by Jaan Einasto
Fig. 4.4 Author with Vladimir Nikonov and Evgeny Kharadze in Sounion near Athens during the First European Astronomy Meeting in 1972. Galactic models with dark coronas announced (author’s photo).
Smith (1936) measured radial velocities of 30 galaxies in the Virgo cluster and confirmed the Zwicky result that the total dynamical mass of this cluster considerably exceeds the estimated total mass of galaxies. This conclusion was again confirmed by Zwicky (1937), who discussed masses of galaxies and clusters in detail. As characteristic for papers which change our world view, early indications of problems in the current world view are ignored by the community; this happened also with the Zwicky’s discovery. Astronomers were interested in the structure and evolution of stars, and Zwicky’s work seemed to be remote and uninteresting.
However, slowly more dynamical data on clusters of galaxies were collected, and the discrepancy between the cluster galaxy measured velocities and expected velocities for a stable cluster could not be ignored. To explain high velocities of cluster galaxies Ambartsumian (1961) suggested the idea that clusters are recently formed and are now expanding. In 1961 during the International Astronomical Union (IAU) General Assembly a special meeting to discuss the stability of clusters of galaxies was organised (Neyman et al., 1961).
I applied to attend the 1961 IAU General Assembly. Recently in sorting old documents I discovered a letter from the Astronomical Council where it was confirmed that I am included in the team of Soviet delegates to the Assembly. However, as characteristic for this time, somewhere my application was stopped, and from Tartu Observatory only Professors Kipper and Keres attended. Thus I missed the opportunity to participate in the first wide discussion of the mass discrepancy in clusters of galaxies.
When reading the Proceedings of the instability of clusters of galaxies I was impressed by the talk of Sidney van den Bergh (1961), who drew attention to the fact that the dominating population in elliptical galaxies is the bulge consisting of old stars, indicating that cluster galaxies are old. It is very difficult to imagine how old cluster galaxies could form an unstable and expanding system. The background of this meeting and views of astronomers supporting these and some alternative solutions were described by Trimble (1995), van den Bergh (2001), Sanders (2010) and Trimble (2010). I fully agree with arguments by van den Bergh (1961, 1962) that clusters of galaxies are old and stable systems.
A similar problem exists in double elliptical galaxies. The mean mass-to- luminosity ratio of double elliptical galaxies is M/L ≈ 66 (Page, 1952, 1959). A certain discrepancy was detected also between masses of individual galaxies and masses of groups of galaxies (Holmberg, 1937; Page, 1960). The conventional approach for the mass determination of pairs and groups of galaxies is statistical. The method is based on the virial theorem and is almost identical to the procedure used to calculate masses of clusters of galaxies. Instead of a single pair or group often a synthetic group is used, consisting of a number of individual pairs or groups. These determinations yield for the mass-to-luminosity ratio (in blue light) values M/LB = 1… 20 for spiral galaxy dominated pairs, and M/LB =5… 90 for elliptical galaxy dominated pairs (for a review see Faber & Gallagher (1979)). These ratios are larger than found from local mass indicators of galaxies — velocity dispersions at the centres of elliptical galaxies and rotation curves of spiral galaxies.
A completely new and innovative approach in the study of masses of systems of galaxies was applied by Kahn & Woltjer (1959). The authors paid attention to the fact that most galaxies have positive redshifts as a result of the expansion of the Universe; only the Andromeda galaxy M31 has a negative redshift of about 120km/s, directed toward our Galaxy. This fact can be explained if both galaxies, M31 and our Galaxy, form a physical system. The negative radial velocity indicates that these galaxies have already passed the apogalacticon of their relative orbit and are presently approaching each other. From the approaching velocity, the mutual distance, and the time since passing the perigalacticon (taken equal to the present age of the Universe), the authors calculated the total mass of the double system. They found that Mtot ≥ 1.8 × 1012 M. The conventional masses of the Galaxy and M31 were estimated to be of the order of 2 × 1011 M. In other words, the authors found evidence for the presence of additional mass in the Local Group of galaxies. The authors suggested that the extra mass is probably in the form of hot gas of temperature about 5 × 105 K. Using modern data Einasto & Lynden-Bell (1982) made a new estimate of the total mass of the Local Group, using the same method, and found for the total mass an even higher value, 4.5 ± 0.5 × 1012M.
