Groups of Galaxies

The mass discrepancy in clusters of galaxies was found by Zwicky (1933). He measured redshifts of galaxies in the Coma cluster and found that the total mass of the cluster calculated from the velocity dispersion using the virial theorem exceeds the sum of masses of visible galaxies more than tenfolds. He concluded that the cluster contains large amounts of invisible dark matter. 

The classical models of spiral galaxies were constructed using rotation velocities. In contrast, the models of elliptical galaxies were found from luminosity profiles and calibrated using central velocity dispersions or motions of companion galaxies. An overview of classical methods to construct models of galaxies is given by Perek (1962). 

Mass-to-luminosity Ratios and Models of Physical Evolution of Stellar Populations 

The next problem was to find internally constitent values of physical parameters of stellar populations of different age and composition. For this purpose I developed a model of physical evolution of stellar populations (Einasto 1971). When I started the modelling of physical evolution of galaxies I was not aware of similar work by Beatrice Tinsley (1968). When my work was almost finished I had the opportunity to read the PhD thesis by Beatrice. Both studies were rather similar, in some aspects my model was a bit more accurate (evolution was calculated as a continuous function of time whereas Beatrice found it for steps of 1 Gyr, also some initial parameters were different). Both models used the evolutionary tracks of stars of various composition (metallicity) and age, and the star formation rate by Salpeter (1955). I accepted a low-mass limit of star formation, Mo 0.03 Msun, whereas Beatrice used a much lower mass limit to get higher mass-to-luminosity ratio for elliptical galaxies. My model 

The second problem encountered in the modelling of M31 was the rotation and density distribution on the periphery. If the rotation data were taken at face value, then it was impossible to represent the rotational velocity with the sum of gravitational attractions by known stellar populations. The local value of M/L increases toward the periphery of M31 very rapidly if the mass distribution is calculated directly from the rotation velocity. All known old metal-poor halo-type stellar populations have a low M/L tv 1 in contrast, on the basis of rotation data we got M/L 1000 on the periphery of the galaxy, near the last point with a measured rotational velocity. 

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The existence of dark matter (either baryonic or non-baryonic) is inferred from its gravitational effects on galactic rotation curves, the velocity dispersions and hydrostatic equilibrium of hot (X-ray) gas in clusters and groups of galaxies, gravitational lensing and departures from the smooth Hubble flow described by Eq. (4.1). This dark matter resides at least partly in the halos of galaxies such as our... 

Groups of galaxies are bound with conventional masses the mean mass-to-luminosity ratios of groups are 4 and 30 for spiral and elliptical dominated groups, respectively ... 

Our own galaxy is a member of a small local group of galaxies which also includes the great spiral in the constellation Andromeda, shown on page 206. 

Local Group The group of galaxies of which our own Galaxy is a member, it consists of some 30-40 known members, the most massive of which are the Galaxy and the Andromeda galaxy. 

Studies in chemistry or any realm of science commonly consist of a series of directed examinations of parts of nature s realm called systems. A system is an identifiable fragment of the world that is recognizable and that has attributes that one can identify in terms of form and/or function. We can give examples at any level of size and complexity and in essentially any context. Indeed, a dog is a system at a pet show whereas the human heart is a system to the cardiologist a tumor cell is a system to the cancer specialist a star or planet or galaxy is a system to an astronomer a molecule or a collection of molecules is a system to a chemist and an atom or group of atoms is a system to a physicist. A system is, then, whatever we focus our attention upon for study and examination. 

The Milky Way belongs to a cluster of galaxies called the Local Group that occupies about 3 Gly in space and contains over 30 galaxies, although the precise number is hard to determine. Placing the Milky Way at the centre (Figure 2.14),... 

Local group The collection of galaxies, including the Milky Way and Andromeda, that form part of the Virgo cluster as part of the local supercluster of galaxies. 

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