In 1918, the American astronomer Heber Curtis used the Crossley Reflector at Lick Observatory in California to take optical photographs of nearby galaxies. When observing M87, the central elliptical galaxy in the Virgo galaxy cluster 53 million light years from the Sun, Curtis noted the lack of a spiral structure and observed a “curious straight ray … apparently connected with the nucleus by a thin line of matter”. This measurement heralded the discovery of radio jets, the ejection of highly collimated, relativistic plasma streams or “blobs”, from a central energy source. Forty years later radio astronomers in the UK and Australia discovered a new class of “radio stars” not obviously connected with bright optical nebulae. Caltech astronomer Maarten Schmidt then used the large 5 m Hale telescope on Palomar to identify a faint, compact optical nebula associated with one of these radio stars, 3C273. Schmidt interpreted the substantial (16%) redshift of all spectral lines in the optical spectrum as being caused by the expansion of the Universe. If so, 3C273 must be 2.4 billion light years from us, 45 times further away than M87, and the optical luminosity of the faint compact speck would have to be about one thousand times greater than the 100 billion stars of our Milky Way. Thus began the subject of quasars, quasi-stellar radio sources. Many quasars and other somewhat less spectacular “active galactic nuclei” (AGN) exhibit powerful radio jets.
Detailed observational and theoretical research since these discoveries have led to a widely accepted standard model in which most galactic nuclei contain spinning charged black holes, with huge masses ranging from 1 million to 10 billion times that of the Sun. When matter in the form of gas or stars is gravitationally attracted by these black holes, a rapidly rotating, hot plasma accretion disk or torus forms around the hole just outside its event horizon, probably pervaded by strong magnetic fields. The interaction of the spinning black hole, the magnetic field lines anchored in the black hole and the inner accretion disk, and the accretion disk plasma, drive matter rapidly out along the rotation axis of hole and disk. The 2020 Shaw Laureate in Astronomy, Professor Roger Blandford, has been one of the most important architects and drivers in constructing the theoretical framework of this active galactic nuclei + jet paradigm. These same processes are also relevant to γ-ray bursts and stellar-mass black holes.
Blandford originated key ideas leading to the spectacular multi-scale acceleration and collimation of relativistic jets, involving complex fluid-dynamical and electro-dynamical processes (with Königl, Begelman, Ostriker, Rees). Perhaps his most prescient contribution was his recognition (with Znajek) that magnetic torques could extract energy from a spinning (Kerr–Newman) black hole, and thus efficiently drive jets. This paper as well as others on the creation of fast winds from accretion disks around massive black holes (with Payne, Begelman and others) have in recent years become even more relevant and widely cited than when they were originally written. This is because high resolution radio and infrared interferometric observations are just now beginning to directly probe and reveal the innermost accretion and jet formation zones around massive black holes, which Blandford analysed in his prescient theoretical work. The disk winds are also relevant for outflows from protostars. His theoretical work impresses by its focus on key physical aspects, such as energy transport and shock-based mechanisms for cosmic ray acceleration in highly complex, magnetised plasmas.
Another paper that is now more cited than ever deals with the fate of binary black holes, which arise as the outcome of mergers between galaxies (with Begelman & Rees). With McKee he showed how time-variable line and continuum emission can be used to explore the spatial structure of broad-line regions around AGN, a now standard technique used by many observers.
Blandford has made major contributions to an extremely broad range of astrophysical problems, arguably placing him among the rare group of “universal” scientists. He has been one of the leaders in the modelling and interpretation of gravitational lensing. He has contributed to the interpretation of γ-ray data from the Fermi spacecraft and to the study of gravitational waves. Blandford’s contributions began and often are rooted in analytic work, but in recent papers he and his collaborators have exploited increasingly sophisticated numerical techniques to capture realistically the complex physics in the strong gravity environment of spinning and accreting black holes (with McKinney, Tchekovskoy, Anantua and many others).
In addition to his research, Roger Blandford stands out because of his tireless participation in community service, culminating in the leadership of the 2010 US decadal survey in astrophysics.
Blandford’s many profound contributions to theoretical astrophysics and his continuing originality and towering presence make him a worthy recipient of the 2020 Shaw Prize in Astronomy.
20 May 2021 Hong Kong
