Washington: Astronomers have stumbled on the most massive black holes, each as big as 10 billion suns, in two separate galaxies about 300 million light years away from earth.

'We knew that really large quasars, which are powered by matter falling into black holes, existed in the early universe,' said Chung-Pei Ma, astrophysicist at the University of California, Berkeley, and study co-author.

'What we hadn't yet found was where the remnants of those quasars -- equally large black holes -- were in the current universe,' she said, the journal Nature reports.

'The boisterous quasars may have passed through a turbulent youth to become the quiescent giant elliptical galaxies we see today, harbouring hidden black holes at their centres,' added Chung-Pei Ma, according to a university statement.

Black holes are so dense that even light can't escape their intense gravitational fields. Exploding stars, supernovae, can create relatively small black holes only a few times more massive than the sun.

But researchers think these monster black holes are formed in different ways, such as multiple smaller black holes merging into one, or voracious growth by swallowing vast amounts of stars and gas while galaxies are forming.

The gigantic black holes discovered by Chung-Pei Ma and her colleagues are so enormous that they are capable of consuming anything within a region five times the size of earth's solar system.

Researchers think that most, if not all, galaxies have a black hole at the centre. The larger the galaxy, the larger the black hole it contains.

Researchers suggest these blacks holes consume tremendous quantities of gas and dust from the central regions of the galaxy, at which point they become 'dormant'. The surviving gas may become stars that orbit peacefully within the galaxy.

Their quiet nature is part of what makes these sleeping giants so difficult to observe. 'Since black holes cannot be seen, we have to detect them by carefully observing their immediate surroundings,' said Nicholas McConnell, study co-author.

'These galaxies contained enormous masses within a small central volume -- too much mass to come from stars alone,' he added.

(Agencies)