Missing black holes give clues as to how galaxies evolve
European astronomers, working with NASA’s Spitzer and Chandra space telescopes, have found hundreds of massive, growing black holes hiding deep inside dusty galaxies billions of light years away. They represent a large fraction of a missing population of black holes that have been sought after by scientists for three decades. Their discovery leads astronomers to conclude that hundreds of millions of additional growing black holes exist in our young universe, more than doubling the total amount known at that distance. These new observations were carried out as part of the Great Observatories Origins Deep Survey (GOODS), the most sensitive survey to date of the distant universe at multiple wave lengths.
Supermassive black holes produce highly energetic structures called quasars.
The research leader was Dr Emanuele Daddi of the Commissariat a l’Energie Atomique in France and two papers were due to appear in the 10 November issue of the Astrophysical Journal. Co-author Dr David Alexander, from the Department of Physics at Durham University, said: 'The findings are the first direct evidence that most, if not all, massive galaxies in the distant universe spend their youths building black holes at their cores.'
These supermassive black holes produce highly energetic structures, known as quasars. They comprise doughnut shaped clouds of gas and dust that envelop and feed the black hole, causing it to heat up and shoot out X-rays. The X-rays can be detected as a general glow in space, but the quasars are often hidden from direct view by dust and gas. 'We know from other studies from about 30 years ago that there must be more quasars in the universe, but we didn’t know where to find them until know,' said Dr Daddi.
The group began by investigating 1 000 dusty, massive galaxies that are busy producing stars, but were thought to be lacking quasars. The galaxies are about the same mass as our own spiral Milky Way galaxy, but irregular in shape. They are 9 to 11 billion light-years away, existing at a time when the universe was between 2.5 to 4.5 billion years old.
When scientists studied the galaxies more closely — using the Spitzer infrared telescope — they observed that around 200 of the galaxies gave off an unusual amount of infrared light. X-ray data from Chandra and a technique known as ''stacking'' showed that the galaxies had quasars hidden inside them. It is now thought that the quasars heat the dust in their surrounding clouds, releasing the excess infrared light. 'We found most of the population of hidden quasars in the early universe', explained Dr Daddi.
The newfound quasars give insights into how massive galaxies evolve. Astronomers have learnt that most massive galaxies build up their stars and black holes simultaneously until they become too large and their black holes suppress star formation. They also believe that collisions between galaxies might not play such a large role in galaxy formation as was previously believed. 'Theorists thought that mergers between galaxies were required to initiate this quasar activity, but we now see that quasars can be active in unharassed galaxies,' underlined Dr Alexander.