Dark Matter and Cosmic Web Story

by Jaan Einasto

  Also we found that dynamical and morphological properties of primary galaxies are well correlated with properties of their companions (Einasto et al., 1976d), see right panel of Fig 4.7. This suggests the presence of a physical link between primary and satellite galaxies.

  A further evidence of the large mass of the corona of our Galaxy came from the study of the dynamics of the Magellanic Stream (Einasto et al., 1976a). On the other hand, as already discussed at the Winter School, coronae cannot be fully gaseous (Komberg & Novikov, 1975). Thus the nature of coronae remained unclear.

  4.1.4 Tallinn and Tbilisi dark matter discussions

  To discuss the existence and the physical nature of dark matter, we organised in January 28–30,1975 a conference in Tallinn, Estonia, devoted solely to dark matter (Doroshkevich et al., 1975). One of the goals was to prepare for the Third European Astronomical Meeting in Tbilisi. The rumour on dark matter had spread around the astronomical and physics community and, in contrast to conventional regional astronomy conferences, leading Soviet astronomers and physicists attended. This conference is not well known, so I give here the list of major talks:

  • Zeldovich: “Deuterium nucleosynthesis in the hot Universe and the density of matter”;

  • Einasto: “Dynamical and morphological properties of galaxy systems”;

  • Ozernoi: “Theory of galaxy formation”;

  • Zasov: “Masses of spiral galaxies”;

  • Novikov: “Physical nature of galactic coronas”;

  • Saar: “Properties of stellar halos”;

  • Doroshkevich: “Problems of the origin of galaxies and galaxy systems”;

  • Komberg: “Properties of central regions of clusters of galaxies”;

  • Vorontsov-Velyaminov: “New data on fragmenting galaxies”.

  The list shows that central problems discussed in Tallinn were: What is the physical nature of the dark matter? and: What is its role in the evolution of the Universe? Two basic models were suggested for coronae: faint stars or hot gas. It was found that both models have serious difficulties (Jaaniste & Saar, 1975; Komberg & Novikov, 1975).

  I was a member of the Scientific Organising Committee of the Third European Astronomical Meeting, and was invited to Abastumani Observatory by Evgeny Kharadze, the Chairman of SOC, to discuss the program. The SOC meeting was held in October 1974. We had our first results of the study of dark matter, but the topic was still rather new and controversial. Initially Evgeny Kharadze and Richard West, another member of SOC present in Abastumani, had doubts of including this topic to the program. The Meeting was aimed at observational aspects of the structure of stars, galaxies and Universe, but dark matter was initially considered as a theoretical problem. It took some time to convince other members of SOC that the problem is presently an observational one. Finally we agreed that a full session shall be devoted to the dark matter problem. However, my suggestion to include a talk by Zeldovich on the theory of galaxy and structure formation was not accepted. As a compromise an agreement was achieved that Zeldovich can give a special lecture, but after the official program is finished.

  In the dark matter session the principal discussion was between the supporters of the classical paradigm with conventional mass estimates of galaxies, and of the new one with dark matter. Statistical arguments against the dark matter concept were presented by Fesenko (1976). Arguments for the presence of dark matter were presented in my lecture (Einasto et al., 1976e), arguments favouring the classical paradigm by Materne & Tammann (1976).

  Historically, the Tbilisi Meeting was the first well documented international discussion between supporters and opponents of the dark matter concept. Using their own statistical data Materne and Tammann concluded that systems of galaxies are stable with conventional masses. However, their most serious argument was: Big Bang nucleosynthesis suggests a low-density Universe with the density parameter Ω ≈ 0.05; the smoothness of the Hubble flow also favours a low-density Universe. Allan Sandage in collaboration with GustavAndreas Tammann has made great efforts to develop the classical cosmological paradigm. This cosmological paradigm is sometimes called “the science of two constants” — the Hubble constant and the density parameter. If one excludes inconvenient data by Zwicky, Kahn and Woltjer, and recent data on flat rotation curves of galaxies and dynamics of double galaxies, then everything fits well into this classical cosmological paradigm.

