Mark Birkinshaw

Mark was born in London in December 1954. The family later moved to Hayling Island, Hampshire. He attended Portsmouth Grammar School, where his great talent for Physics was recognised and nurtured. After completing school in 1973, Mark went up to St. John's College, Cambridge on a Scholarship to read Natural Sciences. He graduated with a First Class Degree in Theoretical Physics. In 1976 he moved across Cambridge to the Mullard Radio Astronomy Observatory (MRAO), Cavendish Laboratory for his PhD, completing his thesis on "The radio and X-ray properties of clusters of galaxies" in 1979. He published nine first author papers based on his PhD work. He then moved to the USA, where he held a two-year, Miller Research Fellowship at the University of California, Berkeley. In 1982 Mark met fellow British astrophysicist Diana Worrall, who was then working in California.

Following his time at Berkeley, Mark returned to MRAO, Cambridge as a Research Fellow at Gonville and Caius College. During this time, he established himself as a world leader in studies of the Cosmic Microwave Background (CMB). In 1983/4 he observed (with Gull and Hardebeck) for ~230 hours with a new receiver on the 40m telescope at the Owens Valley Radio Observatory in California, where Diana would sometimes join him. The resulting paper, led by Mark, reported the first solid detections of the Sunyaev-Zel'dovich (SZ) Effect: the lower temperature observed when CMB photons pass through foreground clusters of galaxies on route to Earth¹. His application of the SZ-effect to determine the scale of the Universe subsequently founded a new and still thriving field of study. Mark's most highly cited paper is a beautifully written and comprehensive 137 page review of the SZ effect².

Mark and Diana married in 1984. Mark once again crossed the Atlantic, this time to join the Department of Astronomy at Harvard University, Cambridge MA, where he also held an Alfred P. Sloan Foundation Research Fellow. Diana took up a position in the High Energy Astrophysics Division, Smithsonian Astrophysical Observatory (SAO, also located in Cambridge), to work at the AXAF Science Center (ASC, renamed the Chandra X-ray Center, CXC following launch in 1999). Mark moved from Harvard to SAO in 1992 to work at the ASC. During his time at SAO, Mark recognized an opportunity to calibrate the Chandra Aspect Camera on a substantially finer grid than specified in the requirements. His excellent and detailed work substantially reduced the uncertainty in positioning individual X-rays on the sky, leading to sub-arcsecond source locations and more detailed structure measurements for extended sources. During this time in "the other" Cambridge, Mark continued to publish widely, often in collaboration with Diana, on radio, optical and X-ray observations of galaxies and clusters, and on the physics of radio jets. He also added theoretical work, and papers on gravitational lensing.

Mark and Diana returned to the UK in 1995, where Mark was appointed the inaugural William P. Coldrick Professor of Cosmology and Astrophysics at the University of Bristol. He also became head of the Astrophysics Group in the Department of Physics. Combining his research with undergraduate teaching, post-graduate supervision and the administration of the group, particularly the acquisition of research grants, he oversaw the significant increase in the size and prestige of Bristol Astrophysics. He was also joint head of the Physics Department for a time. Beyond his excellence in research, Mark was a gifted teacher, with the deep understanding and skill to explain complex topics clearly. He inspired his students with his infectious enthusiasm for his subject, treating them as equals, challenging them, and sharing both ideas and mistakes. He also readily shared his passion for astrophysics with the wider community, giving talks to astronomical societies and participating in outreach events of all kinds. Mark was also a very impressive, light-touch leader. He ran the Astrophysics Group with the minimum of formal meetings, while ensuring that his colleagues were fully consulted, informed, and involved when necessary. He was assiduous at shielding the team from a great deal of the administration, allowing them to concentrate on teaching and research. His unwavering enthusiasm for the science made him an inspirational colleague and leader.

Mark was highly respected across the Astrophysics community. He was known and respected as a well-informed, fair and professional reviewer, and was greatly in demand to serve on national and international research committees and boards. He served on multiple committees at any given time, often as chair. He was extremely diligent, bringing his wide experience and deep, broad knowledge of the field to all his community service roles, and was respected by all those involved. He also forged connections with scientists and students abroad, including Sweden, South Africa, Korea and beyond, often supervising students at Bristol, either remotely or in person. He and Diana regularly spent part of their summers back in the USA at SAO, working with multiple colleagues in the High Energy Astrophysics and Radio Astronomy divisions.

Mark loved Astrophysics! His many pivotal contributions encompass broad and exciting topics including extreme conditions around black holes and the energetic jets they power, the hot gas that permeates rich clusters of galaxies, gravitational lensing, and the role of magnetic fields. He brought his energy, enthusiasm, deep knowledge, and clarity of thought to all aspects of his work. His knowledge of radio galaxies was encyclopaedic, with an ability to immediately provide key parameters and properties of any radio source mentioned in passing. He was a gifted mathematician and teacher, in particular lecturing the challenging General Relativity course which was popular with students. Mark was an inspiring and exacting collaborator whose input was always insightful and constructive, and a dedicated, and supportive supervisor and mentor of students at all levels. He loved interacting with colleagues and was always generous with his time and ideas. His enthusiasm was infectious. The significance of his contributions to our field is huge, both scientifically and socially.

Beyond science, Mark had an unquenchable thirst for knowledge and experiences. He travelled the world, taught himself the language of many of the countries he visited, including China, and Sweden, where he held an affiliated professorship from 2019. He was extremely well-read. When time was available, he enjoyed playing golf, demonstrating significant talent with an impressive handicap. He and Diana were bellringers who rang regularly for their local church in Long Ashton.

Mark was an internationally renowned leader in his multiple fields. He has published 275 (to date) refereed papers with 14,000 citations, covering a broad range of topics and with many different collaborators. His paper count continues to increase as his many collaborators acknowledge his contributions to their current work. He was hugely valued by his colleagues and students in the School of Physics in Bristol, who feel his loss acutely. His SAO collaborators miss his annual visits and reliable injection of energy and ideas. Beyond research, his broad contributions and dedication to advancing UK science generally have been invaluable.

Mark Birkinshaw died of cancer in hospital on 23rd July 2023 after a brief illness. His wife and long-time collaborator, Emeritus Professor Diana Worrall, was at his side. Mark's early and unexpected death represents a huge loss to the University of Bristol, to the UK, and to the worldwide astrophysics and science community.

References:

• 1: Birkinshaw, M., Gull, S.F., & Hardebeck, H, 1984, Nature, 309, 34B
• 2: Birkinshaw, M., 1999, Physical Review, 310, 97