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Dark Matter in the Universe: 60 Years Later

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Dark Matter in the Universe: 60 Years Later Dark Matter in the Universe: 60 Years Later J aan Einasto Tartu Astrophysical Observatory, EE-2444, Estonia 1 Introduction The traditional character of cosmology has been changed by the discoveries of dark matter in the Universe and of the filamentary structure of the Universe with euormous empty regions devoid of any kind of visible matter between filaments. In this year we celebrate the 60 anniversary of the disc.overy of the dark matter. 1 devote this review to some aspects of the dark ma.tter problem. The term dark matter is used here for any form of matter whose exi­ stence ca.n be detected only through its gravitational effect~. The presence of dark matter, its quantity and extent can be determined from the dif­ ference between the dynamical mass estimates in stellar systems and the directly counted masses of visible objects in them. If a difference between dynamical masses and directly counted masses is detected we can speak on the mass paradox or on the "missing mass". Mass determinations in smaU astronomie.al systems as the solar system, binary stars and sma.11 stellar systems show good mutual consistency bet­ ween results obtained by different methods and there is no room left for dark matter. A mass paradox is ·found in gala.ctic disks, around galaxies, in gro­ ups, clusters and superclusters of galaxies. The mass paradox is solved by assuming the existence of dark ma.tter in respective systems. Dark mat­ ter exists probably a.lso in large voids between superclusters. Dynamically dark matter is dominant on a.11 se.ales larger than the sca.le-length of ordina- 17 19 18 Jaan Einasto Dark Mat ter in the Uni verse: 60 Years La ter ry galactic populations. Thus the formation of galaxies and the structure If velocitie and orbital planes of test particles are distributed rando~y, of the Universe is essentially determined by the properties of dark matter. then we have another simple case. From measurements of Dopple~ shifts This makes the problem of the amount and nature of dark matter to the we can determine the velocity dispersion in one coordinate, Un direct~d central one in the whole cosmology. towards tlLe observer. Now instead of the velocity, V, we have the radial In this review I consider first shortly the methods of mass determina­ velocity dispersion, u, in this case f3 = 1/3, and we get tions in various stellar systems. Then a short history of the study of the 2 - 1/3GM(R) (3) dark matter is given. Current topical problems of the dark matter are ur - R . considered next: What is the amount of matter associated with galaxies? How much matter can be located in voids? What is the mean density of This case can be appliod for spheri al lusters of stars or galaxies. U sing t h.e matter in the Universe? What is the nature of dark matter? associated luminosity obtained by photometry one can deduce the mass­ to-luminosity ratio, M / L, which characterizes the physical n.ature of the 2 Mass Determinations of Stellar Systems system. The velocities ca.n be measUTed cLirectly from redsrufts, ~ut the ·t· ·lZ R aii<l the total luminosity L depend on the distance, e tr ec ive s , , ' ' _1 Ali astronomical mass determinations are based on measurements of rela­ D wJûch is inversely proportional ta th Hubble constant: D ex H. · tive velocities, V, and relative distances, R, of members of a system, using It, is convenient to express the Hubble constant in dimensionless uruts: the condition of gravitational equilibrium h = H /100, where H is given in km/s/Mpc. Observational estima.tes-~{ the Hubble constant lie in the interval 0.5 $ h $ 1. We have R ex h , V 2 = (3GM(R)/R (1) M ex 1i-1, L cx h-2, and M/L oc h. Masses, luminosities and M/L are usually measured in solar units. _ 2 where M(R) is the mass inside the orbit of test particles of radius R, Gis In cosm logy it is customa.ry to calculate the quant1ty n =87rGe/(3JI_ ), the gravitational constant, and f3 is a dimensionless constant of the order the mean mass density, ë, roeasured in units of the critical closure _<l~ns1ty. of unity, depending on the form and orientation of mutual orbits and the This quantity is independent of the Hubble constant, since the cnt1cal as 2 mass distribution. well as th meân mass dènsity are p1:oportional to h • If test particles move in nearly circular orbits, as planets around the The mass s of syst~i~ of gala.x.ies can be determined also in an j~direct Sun, then the velocity V is equal to the circular velocity and f3 = 1. In way, by counting the· number of their subsystems and