From Royal Astronomical Society's citation:
... in recognition of his outstanding personal research and leadership in astronomy that make him one of the most influential British astronomers of the last three decades.
During his career, Richard Ellis has played a key role in cosmology and astronomical instrumentation. In the 1990s he used the Hubble Space Telescope to solve the 'faint blue galaxy' problem, identifying the transformation of irregular galaxies into more quiescent systems. Since then he has made major progress in understanding why galaxies are grouped into the 'Hubble sequence' and in recent years has used gravitational lensing to find some of the most distant objects in the universe, with redshifts from 6 to 10.
Prior to Caltech, Richard held posts including Director of the Institute of Astronomy in Cambridge and Professor of Astronomy at Durham University. During this time he led the 2-degree Field Galaxy Redshift Survey (2dFGRS), the first large-scale (and highly
successful) cosmology project. Subsequently he has gone on to lead a team proposing a wide-field spectrograph for the Subaru telescope on Hawaii that would be the natural successor to 2dFGRS and is playing a key role in the efforts to build a 30-m telescope.
Richard is the co-author of 340 refereed papers receiving more than 41000 citations, with three of his papers each having more than 1000 citations, giving him a publication record of immense stature. With all-round vision, science leadership and a rich legacy of contributions to cosmology, he is a very worthy recipient of the Gold Medal.