March 24, 2010
A quasar is the central region of a galaxy that contains an active black hole. The black hole is surrounded in turn by a brightly glowing disk of gas and dust. Matter swirls towards the black hole and emits radiation before disappearing into it. As a result, these accretion disks are among the brightest objects in the entire universe. Quasars shine so brightly that it is possible to obtain information about their physical characteristics from enormous distances.
It takes around 13 billion years for the light from the most distant known quasars to reach us. Therefore, when we observe these objects we go back 13 billion years into the past. Based on this, the scientists would expect to discover comparatively primitive precursors of the modern quasars, which are only in the process of formation in this period in the immediate aftermath of the Big Bang. However, observations made in 2003 revealed that the most distant quasars do not differ significantly from their contemporary counterparts, which are located closer to Earth.
A team of astronomers working with Linhua Jiang from the University of Arizona, which includes researchers from the Max Planck Institute for Astronomy in Heidelberg and the Max Planck Institute for Extraterrestrial Physics in Garching, has observed for the first time objects that appear to be very primitive early forms of modern quasars. The team used NASA’s Spitzer Space Telescope, which can see infrared light. The characteristic radiation of hot dust can be identified on the basis of observations in this spectral range - and this kind of dust is a typical component of modern quasars.
In such quasars, the brightly glowing accretion disk (which is around the same size as our solar system) is surrounded by a gigantic dust torus, which is around one thousand time bigger than the disk itself. The dust signature was lacking in two of the observed 20 quasars. According to the researchers, this indicates that these are early primitive quasars as there was no dust whatsoever in the early universe. Therefore, the first quasars would be very hot and bright, but would not contain any dust particles: a case of fire without smoke. The existence of such dust-free quasars had long been suspected. However, it had not been possible to observe them up to now.
The astronomers then studied all of the available data on the distant quasars and compared them with the measurement data available on modern quasars. It emerged that none of the other quasars - in particular the modern quasars - are anywhere near as dust-free as the two primitive ones. In addition, the astronomers discovered a correlation between the mass of the central black hole and the dust content in the most distant quasars: the greater the mass of the central black hole, the more dust the quasar contains. This would indicate a development process whereby the central black hole grows rapidly by swallowing up matter, while more and more hot dust is produced at the same time,.
"All of the evidence would suggest that, based on our observations, we have succeeded in gaining an initial insight into the evolution of the early quasars, and that the most primitive black hole systems that we know of are concealed within the two dust-free quasars," says Fabian Walter from the Max Planck Institute for Astronomy and co-author of the article in Nature. The observations showed quasars in an early stage of their development which were too young to have formed detectable amounts of dust around them.