4.1. Astronomers' applause
Thus the credit goes to Lin who not only developed the theory
of spiral waves in much more detail, but also presented it in
a relatively simple form that made it acceptable to the rest of
the astronomical world. The response of the work of Lin and
his associates has been an ever-growing wave research in this
area, that has produced many important new results.
Contopoulos 1970a, p.303
The August 1969 Basel IAU Symposium "The Spiral Structure of Our Galaxy" was a significant event in the astronomical life, "the first international gathering ever of optical astronomers, of experts in galactic dynamics, and of the world's greatest radio astrologers". 59 Bok, Contopoulos, Kerr and Lin were the mainstay of its organization presided over by Woltjer who deserved "great credit for planning the symposium to reflect the current status of our knowledge in this field, and for the selection of speakers" (Lin 1971, p.35).
Opening the meeting, Oort conveyed his pleasure that Lindblad's spiral-wave ideas had in recent years been "further worked out by Lin, Shu and Yuan, who showed among other things how such a density wave causing a spiral pattern could be sustained by its own spiral gravitational field superposed on the general axisymmetrical field of the galaxy" (Oort 1970, p.1). This gracious view the speaker supplemented with a prudent, if not veiledly critical, comment.
"The theory explains the maintenance but not the origin of spiral structure. I do not think this is an important shortcoming, for it is easy to conceive of processes which would start a spiral structure. [...] A more serious problem seems that of the long-term permanence of the spiral waves. Can they continue to run round during 50 revolutions without fatal damage to their regularity? Looking at the irregularities in the actual spiral galaxies one wonders whether the present spirals could continue to exist for such a large number of revolutions. [...]
Dr. Lin has sometime quoted me as having stated [...] that in so many cases spiral arms can be followed more or less continuously through the entire galaxy. I do not want to withdraw this statement, but I must point out that it should be supplemented by two essential additions. First, that in about half of the spirals the structure is either unclear, or there are more than two arms. Second, that even in the half that can be classed among the two-armed spirals there are invariably important additional features between the two principal arms, while the latter have often a number of secondary branches coming off their outer rims." (Oort 1970, p.2)
On its empirical side, the meeting revealed strong excitement and desire of astronomers about establishing the Galaxy's spiral structure, at least in general. Their demonstrations were a mixed collection, however. Even the cutting-edge radio data instilled a scanty unanimity at best. Kerr (1970) inferred the Perseus, Sagittarius, Norma-Scutum and Cygnus-Carina Arms as spiral fundamentals, all of pitch angles i = 5° - 7° (the latter having our Sun at its inside, and the Orion Spur emanating from it), but Weaver (1970) agreed on only the first two of them, and then with i = 12° - 14°. Was it to be wondered at the scatter of opinions of `ordinary' optical reporters? Metzger (1970) found no definite spiral pattern at all upon the distribution of HII regions. Courtes et al (1970) re-interpreted data on radial velocities for about 6000 HII regions and concluded an i 20° four-armed spiral. Pavlovskaya and Sharov (1970) gathered a 14-armed (!) spiral from their studies of surface brightness distribution in the Milky Way plane. Vorontsov-Velyaminov (1970, p.17) reminded that the largely discussed tightly wrapped two-armed spiral proposal for our difficult Galaxy called for quite a number of full turns inconsonant to the views of other galaxies, and he advised "not to be in a haste to construct a model of our Galaxy, but to search the real patterns without bias". Vaucouleurs (1970) interpreted the remarkable `3-kpc arm' as a bar particularly oriented to the line of sight, for which he adduced even more bar-favoring `statistical' arguments. Kerr (1970), to close this chain, supported an oval distortion of the galactic plane as a plausible cause for marked asymmetry of the observed rotation curve in the North and South quadrants. At the same time, he expressed general concern over a large uncertainty in the determining of galactic distances, which undermined the cogency of any `large-scale' statements.
In this climate of empirical scatter and vagueness, Lin stated his "bird's eye view of theoretical developments", as enlightened by his focal-point QSSS hypothesis (Lin 1970). That view captured: "ten general observational features which one must consider in dealing with spiral features in galaxies" (p.377); "deep implications on the physical processes in the interstellar medium, and in particular on the formation of new stars" (p.379); "a better opportunity for the understanding of the physical processes, including such microscopic behavior as the formation of molecules and dust grains" (p.389); "a deep mystery of the 3-kpc arm" that on fact might be "a part of a reflected leading wave, of an evanescent type" (p.383); the necessity of recognition that the agreement with observations "should not be perfect, since the galactic disk is perhaps not perfectly circular and the actual structure may not be a pure mode in the theory" (p.381); "the success of the theory" as being expected "to embolden us to apply the theory to external galaxies" (p.379). But Lin's special emphasis was there and on "one important theme to be kept in mind" - coexistence.
"The complicated spiral structure of the galaxies indicates the coexistence of material arms and density waves, - and indeed of the possible coexistence of several wave patterns. When conflicting results appear to be suggested by observations, the truth might indeed lie in the coexistence of several patterns. Before taking this `easy way out', one should of course try to examine each interpretation of the observational data as critically as possible.
