ASTRONOMY/HERSCHEL
Meetings chaired by the convenor Richard Phillips unless otherwise stated

Dan Evans - Dept of Physics, Bristol University

Friday 4 June 2004

The speaker began by describing what a ‘black hole’ is – briefly a large mass (a star, for instance) which has collapsed into such a small radius that the speed of escape from its ‘surface’ is equal to the speed of light. The speed of escape from the Earth is 7 miles per second. As nothing can travel faster than light, it means that a black hole emits little radiation - hence its ‘black’ nature. Relatively light black holes, with a mass around a few to 10 times that of our Sun, are thought to be formed as a result of a large star, at the end of its life, collapsing under its own gravity to form a supernova explosion. Much of its material is thrown outward but there is always a remnant which becomes a neutron star or a black hole.

The ‘surface’ of a black hole is called its ‘event horizon’, the radius R of which depends on the mass M such that R = 2GM/c², where G is the Universal Gravitational Constant and c is the speed of light. A star with a mass 3 times that of the Sun could collapse into a black hole with an event horizon of only 3 km radius. A black hole filling the orbit of the Earth would have a mass of one billion solar masses (i.e. about one hundredth the mass of the Galaxy).

Material falls into such black holes and is heated to millions of degrees and emits X-rays. The sort of telescopes which provide the data on which the speaker works are space telescopes which operate in the X-ray region of the electromagnetic spectrum. The first of these was XMM-Newton, launched by the ESA in 1999. At 3.8 tonnes it is largest European satellite.

Chandra, also launched in 1999, was constructed by NASA and is operated by the Smithsonian Astrophysical Observatory in Cambridge, MA. Chandra is the largest object ever deployed by the space shuttle, and cost $10 billion. It has a very high resolution of ½ second of arc. The first object studied was the supernova remnant in Cassiopeia called Cas A.

(We were shown film-clips of the launches of both these telescopes.)

X-rays are focussed by a number of parabolic and hyperbolic surfaces that are at a small angle to the incoming rays to form an image. (We were shown, by an animated diagram, how the radiation is collected.)

A photograph of the largest galaxy in the cluster in Virgo, M87 shows a jet emanating from its core, which in itself is very massive. The light from the jet is different from that from stars.

Studies of our own galaxy, the Milky Way, have revealed that stars at its centre are orbiting a central mass (we were intrigued by a time-accelerated clip of these stars and saw them move in elliptical orbits – one with a major axis of only 10 light-days long). Broadening of spectral lines further indicates that something very massive at the centre of active galaxies. The central object must be very small as its light flickers on relatively short timescales.

What could it be? A super-massive star is ruled out as such a star would be unstable and fragment. A starburst galaxy (a chain of supernovae) is unlikely as we observe well-ordered jets. A supermassive black hole is the most likely explanation, and fulfils all observational requirements.

The speaker than turned his attention to the object he has studied most: the peculiar galaxy in the constellation of Centaurus called ‘Cen A’. This is the brightest X-ray object in the sky, emitting 10³⁵ watts. A Hubble Space Telescope view shows what is essentially an elliptical galaxy crossed by a dust-lane and containing shock-waves which are causing the formation of hot blue stars. The disc is really the remnant of a small galaxy which has been ‘eaten’ by the large 130 light-year diameter elliptical galaxy.

The radio emission detected by the Very Large Array (VLA) from Cen A is enormous, spanning several degrees on the sky, and has the spectral fingerprint of synchrotron emission. Ten years of study has revealed a jet emanating from the centre, travelling at half the speed of light!

The spectrum of the X-ray emission shows much radiation at long wavelengths coming from a jet, a fall-off in the intermediate wavelengths and then high emission at short wavelengths which originate in the active nucleus.

Although almost nothing comes out of a black hole, its spin and magnetic field can interact with the rapidly orbiting accretion disc just outside the event horizon and channel fundamental particles to its poles to produce a jet from each pole.

The observed spectrum and jet point very strongly to the existence of a massive black hole at the centre of Cen A, and at the centre of M87 and NGC 4258. In fact, it is extremely likely that there is a massive black hole at the centre of every active galaxy, and even in our own Milky Way.

Artist’s impression of a black hole, courtesy NASA. Richard H Phillips

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