1.2. Gas and dust

The difficulty of cosmogonical theories lies in the
interconnection of the facts.
Weizsacker 1951, p.165

Where a few years ago we seemed to be up against a blank wall
of discouragement, we are now in an era of rapidly developing
research.
Bok & Bok 1957, p.244

Stellar dynamics of the 1940s - early 1950s was essentially the theory of a stationary galaxy arranged by the regular forces (see Ogorodnikov 1958) and the theory of quasi-stationary systems open to slow relaxation processes (Ambartsumian 1938; Chandrasekhar 1942, 1943). Together, they provided a basis serving well for getting certain practical dividends but still of little use for conceiving the underlying dynamical problems.

"While these methods have contributed substantially toward the clarification of the peculiarly characteristic aspects of stellar dynamics, an impartial survey of the ground already traversed suggests that we are perhaps still very far from having constructed an adequate theoretical framework in which the physical problems can be discussed satisfactorily. In any case we can expect that the near future will see the initiation of further methods of attack on the problems of stellar dynamics" (Chandrasekhar 1942, p. vii-viii). ¹⁶

The envisaged future did not happen to lie as immediately near, however. The theoretical thought kept on whirling around the idea of galaxies evolutionarily tracking over the Hubble diagram, one way or the other, and that opened in quite a few attempts at a synthesis of the available strict knowledge about gravitating figures in a softer (then bulkier) spirit of cosmogonical inclusion. ¹⁷ Accordingly, non-stationary - dynamical - problems of deformation of the systems and of density disturbances in them seemed difficult and therefore premature, while stationary problems were held as "natural and necessary" at that preliminary point, for "it is hard to imagine that at all stages the evolution of stellar systems has the violently catastrophic character" (Ogorodnikov 1958, p.13). ¹⁸ In this illumination, Lindblad's theory of unstable bar-modes was typically deemed extravagant and unacceptable (Lebedinski 1954, p. 31).

"Such theories cannot yet help the progress of cosmogony, since uncertainty in them still prevails validity" (Schatzman 1954, p.279).

The delicacy of this sort of expert judgment - let alone its other virtues - reflected clearly that it was the issue of gas and dust that became a common focus of galaxy astronomy despite its stellar past. ¹⁹ By the 1950s, Baade discovered in M31 many hundreds of emission nebulosities (HII regions), having concluded that "they are strung out like pearls along the arms" (Baade 1963, p.63). Gas and dust, he stated, are also distributed in this galaxy highly unevenly, grouping in its spiral arms. ²⁰ Besides, no one already doubted the youth of high-luminosity stars since they were ascertained to still form in abundance, e.g. in the Orion nebula. The sheer weight of these individually weak facts convinced many workers that

"the primary phenomenon in the spiral structure is the dust and gas, and that we could forget about the vain attempts at explaining spiral structure by particle dynamics. It must be understood in terms of gas dynamics and magnetic fields" (Baade 1963, p.67). ²¹

The lion's share of these discoveries was made possible due to the 200-inch Palomar reflector put into operation in 1949, although from 1951 onwards the interstellar gas was unprecedentedly attacked also by the 21-cm-line methods. Dutch radio astronomers presented "one of the truly historic diagrams of Milky Way research" (Bok & Bok 1957, p.244) - a detailed map of atomic hydrogen distribution (Hulst et al 1954). ²² It displayed extended fragments of tightly-wrapped spiral arms which in the solar vicinity matched `local arms' in Sagittarius, Orion and Perseus. ²³ Gas kinematics routinely analyzed, a synthesized rotation curve of the Galaxy was pictured (Kwee et al 1954), and the "primary task for the next few years" was claimed to get improved radio equipment "capable of tracing with precision the spiral structure of our Galaxy".

"While there is always room for theorizing, the emphasis must first of all be on careful observation and unbiased analysis of observations" (Bok & Bok 1957, p.248).

