matter, including axions, at Imperial College London in the UK. "But it obviously needs a lot more evidence and assessment before it can really displace the more established scenarios."

One big assumption in Svidzinsky's model is that gravity starts to repel as the gravitational field gets stronger – a tweak to general relativity proposed by physicist Huseyin Yilmaz in the 1990s. And this is what causes the bubbles to oscillate. As the bubble grows, its surface tension pulls it back. As it collapses, its gravity eventually becomes repulsive and the bubble expands again.

The fact that Svidzinsky's model relies on this controversial version of gravity doesn't necessarily count against it, says Konstantin Zioutas of the particle physics laboratory CERN in Geneva, Switzerland. "There are various studies in progress around the world which suggest that Einstein did not speak the last word on gravity," says Zioutas. For example, extra dimensions can change the way that gravity behaves in extreme cases.

The other important question is whether axions really exist. There have been attempts to create them in the lab (New Scientist, 14 July, p 35) and even a possible indirect sighting in the sun's halo by Zioutas (New Scientist, 17 April 2004, p 8). "Until their existence is confirmed, axions will appear to be a deus ex machina," says Zioutas.

Still, Zioutas adds, if Svidzinsky is correct, his idea could solve another mystery perplexing astronomers. As well as X-ray flares, astronomers can see diffuse X-rays emanating from our galactic centre. "We don't know what can be causing these," says Zioutas. "Any gas that would be hot enough to emit this radiation would be moving too fast to be held in our galaxy." Dark matter axions, however, could be releasing these X-rays as they decay, he says.

Evidence one way or the other may be just around the corner. "Within a few years astronomers will be able to resolve compact objects at the centre of galaxies with radio interferometers," says Svidzinsky. Black holes will have a constant size, whereas an axion bubble's radius will oscillate, he says. Zioutas is looking forward to the answer. "There is so much at stake here – rewriting both Einstein and dark matter," he says.