  Tammann is a world class astronomer, and his arguments were serious. In the framework of the classical cosmological paradigm it is really impossible to accept the presence of dark matter in quantities as suggested by Tartu and Princeton analyses (assuming the baryonic nature of DM). In the report to the Astronomical Council of the results of the Tbilisi Meeting the Chairman of the Organising Committee Kharadze noticed that the dark matter concept did not find support. Even Zeldovich started to doubt in the existence of dark matter. He asked “show me at least one galaxy where the existence of dark matter is proven reliably”. If dark matter exists in quantities as suggested by new data, then the arguments by Materne and Tammann must be explained in some other way.

  In Tbilisi I defended a view opposite to the view by Gustav Andreas Tammann. This did not violate our good relationship. He has Estonian roots: his grandfather Gustav Tammann was a professor of physical chemistry at Tartu University before World War I. Their roots are in Sangaste, South Estonia, their relatives still live in Estonia. In 1903 Gustav Tammann emigrated to Germany and became director of the Inorganic Chemistry Institute of the Göttingen University. Gustav Andreas Tammann was the director of the Astronomical Institute of the Basel University. In 1990 the first Meeting of the European Astronomical Society was held in Davos, Switzerland. At this time it was already easier to travel, and I attended the Meeting together with my younger collaborators, my daughter Maret, and Mirt Gramann. After the Society Meeting we drove with my first western car (an old Mercedes) to Basel, following the invitation by Gustav Andreas Tammann. We gave talks at the Astronomical Institute and had very interesting discussions. After Estonia declared independence Gustav Andreas visited Estonia several times to meet his relatives and to see Sangaste.

  The dark matter problem was also discussed during the IAU General Assembly in Grenoble, 1976, at the Commission 33 Meeting. Here arguments for the non-stellar nature of dark coronae were again presented (Einasto et al., 1976c). I remember that after my talk Ivan King quietly said from the audience “Perhaps really there are two halos of galaxies” — the conventional halo of old metal-poor stars and the extended and non-luminous corona. After the lecture Ivan came to me and asked me to repeat main arguments against the stellar origin of dark matter. A year later at Yale a conference was held on the topic “Evolution of galaxies and stellar populations”, and Ivan in his introductory talk listed the dark matter or “missing mass” problem as one of the most disquieting ones (King, 1977). However, no new independent data were presented in Grenoble nor atYale. It was clear that by dispute only it is not possible to solve the problem — new data were needed.

  4.2 The confirmation of the presence of global dark matter

  4.2.1 Rotation curves of galaxies

  As discussed above, a problem with the distribution of mass and mass-to-luminosity ratio was detected in spiral galaxies. Already Babcock (1939) obtained spectra of the Andromeda galaxy M31, and found that in the outer regions the galaxy is rotating with an unexpectedly high velocity, far above the expected Keplerian velocity. He interpreted this result either as a high mass-to-luminosity ratio on the periphery or as strong dust absorption. Oort (1940) studied the rotation and surface brightness of the edge-on S0 galaxy NGC 3115, and found in the outer regions a mass-to-luminosity ratio ≈250.

  Further evidence came from radio observations of the rotation of galaxies using neutral hydrogen. Strom (2012) describes how the atomic hydrogen 21-cm line in space was discovered. The information on radio emission from space became known to Dutch astronomers in late 1940, and Jan Oort started to think about how t
o use this emission to investigate astronomical objects. He organized a colloquium on radio astronomy in 1944 in Leiden, insisting that radio astronomy can really become very important if there were at least one line in the radio spectrum. His student van de Hulst calculated that hydrogen emits radio waves at 21-cm, and that this emission can be used to detect interstellar hydrogen, as well as to measure its velocity.

  After World War II there were numerous abandoned German Wurzburg radar dishes in the Dutch territory. The head of the Post Office Radio Division, de Voogt, appropriated a number of these radar dishes to set up the first radio astronomy observatory near Kootwijk. De Voogt suggested that Oort might use these antennas for the 21-cm line search. The main problem was building a low-noise receiver able to detect the 21-cm line. The Dutch team detected the line 6 weeks after Harvard astronomers Ewen and Purcell; both results were published simultaneously. Ewen and Purcell did not follow up their detection, but for Oort and his team, it was the beginning of systematic studies.