There is also coexistence in the problem of origin of spiral structure. From our experience with plasma physics, we learned that there are many types of instabilities. Since a stellar system is basically a plasmoidal system, various types of instability can also occur in the problem of the galactic disk." (Lin 1970, p.379)
Under the auspices of the coexistence theme and in grudging admiration for Toomre's group-velocity work (as yet unpublished 60) that "brings the problem even into sharper focus" (Lin 1970, p.383), Lin let the sheared waves coexist in his grand-design view in order to provide it with one of several possibilities of an instability mechanism. He presumed that such waves naturally occur on the outskirts of a galaxy disk where stars are sparse and the well-cooled gas is dominant; that this calls up "the Jeans instability of the galactic disk" and thus a random occurence of local condensations developing in the GLB fashion into trailing spiral-shaped segments; and that each such `material arm' produces its own effect, now in the JT manner, and "eventually becomes a roughly self-sustained entity, somewhat like the self-sustained density waves (Lin & Shu 1964, 1966; Lin 1966a) with inherent frequency = 0 (corotating waves)" (Lin 1970, p.384). But in fact such `entities', established in local frames though, were on an intermediate scale at the least, and far enough from their producing source they could indeed appear "somewhat like the self-sustained density waves", but with non-zero frequency . Thus it was not entirely unreasonable to suspect that a really massive outside perturber might be capable of bringing to life some grand galactic spiral. 61 Lin, however, was "inclined to discount the roles of the satellite galaxies in creating spiral patterns". Believing instead that "indeed, the corotation of wave pattern and material objects in the outer parts of external galaxies ha[d] been confirmed for M33, M51 and M81 by Shu and his associates (Shu et al 1971)", he favored a picture where M51-type spiral patterns well originated in remotely corotating local `self-sustained entities' and one of the arms "would join naturally to the intergalactic bridge" (Lin 1971, p.36-37).
"Owing to resonance, the two-armed structure will prevail as the disturbances propagate inwards as a group of waves, which extracts energy from the basic rotation of the galaxy. [...] The reflection of the waves from the central region then stabilizes the wave pattern into a quasi-stationary form by transmitting the signal, via long-range forces, back to the outer regions where the waves originated. Thus, there is necessarily the coexistence of a very loose spiral structure and a tight spiral structure. Population I objects stand out sharply in the tight pattern while stars with large dispersive motion would primarily participate in the very loose pattern." (Lin 1970, p.383) 62
Lin's views of spiral structure, generously illustrated at the Basel symposium in the coordinated presentations of his associates (Roberts 1970; Shu 1970a; Yuan 1970), evoked in quite a few of the astronomers a sort of delight imparted so eloquently by Bok in his `Summary and Outlook'. 63
"Until half of a decade ago, most of us in this field were of the opinion that the magnetic fields near the galactic plane [...] would probably have proved sufficiently strong to hold the spiral arms together as magnetic tubes. [...] Theory took a new turn about five years ago, when Lin and Shu entered the field with the density wave theory. [...] The magnificent work of the MIT group loosely headed by C.C. Lin has made the pendulum of interpretation swing toward Bertil Lindblad's gravitational approach, and this is wonderful indeed. [... It] is now in full bloom, but we must not fool ourselves and think that all is done except the mopping up. [...] There is controversy aplenty even within the MIT-Harvard family and this is all to the good.
We are fortunate indeed that the theorists attended our Symposium in force. [...] The observational astronomer is especially pleased to learn about the interest our theoretical colleagues are showing in observations, and it is a source of regret to the observers, optical and radio alike, that we cannot agree as yet on the full outlines of spiral structure for our Galaxy. Give us a few more years, and we shall be able to tell you all right!" (Bok 1970, pp.457-462) 64
59 An extract from Baseler Nachrichten quoted in the Symposium proceedings (Bok 1970).
During the IAU General Assembly in Prague, 1967, various theoretical and observational papers were presented at a special meeting of Commission 33 on Spiral Structure, most notably including "The density wave theory of galactic spirals" by Lin, "Magnetic approaches to spiral structure" by Pikelner and "Self-gravitating spiral models of the galaxy" by Fujimoto. The participants' interest was obvious, and Contopoulos proposed a special thematic symposium for 1969. It was agreed to hold it "in Basel, a center of galactic research in the center of Europe" (Becker & Contopoulos 1970, p.vii). Back.
60 Contopoulos also mentioned it in Basel, reviewing theoretical spiral developments (Contopoulos 1970a). Back.
61 Commenting on the fact that "the M51 type spirals in Vorontsov-Velyaminov's catalogue all have the companion galaxy or galaxies on the arm", Lynden-Bell well admitted that there "the distortion gravity field of the companion is very important and something on the lines of [Toomre's] work with Julian ought to apply." (Lynden-Bell 1965b) Back.
62 In one year or so Lin's enthusiasm for this scenario will be tempered. He will concentrate on mechanisms of spiral persistence, and direct his associates' efforts to exploring the feedback cycles. Remote corotation will be as important there as before, but now without reference to the GLB and JT ideas. The main point will be the WKBJ-wave excursions to and from the center, and the associated role of a central bar. Back.
63 Very instructive is the view of the contemporary spiral progress by Goldreich who after leaving the subject by mid-1960s "remained an interested spectator to the battles between Alar Toomre and C.C. Lin". "Although I generally favored the arguments of the former - he recalls - the latter's campaign was more successful." (Goldreich) Back.
64 Possibly, such a generous support of Lin's initiative by several leading astronomers of the day partly reflected their desire to see in him a direct follower of Lindblad, their previous indifference to whose efforts might have evoked in them feelings of regret and some guilt. "I do not believe it - Contopoulos comments on this guess. - In particular Bok wanted a simple theory to explain star formation and migration. I remember that when I presented the work of Fujimoto in Prague (1967) and wrote down only two formulae he told me: "Very good George, but too mathematical". A few years later, Bok expressed his disappointment to me, because the density wave theory had become rather complicated. I do not think that Bok appreciated the more formal work of Lindblad." (Contopoulos) Back.