by calculat~g the the solar system practically the whole mass is located in the Sun, and very total mass by a.dding the masses of these su bsystems. As noted. m the little is added by planets. In this case M(R) = M and we have for the Intr duction, when these two different mass determinations co11trad1c~ each circular velocity the Kepler law other then we have a mass para.dox or the "missing mass" problem m the system. v2 =CM. (2) R This law is valid in any case when the whole mass is inside orbits of test 3 First Evidence on the Missing Mass bodies. All test particles measure the same mass and, if par.ticles are Historically the first attempt to estima.te the density of dark matter from located at different distances from the central body, we have a number of the difference between the dynamical mass .~nd directly counted mass was independent mass determinations, as in the case of the Solar system using made by Ernst Ôpik (1915). In this study Opik tried to es~i~ate the d~n­ the motion of different planets to derive the mass of the Sun. sity of dark absorbing matter in the Milky Way. Later a snmlar question 20 Jaan Einasto Dark Matter in the Universe: 60 Years Later 21 was asked by Oort (1932). New determinations were made by Hill (1960) tluough in the dark matter problem came when Tartu and Princeton gro­ and 0.ort .(1960). They noticed that the dynamical density of mass in the ups demonstrated that dark coronas or haloes around giant galaxies have galactic d1sk exceeds the density of directly observed matter, the discre­ an extent of several hundred kiloparsecs and total masses of galaxies exceed pancy was only·by two times. Thls invisible component must form a disk · conventional masses tenfold (Einasto, Kaasik Saar 1974, Ostriker, Peebles, otherwise it distorts the whole rotation curve of the Galaxy. Presentl; Yahil 197 4 ). This discrepancy can be explained by the presence of a mas­ the problem of the presence of such "local dark matter" is not finally sol­ sive invisible corona around the galaxy. These results were confirmed by ved: observation~ determinations are rather uncertain, and the discussion, a systematic survey of rota.tian curves of galaxies by Rubin et al. (1980, whether there eXJsts local dark matter in the Galaxy or not, continue. 1'982). Reviews of the dark matter problem in various systems of galaxies . The ~ctual discovery of the "missing mass" concerns clusters of gala­ are written by Fa.ber and Gallagher (1979) and Trimble (1987). XJes: Zwicky (1933), basing on a virial analysis of 7 galaxies in the Coma There may exist a general srnoothly distributed dark matter backgro­ dus ter, argued for a discrepancy by a factor of rv 400 between the viri­ und too. Studies of the large-scale structure of the Universe demonstrate al M/ L in Coma and that of nearby galaxies. This was an overestimate that galaxies and clusters of galaxies are located in :filaments which form because it was based on a wrong value of the Hubble constant, H = 500 a connected 3-dirnensional network (a review is given by Oort 1983). The km~s/Mpc. According to modern data the mass-to-luminosity ratio in Co­ space between filaments is void of any visible matter. Galaxy systems :fi.11 ma 1s by a factor of~ 100 larger than in typical galaxies. This measures only a few per cent of the total volume of the space (Einasto, Jôeveer, Sa­ the amount of the "missing mass" in clusters of galaxies. ar 1980) a.nd diameter of voids reach tens megaparsecs (Jôeveer, Einasto, A similar discrepancy has been discovered in groups of galaxies. Kahn Tago 1978, Kirshner et al. 1981). A complete evacuation of voids of such and Woltjer (1959) noticed that the mass of the Local Group of galaxies, large diameter is impossible since at the epoch of recombination the den­ as determined from the motion of our Galaxy in respect to the Andromeda sity distribution was still almost homogeneous, and gravitation, the only 12 galaxy, is of the order of 10 M0 whereas the masses of bath galaxies are· 11 force which eau evacuate voids, works slowly. We corne to the conclusion of the order of 10 M0. We see a discrepancy of the order of ten. that some dark matter should exist in voids. The next stage of the dark matter story was connected with individual Summarizing the dynamica.l evidence we can say that there are actually galaxies. The mass distribution in galaxies can be determined directly from three dark matter problems: (1) dark matter in galactic disks; (2) dark their rotation curves and indirectly from the light distribution in visible ma.tter associated with galaxies and clusters of galaxies; (3) a smoothly populations using photometric data.
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