  The first goal was to measure radio emission from our own Galaxy (van de Hulst et al., 1954). The next goal was the Andromeda galaxy M31. van de Hulst et al. (1957) found that the neutral hydrogen emitting the 21-cm line extends much farther than the optical image. They were able to measure the rotation curve of M31 up to about 30 kpc from the centre, confirming the global value of M/L ≈ 20 versus M/L ≈ 2 in the central region.

  About ten years later Morton Roberts (1966) made a new 21-cm hydrogen line survey of M31 using the National Radio Astronomy Observatory’s 300-foot telescope. The flat rotation curve at large radii was confirmed with much higher accuracy. He constructed also a mass distribution model of M31.

  Astronomers with access to large optical telescopes continued to collect dynamical data on galaxies. The most extensive series of optical rotation curves of galaxies was made by Margaret and Geoffrey Burbidge, starting from Burbidge & Burbidge (1959); Burbidge et al. (1959), and including normal and barred spirals as well as some ellipticals. For all galaxies the authors calculated mass distribution models; for spiral galaxies rotation velocities were approximated by a polynomial. They found that in most galaxies within visible images the mean M/L ≈ 3.

  Subsequently, Rubin & Ford (1970) and Roberts & Rots (1973) derived the rotation curve of M31 up to a distance ∼30 kpc, using optical and radio data, respectively. The rotation speed rises slowly with increasing distance from the centre of the galaxy and remains almost constant over radial distances of 16–30 kpc.

  The rotation data allowed us to determine the distribution of mass, while the photometric data determined the distribution of light. Comparing both distributions one can calculate the local value of the mass-to-luminosity ratio. On the periphery of M31 and other galaxies studied the local value of M/L, calculated from the rotation and photometric data, increases very rapidly outwards, if the mass distribution is calculated directly from the rotation velocity. On the periphery old metal-poor halo-type stellar populations dominate. These metal-poor populations have a low M/L ≈ 1 (this value can be checked directly in globular clusters which contain similar old metal-poor stars as the halo). On the peripheral region the luminosity of a galaxy drops rather rapidly, thus the expected circular velocity should decrease according to the Keplerian law. In contrast, on the periphery the rotation speed of galaxies is almost constant, which leads to very high local values of M/L > 200 near the last points with a measured rotational velocity.

  All available rotation data were summarised by Roberts (1975) in the IAU Symposium on Dynamics of Stellar Systems held in Besancon (France) in September 1974. Extended rotation curves were available for 14 galaxies; for some galaxies data were available up the galactocentric distance ≈ 40 h−1 kpc. About half of galaxies had flat rotation curves; the rest had rotation velocities that decreased slightly with distance. In all galaxies the local mass-to-luminosity ratio on the periphery reached values over 100 in Solar units. To explain such high M/L values Roberts assumed that late-type dwarf stars dominate the peripheral regions.

  In the mid-1970’s Vera Rubin and her collaborators used new sensitive detectors to measure optically the rotation curves of galaxies at very large galactocentric distances. Their results suggested that practically all spiral galaxies have extended flat rotation curves (Rubin et al., 1978, 1980). The internal mass of galaxies rises with distance almost linearly, up to the last measured point.

  At the same time measurements of spiral galaxies with the Westerbork Synthesis Radio Telescope were completed, and mass distribution models were built for 25 galaxies (Bosma, 1978). Observations confirmed the general trend that the mean rotation curves remain flat over the whole observed range of distances from the centre, up to ≈ 40 kpc for several galaxies. The internal mass within the radius Rincreases over the whole distance interval.

