Ultra-bright black hole found 'wandering' in distant universe


It's straying to the edge of its galaxy – but the galaxy might not be its original host. Belinda Smith reports.


X-ray image of the suspected ultra-bright black hole XJ1417+52 on the edge of the galaxy GJ1417+52.
X-ray: NASA / CXC / UNH / D.Lin et al; Optical: NASA / STScI

Astronomers think they’ve found a wandering supermassive black hole – and it’s the brightest of its kind ever seen.

Dacheng Lin from the University of New Hampshire in the US and an international team of researchers examined an ultra-luminous source of X-rays streaming from the galaxy GJ1417+52 using a number of ground and space-based telescopes and deduced the source must be a black hole, around 10,000 times the mass of the sun, that was stripped of its host galaxy when it merged with a larger one.

The blast of brightness, they think, was produced when a star cruised too close and was torn apart by tidal forces.

Most large galaxies, including our own Milky Way, are thought to host a supermassive black hole in their centre.

But these can get knocked off balance, stripped of stars or even kicked out altogether during a merger with another galaxy.

In the ensuing push and pull, sometimes one of the black holes scoops all the stars and dust from the other galaxy, leaving the other behind and all but naked.

One such wandering black hole was found, by accident, in July 2000. The European Space Agency’s XMM-Newton space telescope spotted a source of bright X-rays off-centre of its galaxy.

The source, called XJ1417+52, is 4.5 billion light-years away in the galaxy GJ1417+52, in the so-called Extended Groth Strip.

After the serendipitous discovery, NASA’s Chandra space telescope, the Hubble space telescope and Gemini Observatory on Earth checked it out over the next 15 years.

They researchers saw the X-ray emissions peak between 2000 and 2002. At that point it was about 10 times brighter than the brightest X-ray seen from a wandering black hole.

That luminosity has since dropped by a factor of four.

The team thinks the short-lived blast occurred when a star strayed too close to XJ1417+52 and was torn apart.

Some of its gas fell towards the black hole and became hot in the process – so hot it glowed bright with X-ray light which manifested itself as the spike XMM-Newton picked up.

Hubble picked up a faint optical smudge that may be associated with the black hole. If so, the researchers posit, XJ1417+52 could have originally belonged to a small galaxy that barrelled into the larger GJ1417+52 galaxy.

"We [...] find it is best explained as a massive black hole embedded in the nucleus of a possibly stripped satellite galaxy," they write.

The work is available on Arxiv and will be published in The Astrophysical Journal.

  1. http://arxiv.org/abs/1603.00455
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