Black Holes May Supply Up to Half the Universe's Energy Output
Massive black holes, long-thought to make only a modest contribution to the universe's total energy output compared with ordinary stars, may actually be responsible for up to half of all the radiation produced in the universe since the Big Bang. Details of this theory, based on measurements of background X-radiation and the growth of massive black holes obscured by gas, are being presented today at the X-ray Astronomy 1999 meeting in Bologna, Italy, by Professor Andrew Fabian, a Royal Society Research Professor at the Institute of Astronomy, University of Cambridge.
The black holes responsible for this energy production are probably present in the centres of most galaxies and contain the mass of millions, or even billions, of suns compressed into a region smaller than the solar system. Distant galaxies with suspected massive black holes within their exceptionally bright cores are commonly known as quasars. They produce energy in an accretion process, when gas approaching the black hole swirls inwards, attaining very high velocities and temperatures in the extremely strong gravitational field.
This hot, fast-moving gas is very luminous, emitting radiation across a broad spectrum -- from visible light, through the ultraviolet to X-rays -- before disappearing within the event horizon of a black hole. The event horizon is the black hole's point of no return, beyond which gravity is so intense that nothing, not even light, can escape. Outside the event horizon, there is a larger region where the black hole exerts a powerful gravitational influence, but not so powerful that nothing can escape. This is the region where accretion occurs and the intense radiation is emitted.
"The background of X-radiation found in space cannot be explained by stars or by ordinary quasars," says Professor Fabian. "What is required, following earlier ideas, is a population of obscured quasars. For every ordinary quasar about ten more obscured ones are needed, meaning that most massive black holes growing by accretion are hidden from the view of observers looking at visible light, or in the ultraviolet and near infrared wavebands."
Visible light and ultraviolet radiation from the accreting gas try to escape from the black hole region but are absorbed by nearby dust and gas. The X-rays are not absorbed and therefore provide a true measure of the total amount of energy being emitted. The absorbed energy, however, can also provide a useful measure of black hole power. This absorbed energy is re-emitted in the form of far infrared radiation (alternatively known as sub-millimetre radiation), which also penetrates the dust and gas.
Energy emitted from the regions close to massive black holes has been underestimated, Fabian says, because orbiting X-ray observatories have so far not been able to detect the dust-penetrating X-rays and also because there are no superior far infrared telescopes to observe the re-emitted black hole radiation. Stars, on the other hand, are well documented because they radiate their energy largely as visible and ultraviolet light. This radiation is measured by world-class telescopes both in orbit and on Earth.
"Recent ground-based observations in the submillimetre band made with the United Kingdom's Submillimetre Common User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope in Hawaii, and the Diffuse Infrared Background Experiment (DIRBE) on NASA's Cosmic Background Explorer (COBE) satellite do support the idea that much of the energy in the universe has been absorbed and re-radiated at much longer wavelengths. However, the process of absorption and re-radiation conceals whether the energy is from stars or black holes," said Fabian.
Observations with NASA's Chandra X-ray Observatory, launched in July, and ESA's XMM X-ray satellite, scheduled for a December launch are expected to provide the first accurate cosmic census of the power of black holes over the age of the Universe, and detect gas-obscured X-ray sources. Professor Fabian is one of the Interdisciplinary Scientists for the Chandra Observatory.
Professor Fabian's paper, "The obscured growth of massive black holes" will be published in the 1 October 1999 issue of the Monthly Notices of the Royal Astronomical Society.
The X-ray Astronomy 1999 meeting in Bologna, running between the 6th and the 10th of September, is sponsored by NASA Goddard Space Flight Center, the University of Bologna, and Consiglio Nazionale delle Ricerche Istituto di Tecnologie e Studio delle Radiazioni Extraterrestri. More information on the meeting is available at the following web site:
Professor Andrew Fabian, Institute of Astronomy, University of Cambridge.
Phone: (0)1223-337509 FAX (0)1223-337523