  4.2.2 Mass-to-luminosity ratios of galaxies

  Another very important measurement was made by Faber & Jackson (1976); Faber et al. (1977). Sandra Faber and her collaborators measured the central velocity dispersions for 25 elliptical galaxies and the rotation velocity of the Sombrero galaxy, a S0 galaxy with a massive bulge and a very weak population of young stars and gas clouds just outside the main body of the bulge. Their data yielded for the bulge of the Sombrero galaxy a mass-to-luminosity ratio M/L = 3, and a mean mass-to-luminosity ratio of about 7 for elliptical galaxies, close to the ratio for early type spiral galaxies.

  Sandra Faber presented her results at the IAU General Assembly in 1976. I had the fortune to be present at her presentation. It was clear how important these results were. After her presentation I congratulated her for this work. Her results showed that our earlier models, based on velocity dispersions in central regions of galaxies, need correction, since our accepted dispersions were too high. Faber used better equipment; new measurements of velocity dispersions indicated that actual velocity dispersions in central regions of galaxies are about a factor of two lower than values accepted earlier. A review of masses and mass-to-luminosity ratios in galaxies is given by Faber & Gallagher (1979).

  New observational results by Vera Rubin and the Westerbork group confirmed the presence of dark halos of galaxies with high confidence. Observations by Sandra Faber suggested that mass-to-luminosity rations of optically visible populations of galaxies are rather low, thus the discrepancy between rotation and photometric data is serious. Now all new results were taken seriously.

  4.2.3 X-ray data

  Hot intra-cluster gas emitting X-rays was detected in almost all nearby clusters and in many groups of galaxies by the UHURU and Einstein X-ray orbiting observatories. Observations confirmed that the hot gas is in hydrodynamical equilibrium, i.e. gas particles move in the general gravitation field of the cluster with velocities which correspond to the mass of the cluster. The distribution of the mass in clusters can be determined if the density and temperature of the intra-cluster gas are known. This method of determining the mass has a number of advantages over the use of the virial theorem. First, the gas is a collisional fluid, and particle velocities are isotropically distributed, which is not true for galaxies used as test particles to find the cluster mass (uncertainties in the velocity anisotropy of galaxies affect mass determinations). Second, the hydrostatic method gives the mass as a function of radius, rather than the total mass alone as given by the virial method.

  Using UHURU satellite data the method was applied to estimate the masses of several clusters of galaxies (Forman et al., 1972; Gursky et al., 1972; Kellogg et al., 1973). The results confirmed previous estimates of masses made with the virial method using galaxies as test particles. The mass of the hot gas itself is only about 0.1 of the total mass (Holberg et al., 1973; Lea et al., 1973). We used this argument to indicate that the amount of hot gas in clusters is insufficient to explain the mass paradox in clusters (Einasto et al., 1974b). The luminous mass in member galaxies is only a fraction of the mass of the cluster X-ray emitting gas.

  In 1979 a conference was held at Princeton University to d
iscuss scientific programs for the Hubble Space Telescope. I had the luck to be a member of the small delegation from the Soviet Union. After the conference we had the opportunity to visit Harvard Observatory. There I had many discussions with John Huchra on the Harvard redshift survey and on the structure of the cosmic web. Also we met Riccardo Giacconi, the head of the Einstein X-ray observatory program. At this time there were no doubts that X-ray data give strong support to the high masses of clusters of galaxies.

  More recently clusters of galaxies have been observed in X-rays using the ROSAT satellite, and the XMM-Newton and Chandra observatories. The ROSAT satellite was used to compile an all-sky catalog of X-ray clusters and galaxies. More than 1000 clusters up to a redshift ∼0.5 were cataloged. Dark matter profiles have been determined in a number of cases.

  4.2.4 Gravitational lensing

  Clusters, galaxies and even stars are so massive that their gravity bends and focuses the light from distant galaxies, quasars and stars that lie far behind. There are three classes of gravitational lensing: (1) strong lensing, where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images; (2) weak lensing, where the distortions of background objects are much smaller and can only be detected by analysing the shape distortions of a large number of objects; and (3) microlensing, where no shape distortion can be seen, but the amount of light received from a background object changes in time. The background source and the lens may be stars in the Milky Way or in nearby galaxies (M31, Magellanic Clouds).