Centre of Milky Way is teeming with black holes

Katie Ramirez
April 5, 2018

A team from Columbia University in NY believe they have proven a decades-old theory that a multitude of smaller black holes surround the "supermassive" black hole at the core of the Milky Way.

She explained that the team's findings were inconsistent with the theory that the Milky Way's bulge is "purely old".

"There's lots of action going on there", said study lead author Chuck Hailey. As many as 20,000 black holes are predicted to settle in the central area of our galaxy (and all spiral galaxies) but so far, these black holes haven't been observed, and neither has their gravitational effect, despite astronomers' best efforts - that is, until now. "In a sense, this is the only laboratory we have to study this phenomenon".

The study is in Wednesday's journal Nature. This confirms the idea that scientists have speculated that black holes sink to the center of galaxies and accumulate.

After poring over archival data from NASA's Chandra X-ray telescope, researchers from Colombia University in the United States found signs of a dozen black holes in the inner circle of the Milky Way.

"There hasn't been much credible evidence". Furthermore, all black holes in the nearby vicinity of Sagittarius A* are held close by its massive gravitational pull. Looking back in time to 16 different epochs between 11 and 13 billion years ago, the researchers discovered nearly 4000 early galaxies, many of which will have evolved into galaxies like our own Milky Way.

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But from Earth they can only be spotted every 100 to 1000 years.

"It would be so easy if black hole binaries routinely gave off big bursts like neutron star binaries do, but they don't, so we had to come up with another way to look for them", Hailey explained. In 2015, he led a team that found the first example of a spectacularly bright galaxy within the Epoch of Reionization, which was thought to be home to first generation stars, and a year later, he found similar galaxies made up of the earliest stars.

"If we could find black holes that are coupled with low-mass stars and we know what fraction of black holes will mate with low-mass stars, we could scientifically infer the population of isolated black holes out there".

"These stars have already expanded to a huge size to produce the gas that flows into the disk that surrounds the black hole and produces the X-rays we observe. And the Galactic Center is so crowded with X-ray-emitting objects and X-ray glowing gas that it is hard to find the black hole binaries that emit X-rays", he continued. This phenomenon is helpful to the Columbia researchers because it makes the black holes detectable with observatories like Chandra.

Instead, the Columbia University team looked for the fainter but steadier X-rays emitted when these binaries are in an inactive state. Most of the black holes remain isolated, but a small minority become attached to the orbits of low mass stars in the region, forming a binary system.

Hailey said the new findings could significantly advance research into one of the most exciting fields of astronomy right now: gravitational waves.

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