Cutting edge

September 25, 1998

The puzzles of cooling gas in clusters of galaxies and black holes are being investigated with X-ray astronomy

After a first in physics at King's London I began a PhD in rocket astronomy at the Mullard Space Science Laboratory of University College, London. I was involved in the planning, building, launch and analysis of data from instruments on two Skylark sounding rockets, which measured the smoothness of the cosmic X-ray background.

X-ray astronomy was very new, and I was fortunate to work on data from a small X-ray instrument package on the new Copernicus satellite soon after my PhD was finished. At Cambridge, I have built up a small group dedicated to the analysis and interpretation of X-ray astronomy data. My interests now concentrate on clusters of galaxies, active galactic nuclei and the X-ray background.

I have been intrigued by the X-ray emission peaks at the centres of many clusters of galaxies and the fact that the thermal energy of the 50 million degree hot gas there might be expected to be radiated away in a few billion years or less. This is less than the expected age of a cluster. The issue that I and others have been puzzling over for the past 20 years is what happens to the cooling gas. Recent data from the United States-Japanese satellite ASCA clearly shows cooler X-ray emitting components in these regions and the cooled gas. The rates at which the gas is inferred to cool mean that the halo of the massive galaxy at the centre of such a cooling flow may mostly be built out of the cooled gas. The problem is that we see little sign of the cooled gas in other wavebands. There is evidence for lots of mass in these regions, but it is mostly dark; we guess that the cooled gas forms some sort of dark matter.

Next January, a major Nasa X-ray satellite, AXAF, will be launched. I will use the AXAF telescope to examine cooling flows, the brightest extended extragalactic regions of X-ray emission. I anticipate spectacular results.

My other great interest is the search for black holes. Work in the radio and optical bands has shown compact masses that can only be black holes. But they probe only the regions beyond about 100,000 gravitational radii (the event horizon marking the extent of a non-spinning black hole is at 2 gravitational radii). The strength of the gravitational field there is not much greater than that on the Sun's surface, much less than the strong field in the near vicinity of the black hole. The variable X-ray emission from the nuclei of active galaxies has long been thought to come from the near region as the black hole accretes surrounding matter via a disk that gradually drains into the hole.

The key point for studying black holes is that X-ray irradiation of the material in the disk produces a fluorescent iron line at a particular wavelength. Doppler shifts, the special relativistic effects of aberration and time dilatation, and the general relativistic redshift combine to make the iron emission line seem to be broad and highly skewed toward the red. With others, I predicted the shape of the line over ten years ago and helped plan and analyse ASCA data, which clearly revealed it four years ago. The extent of the line shows that the emitting region is at about 6 to 20 gravitational radii, and possibly closer at times.

In about a year the European Space Agency will launch its big X-ray mission, and both Nasa and ESA plan larger instruments that will allow the broad iron line to be monitored as the X-rays flicker and flash, so mapping the immediate environment of a black hole. I am hopeful that strong gravity will then become an observational science.

A. C. Fabian is a Royal Society research professor in astronomy at the University of Cambridge.

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