Fast-Growing Black Hole in CID-947 Galaxy Is Unproportionally Huge

November 30, 2015

The discovery of a central supermassive black hole shows that it grew faster than its galaxy, CID-947, casting doubt on our understanding on how galaxies were developed.

Artist's impression of CID-947. Credit: Illustration: M. Helfenbein, Yale University / OPAC

At the centre of most galaxies, if not all, there is a supermassive black hole. Observations of galaxies nearest to us have shown that a black hole usually has a mass of 0.2% to 0.5% of its galaxy’s mass, which is still millions or even billions of solar masses. Since both the black hole and the stars in a galaxy are formed from the same cloud of cold gas, the ratio of black hole’s mass and mass of stars was thought to be constant for all galaxies. 

The other thing that usually happens is that the black hole growth and creation of new stars stops at approximately the same time, when the cold gas is mostly depleted, the rest of it heated by the radiation emitted by the growing black hole. This leaves a galaxy with a constant black hole’s to stars’ mass ratio billion of years after the galaxy was formed.

Neither of these two things are true when it comes to the galaxy called CID-947. It is a galaxy of regular size, but with an unusually huge supermassive black hole that weighs 7 billion solar masses or 10% of the CID-947’s mass. 

"That means this black hole grew much more efficiently than its galaxy – contradicting the models that predicted a hand-in-hand development," explains Benny Trakhtenbron, from ETH Zurich’s Institute for Astronomy in Switzerland. Using the Keck telescope in Hawaii, Trakhtenbron and his team of international astrophysicists made the discovery which they reported in the journal Science.

Since CID-947 is 12 billion light-years away, it also means we are observing it when the Universe was only 2 billion years old. Up until now, we based our understanding on observations of galaxies that are close to us and where the size of the central black hole is proportional to the number of stars. "This is true for the local universe, which merely reflects the situation in the Universe's recent past," adds Trakhtenbrot.

The other interesting thing is that, even though the black hole at its centre has ceased to grow, the stars are still being created across the galaxy. CID-947 is obviously a very young galaxy forming when the Universe was young as well, but it offers an insight into formation of galaxies. Although it is still forming stars, CID-947 will never have what we thought was normal ratio between central supermassive black hole and its own mass.

Since it’s still growing, this galaxy could be a key to explaining why the NGC 1277 galaxy, which is only 220 million light-years away, has a supermassive black hole weighing 5% of the galaxy, over 10 times more than the usual 0.2% to 0.5%.

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