The new empirical facts - the tightly wrapped, nearly ring-like arms of the Milky-Way spiral, the concentration in them of Population I objects, the general shearing character of rotation - were a surprise to Lindblad. He could not neglect them. But they demanded another, more fitting dynamical theory, and Lindblad put aside (but did not deny ²⁴) his business with unstable circular orbits and wave bar-modes. This step was largely favored by first numerical experiments in galaxy dynamics performed in 1955-60 by his son P.O. Lindblad with the big electronic computing machine installed in Stockholm (Lindblad & Lindblad 1958; P.O. Lindblad 1962). Those experiments showed the trailing - not the leading - spiral arms, the ones supported by fresh data on both the form of the Milky-Way spiral and the space orientation of many galaxies (de Vaucouleurs 1958), and, after all, the ones put into orbit way back by Hubble (1943) in the framework of his working hypothesis that galactic spirals always trail. ²⁵

¹⁶ "I remember very vividly the atmosphere in the 50's in stellar dynamics. On the one hand, we had the most general solutions of Liouville's equation by Chandrasekhar. But it was realized that the self-consistent problem required also the solution of Poisson's equation, which was very difficult in general. Thus people were discouraged." (Contopoulos) Back.

¹⁷ See, e.g., the "Critical review of cosmogonical theories prevailing in West Europe and America" by Schatzman (1954). It would be some fuller with an addendum on a theory developed in 1955-56, now in the Soviet Union, by Ogorodnikov. Finding that the works by Lindblad and Chandrasekhar on collisionless dynamics "really bar the way to studying the laws of evolution of stellar systems", he suggested a "more promising" - "synthetic" - hydrodynamical method with elements of statistical mechanics (Ogorodnikov 1958, p.20, 22), and with this he proved theorems on uniform rotation and nearly constant density for "dynamically determinable" systems, at their "most probable phase distribution". This enabled Ogorodnikov to start his supposed evolutionary sequence with the `needle-shaped' galaxies, or strongly elongated ellipsoids in rotation about their shortest axis. Such needles are secularly unstable, above all at their long-axis extremities from where "the stars are detached in two winding arms" giving the picture of a typical barred spiral galaxy. Material released during this gradual bar destruction feeds a spherical halo, while inside the bar a violent process of low-velocity-dispersion star formation starts, and these emerging Population I stars uniformly fill the new equilibrium figure - a thin disk-like Maclaurin spheroid. The remaining diffuse material of the bar (needle) winds up and, being still `frozen' in the disk, forms spiral arms. Due to irregular forces, Population I and II stars get mixed, because of which the spiral galaxy cannot be in equilibrium: its disk dies out through dissipation, and a nuclear remainder drives up an eventual elliptical galaxy (Ogorodnikov 1958, p.29).

As well illustrative appears Weizsacker's theory of galaxies and stars built on a concept of supersonic turbulent motion in the original gaseous mass, the one picturing a general "evolutionary trend as far as it does not depend on the special conditions by which galaxies, intragalactic clouds, stars, planets, etc., are distinguished". The theorist understands the rapid flattening of that gaseous mass (in about one period of rotation) as due to the decay of its original turbulence, and he reduces its further evolution to some secular changes followed by a slow loss of the axial rotation of the galactic systems. In this way, galaxies of the type of the Magellanic Clouds or the M31 companions are to be obviously younger than the universe, and "elliptic galaxies are in a final stage which no longer shows the sort of evolution we consider". "Thus the large galaxies like our own can be as old as the universe, without having yet reached their final stage", the spiral structure being their "most conspicuous semiregular pattern". Weizsacker's judgment on it is twofold. He finds himself in a position to "try to understand spiral structure as a hydrodynamical effect [...] produced by nonuniform rotation", noticing that any local formation - "cloud formed by the turbulence" - will then be distorted into a segment of a spiral. On the other hand, he admits that "the abundance of systems with just two spiral arms is probably caused not by turbulence but by gravitation", which is in fair correlation with the presence of a bar. The bar is understood as an elongated equilibrium figure of rotation similar to Jacobi's liquid ellipsoids; it "can be kinematically stable only if the system rotates uniformly", i.e. in inner galactic regions. But just a little way out, the shearing effect of differential rotation comes into play, in order "not to destroy the `bar' entirely but to distort it strongly", giving it some spiral contours (Weizsacker 1951, p.176-179). Back.

