Dark Matter and Cosmic Web Story

by Jaan Einasto

  I repeated the statistical analysis of astronomical publishing and referencing in 1975 and 1985. The analyses showed that people had understood the inefficiency of the publications of observatories, which had practically disappeared, whereas during these years the volume of journals had increased more than tenfold. I have also advised my students to publish their work in English-language journals, a suggestion that has been of benefit.

  In the 1960’s some physics theorists of the Institute of Physics worked in our new Observatory at Tõravere. Among them was Madis Kõiv, who wrote plays and tractates in his spare time. Now his dramas are played in several theatres in Estonia, but at the time we could not imagine that one of the innovators of Estonian drama was working beside us. At that time we used to prepare plays and gag stories for the observatory’s New Year’s parties, the scenarios authored by Madis Kõiv and Arved Sapar, one of our best in the field of theoretical astrophysics. One of our most successful parodies was based on the movie “Supernova”. The outdoor shots for this movie were taken in Tõravere, and for a couple of months the filmmakers lived with us, dined in the same canteen and, if they chose to, could take notice of the way we lived and worked. What surprised us the most was their complete lack of interest in what we were actually doing. The filmmakers had their own idea of how science is done and they did not let the reality faze them. The movie was finished and the premiere was held in Tõravere. We watched it and marvelled that while everything seemed to be right, it felt completely phony. Madis Kõiv was among the first to speak up, pointing out that the movie was very artificial. So the idea came about to prepare a parody at the NewYear’s party on this subject. It was made as a pantomime and titled “Superprima”. Arved Sapar played a genius young astronomer, who discovers a new star — the Superprima. One young astronomer depicted the telescope; four other young men danced to the music of the little swans from Tchaikovsky’s “Swan Lake”. It was the best parody I have encountered to this day.

  Fig. 3.11 New observatory in autumn 1995 (author’s photo).

  The directing of New Year’s party presentations was later continued by climatologist Ain Kallis, who produced a piece on defending a dissertation in the eighties. Back then we had a council where our own astronomers as well as guests from other centres could come to defend their PhD theses. Now and then there were also mishaps due to our defective knowledge of Russian — these found their way in the play. It panned out well and word of it spread in the Estonian community. A recording of the play was made by a TV team, and aired on Scientists’ Day. The humour was so subtle that many viewers did not realise it was a parody until towards the end. The matter ended with Rein Ristlaan (secretary of the Estonian communist party responsible for ideology) getting angry and ordering the recording to be destroyed. So it came to be that for us only memories of those nights remain.

  3.3.3 Space studies

  When the new observatory was planned, one of the new research directions was space studies. Similar plans were made in the Sternberg Astronomical Institute in Moscow, in the Crimean Astrophysical Observatory and in the Byurakan Astro- physical Observatory. Our astronomers had discussions with leaders of these observatories, and an agreement was reached that the role of Tartu Observatory is to develop UV-sensitive detectors and UV-calibration systems, needed to observe astronomical objects in ultraviolet light. In our observatory Valdur Tiit was the initiator of this research, and soon a special laboratory was formed to develop and build the equipment needed.

  This work was rather successful. Very sensitive UV-detectors were developed and tested first in rocket flights, and then installed on the first astronomical satellite, Kosmos 215, launched in April 18, 1968 at the Soviet site at Kapustin Yar. The goal was to examine ultraviolet and X-ray radiation from stars. This was a common enterprise of the Sternberg Institute, Crimean and Tartu Observatories. Valdur Tiit was the head of the team at the launch site. Preliminary results of the mission were described by Dimov (1970). Over a period of 40 days spectral regions from 1250 to 2700 Å were recorded by identical telescopes of aperture 70 mm, see Fig. 3.12. The telescopes scanned the sky and recorded all stars which crossed the fields of view. UV-photometry was obtained for 36 A and B stars, in addition an X-ray telescope was used to measure radiation between 0.05 and 0.5 nanometers. Several X-ray sources were detected.

  Fig. 3.12 One of the eight telescopes used in the astronomical satellite Kosmos 15 (author’s archive).

  After this very good experience Valdur Tiit had an agreement with space authorities that a launcher shall be reserved to put a larger telescope into space, of aperture 350 mm. The mirror was made in Leningrad and there was an agreement with the Crimean and Sternberg Observatories on how to put all the equipment together. However, quite unexpectedly, Prof. Kipper announced that Tartu Observatory shall not continue with this project. Because Tartu Observatory was the only one with capabilities to build and test UV equipment, the whole project was cancelled. Years later, Kipper explained that he feared that if we are very seriously involved in space studies, then the whole Observatory could be surrounded by barbed wire, and civil scientific work would be difficult. The laboratory led by Valdur Tiit was moved to the Institue of Physics, where he continued to develop UV equipment and other modern devices.

  However, this was not the end of space studies in Tartu Observatory. During the International Geophysical Year 1957 our astronomers participated in one of the projects — the study of noctilucent (or night) clouds. These clouds are located at altitudes of around 80 km, and are visible only when illuminated by sunlight while the lower atmosphere is in the Earth’s shadow during summer nights. Their formation is not clear; they are observed only at Earth latitudes between 50° and 70° North and South of the equator.