Materne & Tammann (1974b,a) tested a number of nearby groups of galaxies for stability. They found that of 15 groups tested about half had either small or very uncertain values of M/Lpg. Of the remaining groups most had 10 < M/Lpg ≤ 30, and only a few had M/Lpg > 30. The galaxy membership in groups with high M/Lpg is not very certain. Thus the authors conclude that there is little evidence for the presence of a large discrepancy between mass-to-luminosity ratios of individual galaxies and groups of galaxies.
These relatively high M/Lpg ratios are in conflict with well-known values calculated from the luminosity function of stars in the Solar vicinity (Kapteyn & van Rhijn, 1920; Öpik, 1922a). The high M/Lpg ratios are also in conflict with values calculated from rotation and photometric data (the Burbidge’s series of papers on rotation curves), which suggest M/Lpg ≈ 3 for main bodies of galaxies. My own recent population models suggested that most stellar populations have 1 ≤ M/L ≤ 30. In particular, for bulges of galaxies our composite galactic model gave M/L ≈ 3–10 in good agreement with models of physical evolution of stellar populations.
On the other hand, already early rotation curves of galaxies suggested very high values of M/L on the periphery of galaxies (Babcock, 1939; Oort, 1940; Roberts, 1966; Rubin & Ford, 1970). However, these rotation curves were not long enough to calculate correctly mass distribution models for larger distances from centres of galaxies. Thus it was not clear how serious the discrepancy between the total masses and mass-to-luminosity ratios, found using different methods, is.
4.1.3 Dynamics and morphology of companion galaxies
Reading these papers on the mass discrepancy in clusters, groups and galaxies I realised how it is possible to check the presence of dark coronae around galaxies. If galactic coronae are large enough, then in pairs of galaxies the companion galaxy can be considered as a test particle to measure the gravitational attraction of the main galaxy. Mean relative velocities, calculated for different distances from the main galaxy, can be used instead of rotation velocities to find the mass distribution of giant galaxies for a much larger range of distances from the center of the main galaxy.
Fig. 4.5 Left: Enn Saar. Right: Mihkel Jõeveer in the 2000’s (author’s archive).
Quickly I collected data for pairs of galaxies. To avoid inclusion of optical pairs only double galaxies with some sign of mutual interaction were chosen. The analysis was ready on January 11,1974. It showed that radii and masses of galactic coronae exceed radii and masses of visible parts of parent galaxies by an order of magnitude! Together with Ants Kaasik and Enn Saar we calculated new models of galaxies including dark coronae. This time we had enough data to find total masses and radii separately for visible parts of galaxies and for their dark coronae.
In those years Soviet astrophysicists had the tradition of gathering in Caucasus Winter Schools. My results from galactic mass modelling were reported in the Arkhõz Winter School in 1972. In 1974 the School was held near the Elbrus mountain in the Terskol winter resort. I had my report on the masses of galaxies on January 29, 1974. I had a chance to discuss the results before the talk with my friend from the Ioffe Institute in Leningrad, Arthur Chernin. He suggested excluding all other stuff from the talk (initially the talk was on new models of galaxies), and to concentrate to the main result and its consequences to our world view on the structure of the Universe. I followed his suggestions.
My message was:
&nb
sp; (1) Data suggest that all giant galaxies have massive coronae, exceeding the mass and the radius of known populations about tenfold;
(2) The presence of massive coronae around galaxies may solve the problem of high masses of clusters of galaxies. X-ray data suggest that the mass of hot gas is not sufficient to stabilise clusters — clusters must be stabilised by dark matter;
(3) According to new estimates the total mass density of matter is 20% of the critical cosmological density, thus dark matter is the dominant population in the whole Universe.
