Artist's impression of an early black hole in a universe without dust. Image credit Nasa
17th March 2010
Black holes are monstrous gravity wells where our understanding of physics breaks down, places where not even light itself, the fastest thing in the universe can escape. The centres of most galaxies are thought to hold a supermassive black hole, including ours. They nearly always sport a disk of gas and dust, which rotates the hole at vast speeds as it spirals in towards the inescapable gravity. The material in the accretion disks emits enormous amounts of energy from X-rays to visible light to radio as it enters the black hole. This enormous radiation output from the rotating disk is called a quasar. These accretion disks are normally surrounded by a doughnut shaped structure called a dust torus.
But when the universe was a younster less than 1 billion years after the Big Bang, dust in the universe would not have even existed because not enough time would have passed for the molecules to come together to form dust grains. So a black hole in this early universe should have a disk of gas, but no dust.
Astronomers using Nasa’s Spitzer Space Telescope have now seen not one, but two of these squeaky clean dust free quasars, 13 billion light years away. They are named as J0005-0006 and J0303-0019, and were first detected in visible light using data from the Sloan Digital Sky Survey. Spitzer was observing a number of quasars looking for the tell tale sign of the infrared glow of hot dust swirling around the black holes. But these two particular quasars together with their central 100 million solar mass black holes, were not showing the usual dusty signatures. Their accretion disks only contained the gas but no dust, meaning they are very young as no dust existed yet, they are also the smallest ever measured.
