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Supermassive black hole dominates titchy star formation

Astronomers have spied a supermassive black hole - of the type theorised by some physicists to be portals out of our universe to elsewhere - in an itsy-bitsy ultracompact dwarf galaxy, the smallest ever known to contain such a gigantic light-sucking feature.

Artist's View of M60-UCD1 Black Hole

"It is the smallest and lightest object that we know of that has a supermassive black hole," says Anil Seth, astrophysicist at the University of Utah. "It's also one of the most black hole-dominated galaxies known."

The astroboffins used the Gemini North 8m telescope on Hawaii’s Mauna Kea and photos from the Hubble Space Telescope to spot that the little galaxy called M60-UCD1 has a black hole with a mass equivalent to 21 million suns.

Our own Milky Way has a central supermassive black hole with the mass of four million suns, which is just a fraction of the galaxy’s entire mass of around 50 billion solar masses.

However, M60-UCD1’s black hole is five times larger and takes up a huge 15 per cent of the small galaxy’s total mass of 140 million suns.

If M60-UCD1 can contain this huge event horizon, it’s likely that other ultracompact dwarf galaxies can too – in fact, they may even be the stripped down remains of larger galaxies torn apart in collisions with other galaxies.

"That is pretty amazing, given that the Milky Way is 500 times larger and more than 1,000 times heavier than the dwarf galaxy M60-UCD1," Seth said.

"We believe this once was a very big galaxy with maybe ten billion stars in it, but then it passed very close to the centre of an even larger galaxy, M60, and in that process all the stars and dark matter in the outer part of the galaxy got torn away and became part of M60," he says. "That was maybe as much as ten billion years ago. We don't know."

The ultimate fate of the tiny galaxy is likely to be obliteration, as it merges inexorably with the M60, among the largest galaxies in the local universe.

"Eventually, this thing may merge with the centre of M60, which has a monster black hole in it, with 4.5 billion solar masses – more than 1,000 times bigger than the supermassive black hole in our galaxy. When that happens, the black hole we found in M60-UCD1 will merge with that monster black hole," Seth explained.

Ultracompact dwarf galaxies are among the densest star systems in the Universe and M60-UCD1 is the most massive of these now known. It lies around 54 million light years from Earth, but only 22,000 light years from the centre of the galaxy M60.

Astroboffins reckon that this type of galaxy is either the stripped centre of larger galaxies that were involved in collisions or globular clusters of hundreds of thousands of stars, all born together. ®

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