Scheme: Royal Society Research Professorship
Organisation: University of Cambridge
Dates: Jan 1982-Sep 2013
Summary: Clusters of Galaxies are the largest gravitationally bound objects in the Universe. The baryons - ordinary matter - in a cluster are mostly in the form of a hot plasma at tens of millions of degrees, and emit X-rays. Significant amounts of cool and cold gas (10,000 to below 100 degrees) are seen in the core of about a third of clusters, often in the form of a filamentary nebulosity stretching many thousands of light years around the central galaxy. The filaments glow with an optical spectrum unlike that of anything in our Galaxy. We have now shown that this is due to heating and ionization of the cold gas by hot gas particles. We are extending the spectral measurements to a much wider bandwidth stretching to the far IR and submm.
The densest hot gas in cluster cores emits the most X-rays. This loss of energy by the gas is partly compensated for by energy supplied by accretion onto a massive central black hole powering jets which blow bubbles in the gas. Understanding how accretion onto black holes actually works is difficult and requires observations of the very innermost regions. We do this by looking at X-ray luminous objects (active galaxies and Galactic black hole binaries) outside clusters where the gravitational energy released by accretion makes the infalling gas luminous by reflected X-rays which carry spectral signatures - iron lines for example - which can be used to determine the radius, velocity, and other properties of the matter.