2008 Kavli
Prize in
Astrophysics

Citation from the committee

In 1963, Maarten Schmidt unlocked the gate to the far reaches of the Universe by correctly identifying emission lines in the optical spectrum of a radio “star” known as 3C273. That insight immediately showed that 3C273 is an extremely luminous, very distant object rather than a star in our own galaxy. Objects like 3C273 are now known as quasars.

Schmidt’s breakthrough interpretation of the spectrum of 3C273 followed essential work by radio astronomers, who discovered quasars by their radio wave emission and measured their positions on the sky. Schmidt extended his discovery by finding quasars even more distant than 3C273 and showing that quasars were much more numerous when the Universe was young. Schmidt also devised powerful statistical techniques to measure the luminosity and space density evolution of quasars.

The extraordinary power emitted by quasars requires an extraordinary engine. Various proposals for the nature of the quasar engine were advanced in the years following Schmidt’s discovery. The watershed in our theoretical understanding of the nature of quasars was Donald Lynden-Bell’s investigation in 1969 of the hypothesis that quasars were powered by gravity, through the accretion of material onto massive black holes.

Although others had suggested that quasars were powered by black hole accretion, Lynden-Bell argued persuasively that most of their luminosity comes from frictional heating in a rotating gaseous disk (the “accretion disk”); developed an approximate model for their spectrum; and suggested that these black holes are to be found in the centers of galaxies. He also pointed out that many nearby galaxies should contain black holes at their centers that do not currently shine (“dead quasars”), and that these could be detected by their gravitational influence on stars orbiting nearby, a prediction that has been observationally confirmed.

Maarten Schmidt’s and Donald Lynden-Bell’s seminal work dramatically expanded the scale of the observable Universe and led to our present view of a violent universe in which massive black holes play a key role.

The 2008 Astrophysics Kavli Prize explained

Many quasars are bright enough to be observed with small telescopes, through which they appear as single points of light. Until around 50 years ago, those who looked up at the night skies had no way to distinguish them from ordinary stars in our galaxy. However, as radio telescopes became more advanced, astronomers identified a group of objects which produced unusually compact patterns of radio wave emissions.

By Nic Fleming

During the late 1950s, scores of these radio sources were recorded, but astronomers were unable to link them with visible objects until 1960. They became known as quasi-stellar, or star-like, radio sources - a phrase later shortened to quasars.

In 1960, Allan Sandage and Thomas Matthews identified quasar 3C48 as a faint, star-like object seen on photographic plates. Two years later, Cyril Hazard in Australia identified the location of quasar 3C273 by timing the blackout of its radio signal as the moon passed in front of it. This allowed Maarten Schmidt, who had emigrated from The Netherlands to the U.S. and was working at the California Institute of Technology, to obtain its visible light spectrum using the 200-inch Hale Telescope on Mount Palomar in California.

The pattern displayed by 3C273’s spectrum was initially puzzling, until Schmidt realized it could be explained by the phenomenon known as red shift - the displacement of the light emitted by an object toward the red end of the spectrum due to motion of the source.

Schmidt’s calculations suggested 3C273 was travelling away from Earth at a rate of 47,000 kms per second (29,000 miles per second). This seemed unbelievable at first, until Schmidt and colleague Jesse Greenstein examined a spectra of 3C48 and found this quasar was moving at more than double this speed. Schmidt correctly interpreted these findings as arising from the expansion of the universe, which in turn allowed him to calculate 3C273 to be approximately 2,000 million light years away. This in turn implied it was emitting more than a million million times the energy of the Sun. Schmidt went on to identify quasars even more distant than 3C273, and to demonstrate they were much more numerous when the universe was young. He also devised powerful statistical methods to measure the luminosity and space density evolution of quasars.

His work left astronomers puzzling over how quasars could emit such enormous quantities of energy. It is now known that quasars like 3C273 emit hundreds of times the energy of the entire Milky Way Galaxy from a volume no larger than the size of our own solar system.

Various theories were proposed to explain this. Shortly after Schmidt’s discovery, astrophysicists Edwin Salpeter in the U.S. and Yakov Zeldovich in the USSR put forward the theory that quasars were powered by black holes.

However, the key breakthrough came in 1969 with the publication of English astrophysicist Donald Lynden-Bell’s investigation of this hypothesis. He argued quasar luminosity arose from frictional heating in a gaseous disk of material rotating around giant black holes at their centers. The prediction that quasars are found at the centers of galaxies was later confirmed by high-resolution telescope observations.

Lynden-Bell, of Cambridge University in the UK, calculated the spectrum this model should produce and compared it to observed quasar spectra. He also said nearby “dead quasar” galaxies should contain black holes that did not shine, and that these could be detected by their gravitational influence on nearby orbiting stars - another prediction later confirmed by observation.

In making their award, the members of the Kavli Astrophysics Prize Committee said, “Maarten Schmidt and Donald Lynden-Bell’s seminal work dramatically expanded the scale of the observable universe and led to our present view of the violent universe in which massive black holes play a key role.”