¹⁸ Zwicky reflected on the `cooperative' effects in gravitating systems (both in stars and galaxy clusters) since the mid-1930s, and he believed that whereas the nuclei of spiral galaxies had already reached their equilibrium the spiral arms and interarm regions were still "transitory configurations" (Zwicky 1957, p.214). He thus did not treat the spiral structure from the natural, for collective phenomena, viewpoint of oscillations and waves in equilibrium media. Back.

¹⁹ "Why do the spirals always show the combination of a disk and a central spheroidal system? It must reflect the original density distribution in gas. [...] Can we imagine that at some era in the past, the central spheroidal system of low rotation and the disk with very fast rotation actually resembled the equilibrium figure of the gas? One should really look into these things" (Baade 1963, p.17).

"The origin of the spiral systems is an unsolved problem as yet. Doubtless the interstellar material plays a major part in it. Therefore the methods [of stellar dynamics ... ] seem to be insufficient for a solution" (Kurth 1957, p.146). Back.

²⁰ This was inferred from the lack of reddening of globular clusters in M31, one half of which lie behind the galaxy disk because of their spherical distribution. As Baade wrote (1963, p.70), initially one did not believe in this finding, since the gas layer in our own Galaxy was still held to be uniform. Back.

²¹ Baade has usually been quoted from his posthumous monograph (Baade 1963). It reproduces his 1958 lectures that vividly transmit the mid-century atmosphere in extragalactic astronomy. Many investigators of the time claimed to have agreed with Baade on the basic role of gas in the spiral arrangement (e.g., Weizsacker 1951, p.178). Back.

²² In 1958 this map was completed with the spiral fragments observed from Australia (Oort et al 1958). Back.

²³ They were inferred in 1951 from data on the distribution of O-B associations and HII regions (Morgan et al 1952; see Gingerich 1985). Back.

²⁴ Via such shifts of opinion, Lindblad found himself on the way towards "a more definite theory" (Lindblad 1962b, p.148). There he might well be judged (Toomre 1977, p.439) as if even having finally conceded that his old leading-arm models were "not reconcilable with modern evidence" (Lindblad 1962b, p.146). Yet he blamed that on some other "early gravitational theories which interpret spiral structure as due to orbital motions of stars starting from a small nucleus" (Lindblad 1962b, p.146). Back.

²⁵ Having completed by the 1930s his theory of asymptotic leading spirals, Lindblad (1934) turned to the empirical component of the problem of the `sense of rotation' of spiral arms. The difficulty was with determining the near and the far sides of a galaxy, as this might be made no other than by way of speculation on the asymmetry of dust absorption along the minor axis of the visible image. There were at the time no reliable data on interstellar dust properties. To Lindblad's way of thinking, a stronger absorption was felt by a farther side (thought also to show sprinkles of dust veins in the bulge region), which maintained leading arms. After a categorical objection by Hubble (1943), he scrutinized the subject anew in his fundamental work with Brahde (Lindblad & Brahde 1946) followed by a succession of smaller articles during a decade or so. To criticize Lindblad for his leading-arm orientation was a commonplace. One agreed with him (and, evidently, with Hubble) in that the sense of spiral winding must be the same for all galaxies, which demanded only one good example of a nearly edge-on galaxy that might be clearly judged on both its spiral form and nearer side. de Vaucouleurs (1958) gave such an example as got a high-quality long-exposure photograph of NGC 7331 taken with the 200-inch reflector. It favored Hubble's camp. Lindblad must have reserved objections on how the spiral form was to be inferred from that crucial case (he and his collaborators Elvius and Jensen had been studying this galaxy photometrically in several papers from 1941 to 1959, and he gave a rather incomplete summary on the topic in Lindblad 1962a), but for the absolute majority of astronomers the empirical component of the sense-of-winding problem was no longer acute. Back.