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3. KAHN & WOLTJER (1959)

The Kahn & Woltjer (1959) paper is about the motion of M31 relative to the Milky Way. This is another important paper that did not have the impact it deserved. M31 is about 750 kpc from the Milky Way and approaches the Milky Way at 118 km s−1. Kahn & Woltjer noted that the two galaxies are now approaching each other. They assumed that (i) the two galaxies were formed close together, (ii) that their combined mass was sufficient to make them a bound unit, and (iii) that they have performed the larger part of at least one orbit with a period of no more than 15 Gyr. They adopted a radial orbit (which turned out to be very close to the truth) and simple Keplerian dynamics to show that the mass of the (M31–Milky Way) system is about 20 times larger than the likely masses of the stars of the two galaxies. Their distance scale and stellar masses were roughly right. Their estimate of the total mass was > 2 × 1012 M (present estimates are about 3 × 1012 M: van der Marel et al. 2012).

Kahn & Woltjer excluded the possibility of intergalactic stars and argued that the extra required matter was in the form of intergalactic ionized gas. Their paper went on to explore the distorting effects of this gas on the Galactic disk. Maybe that was a distraction. I remain surprised that this paper had so little impact. The argument is simple and correct and has survived to the present. Kahn and Woltjer were both very respectable and well-regarded researchers. Their work should have started the dark matter revival in 1959.

My guess is that the situation for the Kahn & Woltjer paper was much as for Zwicky's much earlier paper. There was simply no theoretical framework within which to interpret the observation of such a large total mass for the (M31–Milky Way) system. Their argument is direct and compelling but made almost no impact at the time. Some weak contrary evidence (e.g. Godfredsen 1961) based on virial theorem arguments, that concluded that extra mass was not needed to bind the Local Group, may have provided a welcome escape. I will speculate later that Slipher's work suffered a somewhat parallel situation. His discoveries did not have the impact they deserved in settling the controversy about the nature of the nebulae because of work by van Maanen which convinced much of the community at the time but turned out to be incorrect.

When the dark matter revival came in the 1970s, it was based observationally on the 21-cm rotation curves of spiral galaxies, which showed that they have massive and extended dark halos. This argument was not much more direct than the Kahn & Woltjer argument but it made an impact, even though the evidence for dark matter from rotation curves was not really secure until about 1978 when the high quality 21-cm radio synthesis data became available. The difference was that, by the early 1970s, there was a theoretical framework on the need for dark halos to stabilize disks against bar formation (e.g. Ostriker & Peebles 1973). This framework helped to sustain the quest for dark matter in galaxies through the period of controversy in the 1970s. People appear to be ready to believe observations that fit into some theoretical framework, even if the observations have a sounder basis than the theory. It allows them to come to grips with startling observations, as Eddington pointed out. In this case, the sustaining theory turned out to be only partly relevant. Most disks are now known to have bars, so there is no need to invoke a dark halo to stabilize them. In fact, dark halos are now known to be needed to promote the growth of bars via angular momentum transport.

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