In Memoriam: Bill Mathews (1937–2021)

To: UC Santa Cruz Community

From: Enrico Ramirez Ruiz, Department of Astronomy & Astrophysics

Bill Mathews

William George “Bill” Mathews—astrophysicist, musician, and idea foundry—died on September 24 at the age of 84, but his sparkle lives on.

When Bill passed away, the Department of Astronomy and Astrophysics at UC Santa Cruz lost one of its most versatile and astute astrophysicists. With deep scientific curiosity and an endless enthusiasm for discovery, he vigorously continued his research until his final days. His contributions have had an everlasting impact in our field. Always magnanimous with his time, Bill was an inspirational colleague and teacher who shaped the careers of all of us fortunate enough to reside within his sphere of influence. His passing is deeply felt.

Throughout his career, Bill showed an exceptional ability to identify the roads less traveled. I greatly admired his style of trailblazing reasoning coupled with kindness and endless humility. Bill simply saw, and then brilliantly pursued, promising pathways that others did not. And in many cases others could not even pursue them, as Bill combined his novel ideas with technical mastery in the implementation.

Bill is most well known for his work on understanding the role, state, and history of gas in the universe. Shortly after its birth, the universe was loaded with gas, mainly hydrogen. In due course, far and wide, gravity drew the gas into clouds which transformed them into galaxies whereupon stars lit. Stars glow by nuclear burning of the gas, and those that end their lives in bright explosions help blow the gas back out of galaxies. As such, galaxies are embedded in a tub of gas, the medium from which they were born and which ultimately powers them. They breathe gas in and out, and their stars glow until their gas is gone.

This was one of Bill’s groundbreaking theories, and his classic paper set the course for a lifetime of research. The challenge with verifying his theory was that fifty years ago instruments were unable to detect signs of gas in galaxies, let alone map its comings and goings. With more sensitive instruments and resolute surveys, we now know much more. Persuasive evidence suggests that, as envisioned by Bill, galaxies subsist by recycling gas into and out of stars. And we also have precursory proof for how galaxies might run out of gas, stop forming stars, and die.

Most of what we astronomers understand about the universe originates from what we are able to observe. So our ideas have been biased toward galaxies and stars, which are luminous. But most of the ordinary matter in the universe is in the form of gas, which is faint and difficult to see. Gas, termed the intergalactic medium, floods the space between galaxies, while the gas of the circumgalactic medium resides closer in, encircling galaxies. The gas in both places regulates the birth, life, and death of the galaxies, and holds a detailed narrative of the universe. Only lately have astronomers been able to detect it. Bill always had the remarkable ability to be way ahead of the game, forecasting many key astronomical discoveries.

Observers commonly detect outflows of gas, heavy in metals, flowing in wide swaths out of galaxies. This is because gas in the universe is not unadulterated hydrogen but is spiced sparingly with heavier elements, manufactured primarily when stars explode at the end of their lives. While there is still significant debate about what could be driving outflows in galaxies, the two most widely discussed theories, supernova explosions or the fast streams of gas spurted from the central supermassive black hole, were both scouted by Bill, and ample evidence now endures for both scenarios.

Most of the space between galaxies in clusters is also filled with gas, yet this gas is incredibly hot. This shocked gas is heated to about ten million kelvin and cannot cool efficiently because it is too tenuous and, as such, is predicted to endure, rather than condense into stars and galaxies. Yet, as Bill never tired of pointing out, one of the enduring mysteries about galaxy clusters is that the hot intracluster gas should cool faster than is observed. “These inadequacies suggest that a new physical mechanism, in addition to the basic jet hydrodynamics, should play a critical role,” he wrote (with Fulai Guo) in an article in 2012. Bill suggested that hot gas also might be turbulent, maybe driven by mechanical gas outflows expelled from the supermassive black holes at the centers of the clusters' galaxies, possibly inflating bubbles of high-speed charged particles that shake and warm the intracluster gas. Bill’s seminal work on the subject, though not definitive, implies that no mysterious processes needed to be invoked to explain the persistent hot gas.

Gas enriched with metals is ubiquitous and can be found everywhere in the Universe, both in between galaxies and within them, ripe to be recycled into new stars. Freshly synthesized stars amalgamate from gas loaded with heavy metals together with dust. Surrounding stars, protoplanetary disks are formed, which now and then consolidate into planets, like the one we inhabit. The all-pervading dust, a subject at the center of Bill’s pioneering interests, not only renders life more enjoyable by giving us sunset tints but appears to be wholly indispensable for our existence. The overwhelming relevance of the little stuff in our world has been brought home to us through Bill's extraordinary investigations into the widespread and far-reaching beneficial influences of gas, metals, and dust in the universe.

Bill's passion for life and for science, his fervor, his honesty, his brilliance, his dedication, his relentless resolution, his high expectations for the pursuit of knowledge, his love of family and community, and his wonderful attentiveness for fun, were the trademarks of his life. He was an inspiration, and he became a mentor and a friend. I am eternally grateful that my path crossed his.

Gifts in Bill’s memory can be made to the Bill Mathews Fund for Excellence in Astronomy and Astrophysics, which supports graduate students at UC Santa Cruz.

An in memoriam from the San Francisco Early Music Society and an obituary in the Santa Cruz Sentinel provide additional details about his life.