Also I stressed my arguments suggesting that the corona is probably not a stellar population. What impressed me most was the beauty of the dark matter concept. Once you accept the presence of dark matter, a number of problems are solved — the Zwicky paradox of masses in clusters of galaxies, a similar paradox in groups, and flat rotation curves of galaxies.
In the Winter School prominent Soviet astrophysicists like Zeldovich, Shklovsky, Novikov and others participated. After the talk the atmosphere was as if a bomb had exploded. Everybody realised that, if true, this is a discovery of principal importance. Two questions dominated: What is the physical nature of the dark matter? and What is its role in the evolution of the Universe? Zeldovich and his group had been working over 15 years to find the basic physical processes of the formation and evolution of the structure of the Universe. For them the possible presence of a completely new, massive non-stellar population was a great surprise.
Zeldovich had a habit of waking up very early in the morning — these early hours were the most productive to think about new problems. He had a very good knowledge in physics but did not have all the important astronomical facts in his head. So he often called his students or collaborators almost in the middle of the night asking for some astronomical number or other question. Thus his team of young collaborators was trained to give quick answers to any question. In particular, they had the ability to do very quickly order of magnitude calculations. In such calculations there were only numbers 1 and 3, multiplied by a factor of 10 to some power.
Already during the School Zeldovich’s boys started to make estimates on the possible nature of dark matter. My own opinion was that the coronae are not of stellar origin for reasons discussed above. I also had arguments against neutral gas, thus my preliminary guess was hot gas (Einasto, 1974a).
Komberg & Novikov (1975) studied in more detail the hypothesis that massive coronae around spiral galaxies are composed of hot ionised gas. They found that this is in conflict with X-ray data on the amount of hot gas, already available in early 1970s (see the introduction of my paper on dark coronas (Einasto et al., 1974b)). Moreover, hot gas in such quantities would ionise neutral gas in the galactic disk, but neutral gas exists, thus this process is not dominant. Neutrinos were also considered, but rejected, since quick estimates showed that they can form only superclusterscale coronas, about 1000 times more massive than coronae around galaxies are.
Fig. 4.6 The mean internal mass M(R) as a function of the radius R from the main galaxy in 105 pairs of galaxies (dots). Dashed line shows the contribution of visible populations, dotted line the contribution of the dark corona, solid line the total distribution (Einasto et al., 1974b).
The School caused an avalanche of new studies to find the properties and physical nature of dark coronae. Ozernoi (1974) pointed out that, within rich clusters, the dark matter must mostly belong to the cluster as a whole and not to the individual galaxies, because the galaxy separations were smaller than the dark matter corona sizes. Bobrova & Ozernoi (1975) found that the mass of the hot gas is much smaller than the mass of hidden (dark) matter. If the hidden matter were confined to coronae of galaxies, then one would observe a more spotty structure of the X-ray emitting gas than observed. Chernin (1976) studied possible consequences of gaseous coronae, and Jaaniste & Saar (1975) investigated the possible stellar nature of the corona in more detail (see below). Antonov et al. (1975b,a);Antonov & Chernin (1975) investigated dynamical properties of galactic coronae. Zeldovich (1975) suggested a mechanism to explain the nucleosynthesis constraints on the amount of baryonic matter with the density due to dark matter (see below).
We had to hurry with the publication of our results, since large masses of halos were already discussed by Ostriker & Peebles (1973). Preliminary results of our analysis were published in February 1974 as Einasto et al. (1974d). But Zeldovich insisted that this is not enough: “Major results must be published in major journals”. Thus a more detailed report was sent to “Nature” (Einasto et al., 1974b) and, for the first time, a preprint was made and sent to all observatories. Soon we realised that it was just in time: Ostriker et al. (1974) got similar results using similar arguments; their paper was published several months after our “Nature” paper and has a reference to our preprint.
Since our paper and the paper by Princeton astronomers played a crucial role in the development of the dark matter concept, I shall discuss in a bit more detail the similarities and differences between the two papers.
