Dr. Vera Rubin, a pioneering American astronomer, died on December 25, 2016, at the age of 88. Rubin’s life in astronomy bridged three crucial transitions: the discovery of dark matter, the replacement of photographic plates by more sensitive electronic detectors, and the entrance of significant numbers of female astronomers into the profession. Rubin played a crucial role in advancing all three, but let's look at her dark matter investigations in both gas cloud and star rotation around the central galactic cores of an increasing number of galaxies.
Rubin’s most important scientific contribution was establishing that the orbiting speeds of gas clouds in the outer rims of the galaxies she examined remain constant (i.e., “flat”) to distances well beyond the visible starlight, rather than declining as in the outer parts of our Solar System. High orbital speeds in the outer parts of galaxies imply the existence of extra matter at large radial distances to insure these velocities.
As a result of Dr. Rubin’s work and later studies, we now know that galaxies are surrounded by enormous invisible halos of matter containing 5/6 of their mass which extend ten times farther out than the visible regions. Numerous arguments and thought experiments show that this so-called “dark matter” must be totally different from the ordinary, “baryonic”, matter of the periodic table. Although its nature is still unknown, it is being pursued in numerous experiments in particle accelerators and particle detectors around the world. The eventual realization that baryonic matter is only a partial component of the Universe, following the acceptance of numerous papers by Dr. Rubin and her collaborator, Kent Ford, showed that our understanding of the cosmos was shockingly incomplete and was one of the milestones that ushered in modern cosmology.
Dark matter had a somewhat checkered history before Rubin’s first paper on the subject was published in 1978 (Rubin, Ford, and Thonnard, Astrophysical Journal Letters, 225, 107, 1978). Astronomer Fritz Zwicky opened the subject in 1933 with the claim that galactic clusters would fly apart if extra matter were not present to provide more gravitational pull. A sprinkling of papers followed over the next three decades, culminating in the Santa Barbara Conference on “missing mass” in 1964, but the available data, mostly still confined to clusters and binary galaxies, were hard to analyze. The subject advanced in the early 1970’s with the early radio studies of the 21-cm line of neutral hydrogen to measure rotation speeds in the disks of gas in the outskirts of nearby galaxies. The disks in circular rotation were much simpler to analyze, and these early data hinted at the rotation curve discrepancy, but the number of sampled galaxies was small. A leader in these early radio papers was Morton Roberts at the National Radio Astronomy Observatory, who actively stimulated Rubin’s interest in the subject. The PhD thesis of Albert Bosma, which appeared in 1978 just before Rubin’s first paper, extended radio data to 24 galaxies using the Westerbork interferometer, in the Netherlands, and again saw flat outer rotation curves.
Subsequently, Babcock's optical rotation curve, and that of Rubin and Ford (1970), was extended to even larger radii by Roberts and Whitehurst (1975) using 21 cm line observations that reached a radial distance of ~30 kilo parsecs. These observations clearly showed that the rotation curve of the Andromeda Galaxy, or M31, did not exhibit a Keplerian drop‐off in velocity. In fact, its rotational velocity remained constant over radial distances of 16–30 kpc. These observations indicated that the mass in the outer regions of the Andromeda galaxy increased with the distance from the galactic center, even though the stellar optical luminosity of M31 did not.
Amidst this growing body of data indicating dark matter, Rubin’s work was particularly influential because of three factors. First was the clarity and directness of the papers, including beautiful illustrations of the raw spectra that she was measuring—the flatness of the rotation curves could not be denied. Second was the fact that Rubin and her colleagues followed up with several more papers over the next few years, each one enlarging the sample size and demonstrating the seeming ubiquity of flat curves of rotations. Third were Rubin’s presentations at numerous astronomical conferences, which, like her published papers, were clear, direct, pared down to essentials, and ultimately compelling, driving her dark matter thesis home.
Vera Rubin truly lit the way in dark matter discovery and she began her work with our galactic neighbor, M-31, Andromeda, that massive and beautiful star rich cousin.