The Milky Way's central black hole may not be able to hide for much longer. Observations have been made three times closer to the centre of our galaxy than ever before, strengthening the case that a supermassive black hole lurks there.
The Milky Way's centre hosts a bright object called SgrA*, which may be a disc of swirling gas and dust surrounding a heavyweight black hole.
Measuring the size and shape of SgrA* could help confirm the existence of the black hole. But a blur of clouds between the galactic centre and the Earth has prevented astronomers from getting a clear view of the object's shape.
Now, a team has managed to get three times closer than previous studies in viewing SgrA*. They did it by looking for the object's higher frequency radio signals, which cut through intervening gas and dust.
The size of SgrA* suggests the matter it contains is about 10 times denser than previous measurements, bolstering the idea that the object harbours a black hole, says astronomer Sheperd Doeleman of MIT's Haystack Observatory: "Our results are more evidence that we are looking at a black hole."
To image SgrA*, Doeleman and colleagues linked radio telescopes in Hawaii, Arizona, and California to create a telescope that was effectively 4500 kilometres wide. They used it to zoom in on the bright centre of the Milky Way with a resolution 1000 times that of Hubble Space Telescope.
So far, the team cannot produce dramatic pictures of SgrA*. But Doeleman says the technique could allow astronomers to clearly discern the shape of the disc within three to five years.
That's because future radio telescopes, such as Chile's Atacama Large Millimetre Array, due to become operational in 2010, will allow astronomers to use even shorter wavelengths to pierce through the galaxy's dust.
Black hole shadow
The images could reveal whether SgrA* has a 'shadow', a region in its centre where light is dimmed and reddened because it is pulled towards the black hole.
If SgrA* is indeed a disc of material swirling into the black hole, future observations could reveal how it is aligned.
Astronomers expect it may not spin in the plane of the Milky Way, since that would hint that the black hole at the disc's centre is actually the result of the merger of two galaxies. After the galaxies merged, their central black holes would have united as well, and the galaxy's gravity might not have had enough time to pull the disc around them into alignment with the galactic disc.
Further observations could measure elusive features, like the black hole's spin, by tracking the progress of flares of radiation – created when matter collides with the black hole – that whip around as the black hole spins.
a man lurking in the shadows
He discerned her plan immediately.
He was just able to discern the road in the dark.
I saw him redden with pleasure.
He remained as elusive as ever.