  The leader of the study of noctilucent clouds in Estonia was Charles Villmann, a former amateur astronomer. He was a very good organiser and was invited in the mid 1960’s to the Observatory as vice-director to help organise the building of the new observatory in Tõravere. Soon he understood that it is much easier to observe noctilucent clouds from space. He contacted the manned flight center in Moscow and got permission to install on space stations Salyut 6 and Salyut 7 equipment to scan the upper atmosphere near the Earth limb. He managed to form a team who constructed and built photometers Mikron and Faza (see Fig. 3.13), which were used on these space stations by Soviet kosmonauts Georgy Grechko and Vitaly Sevastyanov. Both kosmonauts often visited Tartu Observatory to discuss results of observations; one picture of such a visit is shown in Fig. 3.14.

  Georgy Grechko wrote his PhD thesis on the basis of data collected in collaboration with the Tartu team by Villmann, Enn Saar acting as consultant in theoretical physics. Initially visits of kosmonauts were made secretly, but in the last years of 1980’s they visited us quite openly. Once kosmonauts even participated in our New Year party where the play on the thesis defence was performed. For our guests the play was performed in Russian.

  Actually the fear of Prof. Kipper concerning the freedom of scientific thinking in the Observatory was not completely unjustified. One office in the Observatory was reserved for a ‘special department’, its door was covered with an ironplate and window with an iron grid, and a KGB officer had her office there.

  Fig. 3.13 The photometer Faza used in Space Station Mir to observe Earth’s atmosphere from space (author’s archive).

  Fig. 3.14 Soviet kosmonauts visiting Tartu Observatory in 1981. From left the wife of Georgy Grechko, Charles Villmann, Georgy Grechko, Aksel Kipper, Vitaly Sevastyanov, Väino Unt (author’s archive).

  All correspondence on the space program went through this office, and only people with special permissions could read and write the correspondence. For these ‘trusted’ people this possibility was a hindrance for foreign travel, because these people knew ‘state secrets’. This office had one more function in the observatory — Soviet officials made all possible efforts to prevent any kind of demonstration on May Day and October Revolution anniversaries
. For this reason all typewriters and xerox-machines were locked, in addition all offices of the observatory were locked and sealed up. All these measures were rather stupid, but we accepted this with humor. We were aware what was possible and what not, we did not have any open hostility against the rulers, and continued our scientific studies and cultural activities as before.

  Chapter 4

  Global dark matter

  During the 1960’s I elaborated, step by step, the main principles on how to calculate models of galaxies which make use of as much observational data as possible. To bring physical data on various populations to a coherent system, models of physical evolution of galaxies and their populations were calculated. And then I ran to difficulties — no combination of stellar populations was able to explain rotation data of galaxies. The solution came in the early 1970’s when my collaborator Enn Saar suggested abandoning the idea that only known populations exist in galaxies. This brought us to the dark matter problem. But then we had another difficulty — there were no suitable candidates for the nature of the dark matter. The whole decade of the 1970’s was needed to finally find a possible candidate for dark matter particles. The candidate was found elsewhere, as we had no good experts on particle physics in our team, but we were able to test its role in the formation of the cosmic web. In this Chapter I shall concentrate on our efforts to find the amount of dark matter, its distribution, and connection with ordinary matter.

  4.1 The discovery of global dark matter

  4.1.1 Galactic coronas

  In 1970 I had the chance to attend the IAU General Assembly in Brighton. I reported my models of galaxies at the meeting of the Commission 33 of IAU on the Structure and Dynamics of the Galaxy. These were essentially models I calculated for my Doctor of Sciences thesis. Galactic evolution was already included, but dark coronas not. I had a chance to meet Ernst Öpik and his wife. I had a lot of discussions also with other astronomers.

  In spring 1972 George Contopoulos invited me to give a review on Galactic models at the First European Astronomy Meeting in Athens. At this time population models of galaxies had been calculated already for 5 galaxies of the Local Group and for the giant elliptical galaxy M87 in the Virgo cluster. More and more data accumulated on rotation velocities of galaxies. New data suggested the presence of almost flat rotation curves on the periphery of galaxies, thus it was increasingly difficult to accept the previous concept of large non-circular motions. On the other hand, recently finished calculations of the physical evolution of stellar populations confirmed our previous view that it is extremely difficult to accept stellar origins for the hypothetical population, responsible for flat rotation curves.

  Fig. 4.1 Ernst Öpik with his wife and author in Brighton during the IAU General Assembly 1970. Presentation of new galactic models, but without dark coronas (author’s photo).

  In summer 1972 I discussed the problem with my collaborator Enn Saar. He suggested abandoning the idea that only known stellar populations exist in galaxies, to assume that there is a population of unknown nature and origin, and to look at which properties it should have using available data on known stellar populations and galaxy rotation data.

  This discussion was one of the decisive moments of the whole dark matter story. It was immediately clear that the assumption of the presence of a new population demands a radical change in our understanding of the structure of galaxies, and that we are dealing with a completely new phenomenon of unknown origin.

