Supermassive black holes underwent periods of exponential growth during which we see them as quasars in the distant Universe. Quasars are some of the brightest objects in the Universe and are seen by the light emitted as gas and dust spiral into the black hole. They are situated in the inner-most regions of galaxies and can consume the equivalent mass of between ten and a thousand stars in one year. It is believed that all quasars are surrounded by a dusty ring which hides them from sight on Earth in about half of cases.
The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured. There are clear examples of obscured black-hole growth in the form of 'type 2' quasars, but their numbers are fewer than expected from modelling of the X-ray background. Objects surrounded by dust are hard to see in visible light, so the astronomers looked at infrared wavelengths. Using NASA's Spitzer Space Telescope data, they selected objects that have mid-infrared and radio emissions characteristic of quasars, but which are faint at near-infrared and optical wavelengths. The researchers found 21 examples of lost quasars in a relatively small patch of sky. All of the objects were confirmed as quasars in radio wavelengths and using ISIS spectrograph on the WHT. This new population of obscured quasars are hidden behind the dust of the galaxy itself rather than just a dust ring. The presence of lots of dust in a galaxy indicates that stars are still forming there.
Therefore, this population of distant type-2 quasars, which is at least comparable in size to the well-known unobscured type-1 population, is responsible for most of the black-hole growth in the young Universe and, throughout cosmic history, black-hole growth has been concentrated in the dusty, gas-rich centres of active galaxies. This is in good agreement with predictions from the X-ray background and implies, from comparisons between the integrated luminosity density of quasars (both type-1 and type-2) and the local space density of relic black holes, that black-hole growth occurs in short, efficient spurts in the cores of forming galaxies.
- "Voracious black holes hide their appetite in dusty galaxies", PPARC Press Release, 4th August 2005.
- A. Martinez-Sansigre, S. Rawlings, M. Lacy, D. Fadda, F. R. Marleau, C. Simpson, C. J. Willott, M. J. Jarvis, 2005, Nature, 436, 666.