Cloud Passing May Reveal Black Holes at Center of our Galaxy

A vast and hidden field of small black holes predicted to be near the centre of our galaxy could be revealed as a giant gas cloud passes by. The G2 cloud is as large as our Solar System, and bound for a "supermassive" black hole at the Milky Way's core. On the way, it should encounter many black holes just tens of km across. They will spin and heat the gas, which will emit a spray of X-ray light that telescopes could see.

The cloud of gas - three times larger than Pluto's orbit but with a total mass just three times that of the Earth - was first spotted on its course toward the galaxy's centre in 2011.  Researchers have been gearing up for the cloud's approach to the galaxy's enormous central black hole, with its closest approach in September.  But Imre Bartos of Columbia University in New York and colleagues hit on the idea of using the cloud's passage for another purpose.
"We know that there is a very massive black hole in the centre of the galaxy, many millions of times heavier than our Sun, and we also suspect that there are thousands and thousands much smaller - a few times the mass of the Sun," explained Dr Bartos.

Black holes

• Black holes are incredibly dense objects with gravity strong enough to trap even light
• A 'medium' black hole could have the mass of 1,000 Suns but be no bigger than Earth

The idea is that as the cloud speeds past these small black holes - some slightly more massive than our Sun but just a few tens of km across - gas will spiral around them faster and faster, heating up to millions of degrees and emitting X-ray light. It is a bit like allowing a giant sink to empty through thousands of tiny drains and looking for any evidence of swirling water.

The team estimates - based on guesses about just how much gas is in the cloud - that as G2 makes its pass around the central black hole, X-ray space telescopes such as Chandra or NuStar should be able to glimpse about 16 interactions with its smaller cousins.

Keeping an eye out for these X-rays may also confirm the existence of what are called "intermediate mass" black holes - a few thousand times the mass of our Sun. Here again, theory predicts their existence - particularly near the centres of galaxies - but none has ever been definitively confirmed.
"I think it's a good idea," said Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics, co-author of the 2012 paper in Nature on the G2 cloud's discovery. "We didn't think of that when we did the original paper - I think it's something worth following up".

The less dense the gas is, the less likely that enough light will be produced that our telescopes can see it. But as Dr Bartos points out, it is the first real chance to get a look at what may be thousands of smaller black holes hidden between us and the galactic core.
"It's a very special opportunity, and it's also lucky that we've now got the capacity to observe these things with X-ray telescopes on satellites," he said.