  First of all — the extended dark population cannot be of the same nature and origin as the dark population in the Solar vicinity near the Galactic plane. In other words, there are two dark matter problems: one of the dark matter in the Solar vicinity, and the other of the dark matter surrounding galaxies and clusters of galaxies. The dark matter in the Solar vicinity is strongly concentrated in the plane of the Galaxy, thus dissipation is needed to form this population. This population probably consists of very faint stars or Jupiter-like objects with no hydrogen- burning in their interiors. This assumption is not new; Jeans (1922) already argued that there are several invisible stars per visible one.

  What concerns the nature of the new extended population then are arguments against its stellar origin. These arguments were mentioned in the previous Chapter and shall be discussed in more detail in the next Chapter. Taking all these considerations into account I realised that we are dealing with a new very extended population of unknown nature, well segregated from known populations. To avoid confusion with the known halo population consisting of old metal-poor stars, I called the new population “corona”. Data also indicated that the presence of dark coronas is a general property of galaxies, at least of giant ones. It may be of the same origin as the dark matter in clusters of galaxies.

  When I understood all this, I had the feeling that we have reached a peak of a mountain, and that behind the mountain there is a new terrain, completely unknown before, with other mountains and valleys, and a new horizon far away. The peak itself had been visible already for some time, but everything on the other side of the peak was unknown.

  Quickly I calculated a second set of models for galaxies, assuming, as a first approximation, that the total mass of the new population is equal to the mass of the sum of known stellar populations. The central density of the new population can be easily found using observed rotation curves and data on known stellar populations. Already first calculations showed that this assumption is too modest, and it improves the rotation velocity law only a little. Indirect arguments, applied to the giant elliptical galaxy M87, suggested that the mass and the radius of the dark population can exceed the total mass and the mean radius of known populations even tenfold. The distribution of mass-to-luminosity ratios of models is shown in Fig. 4.2 for all galaxies studied; the rotation curve for the Andromeda galaxy is given in Fig. 4.3. In both Figures two variants of models are given, the variant A without, and the variant B with the dark corona. For comparison, the right panel of Fig. 4.3 shows the rotation of M31 according to our later model by Tenjes et al. (1994).

  Fig. 4.2 The distribution of mass-to-luminosity ratio, fB = M/LB, in galaxies of the Local Group and M87: models without (A) and with (B) dark corona (Einasto, 1974a).

  My talk in Athens was on September 8, 1972. The main results were (Einasto, 1972a, 1974a):

  (1) There are two dark matter problems: the local and the global one;

  (2) Local dark matter, if it exists, must be of stellar origin, since it is strongly concentrated in the Galactic plane, and dissipation is needed to form such a flat population (in the gaseous phase of the evolution of the Galaxy);

  (3) Global dark matter is of non-stellar origin; it has very low concentration in the plane and centre of the galaxy; its dynamical and physical properties are different from properties of all previously known stellar populations; to avoid confusion with known stellar populations I called the new population ‘corona’;

  (4) Available data are insufficient to determine radii and masses of coronae.

  The nature of the corona was unclear. In the Athens talk I wrote: “The matter in question cannot be in the form of neutral gas, since this gas would be observable. The matter cannot be in the form of stars too. Luminosity decreases in outer galactic regions rapidly, therefore, if the matter is in the form of stars, the latter must be of very low luminosity to be invisible. The presence of low-luminosity stars in outer galactic regions without bright ones would require a powerful process of a large-scale segregation of stars according to mass (low-luminosity stars have smallest masses), but this is highly improbable. There remains the possibility that the unknown matter exists in the form of rarefied ionised gas.”

  Fig. 4.3 Left: the rotation curve of M31 according to the model by Einasto (1974a). Variants A and B correspond to models without and with corona. For comparison the rotation curve by Roberts (1966) is shown. Right: the rotation curve of M31 according to the model by Tenjes et al. (1994). Open circles mark observations, thick line — the best-fit model
, dashed lines — contribution of populations to the rotation curve.

  Kahn & Woltjer (1959) in their study of the dynamics of the system M31- Galaxy also suggested hot gas as the possible form of the unseen matter in the Local group of galaxies.

  My Athens report did not give rise to special excitement. The main reason for this lukewarm reception was probably the absence of a solid proof for the existence of the corona, of its main parameters (mass and radius), and of its nature. Thus I continued the search for further evidence.

  4.1.2 Clusters and groups of galaxies

  In the search for further evidence of dark matter I spent a lot of time searching in the literature and looking at what other people had already done. I noticed that the problem of galactic coronae is the same as discussed already long time ago in clusters and groups of galaxies, starting from the pioneering work by Fritz Zwicky (1933, 1937). He measured redshifts of galaxies in the Coma cluster and found that the velocities of individual galaxies with respect to the cluster mean velocity are much larger than those expected from the estimated total mass of the cluster, calculated from masses of individual galaxies. The only way to hold the cluster from rapid expansion is to assume that the cluster contains huge quantities of some invisible dark matter. According to his estimate the amount of dark matter in this cluster exceeds the total mass of cluster galaxies at least tenfold, probably even more.