Astronomers have spotted an enormous black hole – the second-heaviest ever seen – but it resides in a tiny galaxy.
The galaxy NGC 1277, just a quarter the size of our own Milky Way, hosts a black hole 4,000 times larger than the one at the Milky Way’s centre.
It has a mass some 17 billion times that of our Sun.
The surprise finding is hard to reconcile with existing models of black hole growth, which hold that they evolve in tandem with host galaxies.
Getting to grips with just how large black holes are is a tricky business – after all, since they swallow light in their vicinities, they cannot be seen.
Instead, astronomers measure the black holes’ “sphere of influence” – the gravitational effects they have on surrounding gas and stars.
In the Milky Way, it is possible to observe individual stars as they orbit Sagittarius A, our own local black hole, to guess its mass.
But for the 100 or so far more distant black holes whose masses have been estimated, astronomers have made average measurements of associated stars’ speeds – their “velocity dispersion”.
On a hunt for the Universe’s largest black holes, astronomers using the Hobby-Eberly Telescope in the US state of Texas undertook a survey that brought in a haul of nearly 900 host galaxies.
But Remco van den Bosch, then at the University of Texas at Austin, and his colleagues were surprised to find that some of the largest black holes were to be found in small galaxies. Among them was NGC 1277, 220 million light years away in the constellation Perseus, which happens to appear also in a high-resolution Hubble Space Telescope image, helping the researchers to refine their computer models. “We make a model of the galaxy and compute all the possible stellar orbits,” Dr Van den Bosch explained to BBC News. “Like a big jigsaw, we try to put those orbits together to reproduce that galaxy so it has the same stellar velocities we measure.” What the team found was that the NGC 1277 black hole was enormous – as large as our Solar System, and comprising some 14% of the entire galaxy’s mass.
