Arthur Michael Wolfe, professor emeritus of physics at the University of California, San Diego (UCSD), died of cancer on 17 February 2014 in La Jolla. Art had a vibrant career that spanned nearly five decades; he conducted research in theoretical cosmology, experimental physics, and observational astrophysics. He had a terrific passion for physics that he shared with researchers throughout the community and that inspired scientists young and old.
Art was born on 29 April 1939 in Brooklyn, New York, and spent his formative years in Manhattan. His family then moved to Queens, where he graduated from Forest Hills High School. After completing his BS in physics at Queens College in 1961, he pursued graduate studies at Stevens Institute of Technology in Hoboken, New Jersey. In 1963 he followed his mentor, Rainer Sachs, to the University of Texas at Austin, where he obtained his PhD in physics in 1967.
In his thesis, Art calculated the fluctuations in the cosmic microwave background (CMB) imposed by the gravitational potential of density fluctuations within the surface of last scattering. Now termed the Sachs–Wolfe effect, the idea linked anisotropies in the observable CMB to the underlying distribution of matter. It underpins nearly all calculations for the CMB power spectrum in our cosmological paradigm. The idea was later extended to describe fluctuations in the CMB generated by the integrated mass foreground to the last scattering surface, the integrated Sachs–Wolfe effect.
Between 1967 and 1972, Art held postdoctoral fellowships at UCSD and Cambridge University in the UK. During that time, he made the remarkable transformation from general relativist to radio astronomer. He investigated 21-cm spectroscopy of gas in the early universe, revealed in absorption spectra in the direction of distant, radio-loud quasars, which required him to oversee the design and fabrication of custom, low-noise amplifiers. One application of those data was to test for temporal changes in the fine-structure constant and other fundamental constants over the roughly 10-Gyr time interval between today and the distant gas.
But Art’s primary focus, inspired by analogous observations of present-day galaxies, was to study gas in galaxies of the early universe. His efforts with 21-cm absorption met with modest success, but they were expensive, were limited to the small sample of known radio-loud quasars, and frequently resulted in nondetections. Stymied but undeterred, Art attacked the problem anew by searching for the hydrogen Lyman-α transition. At the surface densities characteristic of galaxies, the neutral hydrogen (HI) Ly-α line exhibits quantum mechanical damping that gives rise to broad absorption wings that are more easily detected. And for gas in the distant past, cosmological expansion shifts the UV Ly-α line into optical passbands, which one can observe using large, ground-based telescopes. Such absorption lines are now termed damped Ly-α systems, or DLAs.
As a professor of physics at the University of Pittsburgh—he was hired in 1973—he reinvented himself yet again, this time as an optical spectroscopist. He launched the field of DLA research in 1986 with a landmark paper that analyzed data collected at the Lick Observatory of the University of California. Art and his coauthors established entirely new lines of research in cosmology and galaxy formation. Their work provided the first cosmological census of hydrogen atoms in the distant universe. In addition, the spectra revealed the distribution of HI surface densities in young galaxies and tagged regions of the universe where one could uniquely probe the progenitors of galaxies like our Milky Way.
Art recognized the far-reaching aspects of those observations and developed the framework for analysis. Furthermore, he expanded the observational program to include new quasars at high redshift and complementary surveys of the modern universe with space-borne UV spectrometers. By 1995 Art provided a cosmological census of HI gas across 10 Gyr of cosmic time and demonstrated that the baryonic budget was nearly sufficient to fuel the formation of all the known stars in the universe.
In 1989 Art accepted a professorship in physics at UCSD, a few years before the commissioning of the first 10-m Keck telescope. Taking advantage of the telescope’s unparalleled aperture and high-resolution echelle spectrometer, Art led a research program to determine the physical characteristics of the gas within the DLA protogalaxies. Those efforts yielded novel and detailed results on the nature of the gas dynamics, the enrichment in heavy elements, nucleosynthesis within those young systems, and the formation and composition of dust.
Art inspired the broader community to join the pursuit, and DLA programs were approved at every major astronomical observatory. His work also motivated theoretical investigations—both analytic and numerical—to develop models for the origin and nature of the DLA galaxies. Interest was sparked, in part, by the challenge to reproduce the galaxies’ properties within the framework of hierarchical cosmology. Indeed, the DLA observations offered the first precise constraints on the physical characteristics of young galaxies, and even today they tightly constrain models of galaxy formation.
As a scientific research practitioner, Art was widely recognized for his deep and intuitive understanding of physics and his passion for discussing it with collaborators, colleagues, and competitors alike. As a professor, Art imparted his breadth of knowledge to students for more than 39 years. He was a humble and dedicated mentor to faculty members, postdoctoral fellows, and graduate students (his own and others); with them, he spent countless hours dissecting the underlying physics of many research problems. He was director of the Center for Astrophysics and Space Sciences at UCSD for 10 years and guided its scientific vision. On a personal level, Art was a caring and loving husband and a doting father. His death is a great loss to us all.
