Galactic lenses cut black holes down to size
Jun 26, 2002
The abundance of primordial ‘supermassive’ black holes – several billion times more massive than the Sun – could have been greatly overestimated, according to Stuart Wyithe and Abraham Loeb of Harvard University in the US. Wyithe and Loeb have calculated that the light emitted by certain quasars – the brightness of which is linked to the size of their parent black holes – has been greatly amplified by gravitational lenses. The claim could boost models of the early Universe, which predict a more modest number of supermassive black holes than some studies have suggested (J Wyithe and A Loeb 2002 Nature 417 923).
Quasars are the brightest objects in the Universe, and exist at the cores of active galaxies. As particles of gas and dust are sucked into the black hole at the centre of the galaxy, they release their gravitational energy as light, and this makes the quasar shine. Previous studies of remote, bright quasars have suggested that supermassive black holes were common in the early Universe.
But existing models cannot explain how so much matter was formed at such an early stage in the development of the Universe. Now Wyithe and Loeb have studied the luminosity of four very bright quasars recently discovered by the Sloan Digital Sky Survey. The quasars have large ‘redshifts’, which means that they are extremely remote and emitted their light when the Universe was very young. The researchers say that the light from them could have been amplified at least ten times by intervening galaxies acting as ‘gravitational lenses’.
Astronomers know that the immense gravitational field of a galaxy can bend the light from any source that lies ‘behind’ it. This effect can produce several images of the background object, but these are usually close together and difficult to resolve. This can make the total light output of the object seem much greater than it really is.
From the density of galaxies in the Universe, Wyithe and Loeb calculated the probability that a galaxy lies in the line-of-sight of the distant quasars. They concluded that the light from 10–30% of similar quasars is magnified by a factor of ten or more.
If the claim is correct, it means that the quasars are dimmer – and that their parent black holes are smaller – than previously thought. This would resolve the theoretical glitch, and could mean that black holes can form in less massive galaxies than astronomers previously thought. “Studies of these lensed quasars could also allow us to ‘weigh’ the intervening galaxies,” Loeb told PhysicsWeb.
About the author
Katie Pennicott is Editor of PhysicsWeb