The basic similarity is that the presence of massive halos/coronae was suggested using rather similar arguments. Both papers suggest that the total cosmological density of the matter in galaxies is about 0.2 of the critical cosmological density. But more interesting are the differences.
First, in our paper it was clearly stated that we have here a new population — corona. The principal goal of our paper was to find the main parameters of the corona. Estimates of radii, masses, and central densities of coronae were given in two Tables, which by mistake were missed in the “Nature” version. Because these Tables contain very important data, they are reproduced here as Table 4.1 and 4.2. Here Ls is the total luminosity of stellar populations, Ms — the total mass of stellar populations, Mc — the mass of the corona, Rcc — the effective radius of the corona, and gc — the central density of the corona. Table 4.2 shows that mean radii and mean masses of coronae exceed mean total masses and mean radii of known populations about tenfold. The segregation of the corona from known stellar populations was not stressed in the text, but is clearly evident from data shown in Tables.
Table 4.1 Parameters of Galactic Populations; Individual Galaxies.
Table 4.2 Parameters of Galactic Populations; Pairs of Galaxies.
Second, opinions about the nature of dark matter were different. In our paper in the introduction it was noted that dark matter in clusters cannot be explained by hot gas, since its mass is insufficient to stabilise clusters. Thus dark matter cannot be identified with hot X-ray emitting gas. The paper ends with a statement that a further discussion of the nature of galactic coronae shall be published elsewhere. Our additional studies were the previous review paper by Einasto (1972a, 1974a), the ongoing analysis by Einasto et al. (1974c) on morphological properties of systems of satellite galaxies around giant ones, and the study by Jaaniste & Saar (1975) on the possible stellar nature of galactic coronae. Ostriker et al. (1974) did not notice that dark matter forms a new population of unknown nature; authors write in the discussion that “the very great extent of spiral galaxies can perhaps most plausibly be understood as due a giant halo of faint stars”.
Soon the first reaction to the results of both papers appeared: Burbidge (1975) formulated difficulties of the dark corona concept. The main problem is in the statistical character of the dynamical determination of masses of double galaxies. If companion galaxies, used in mass determination, are not real physical companions but random interlopers, then the mean velocity dispersion reflects random velocities of field galaxies, and no conclusions on the mass distribution around giant galaxies can be made.
These three publications initiated the dark matter boom. Our paper was written by a previously almost unknown group of astronomers, thus the publication of papers on the subject by leading astronomers was important. From now on the possible presence of dark matter in and around galaxies was taken more seriously, which initiated further studies and discussions of the problem by the astronomical community. As no
ted by Kuhn, a scientific revolution begins when leading scientists in the field start to discuss the problem and arguments in favour of the new over the old paradigm.
Difficulties connected with the statistical character of our arguments were discussed already in the Winter School, thus we started immediately a study of properties of companion galaxies to find evidence for some other regularity in the satellite system, which surrounds giant galaxies. Soon we discovered that companion galaxies are segregated morphologically. Elliptical (non-gaseous) companions lie close to the primary galaxy whereas spiral and irregular (gaseous) companions of the same luminosity have larger distances from the primary galaxy. The distance of the segregation line from the primary galaxy depends on the luminosity of the satellite galaxy, see Fig. 4.7.
Fig. 4.7 Left: Distribution of luminosity of companion galaxies of different morphology vs. distance from the central galaxy; spiral and irregular companions are marked with open circles, elliptical companions with filled circles (Einasto et al., 1974b). Right: Distribution of internal mass in the giant elliptical galaxy M87, giant spiral galaxy NGC6946, and medium luminous spiral galaxy M81, compared with mass distribution in groups of galaxies derived from relative motions of companions of giant and medium bright elliptical and spiral galaxies (Einasto et al., 1976d).
This result shows, first of all, that companions are real members of these systems — random by-flyers cannot have such properties. Second, this result demonstrated that diffuse matter can have a certain role in the evolution of galaxy systems. The role of diffuse matter in galactic coronae was discussed in detail by Chernin et al. (1976). Morphological properties of companion galaxies can be explained if we assume that at least part of the corona is gaseous.
