The least massive members of the Local Group cluster around the dominant galaxies. At a distance of 50 kpc from the centre of a 1011Mo galaxy, the escape velocity is about 140kms-1. Because the dwarf elliptical galaxies in the Local Group move more slowly than this, if, appears fairly certain that they are satellite galaxies orbiting in the gravitational field of giant galaxies. Their orbital periods are about 500 million years, so they must have orbited their central galaxies a score of times during the history of the Universe, rather like globular clusters in galactic haloes. Because we observe only a snapshot of the satellites, evidence of orbital motion must be obtained indirectly.

One effect to be considered is that the central galaxy tends to pull stars away from a satellite. The outer stars, which are weakly bound, run the risk of being peeled oil’, thereby stripping the satellite down to its tidal radium. The existence of this tidal limiting has been confirmed by the study of the distribution of light in the ellipticals. Likewise, we may expect gas from the satellite to be torn away by the gravitational pull of the central galaxy. Strong evidence for such an occurrence is the MAGELLANIC STREAM, a huge lane of neutral hydrogen streaming off the Magellanie Clouds across the south galactic, pole, presumably to end near our Galaxy . Along this stream, the gas velocity equals the velocity of the Clouds at that end, and approaches the velocity of our Galaxy at the other. This is additional evidence that the Magellanic Stream is a tidal bridge of gas, caused by the gravitational interaction between the Galaxy and the Magellanic Clouds in their orbit. Theoretical calculations confirm that long narrow bridges can be formed between a satellite and its central galaxy . Observations of the neutral hydrogen in the outer parts of our Galaxy show that the galactic plane is warped, possibly as much as 1kpc at a radius of 15kpc. This might be due to the disturbing effect of the Magellanic Clouds. A similar effect can explain the warp in the plane of M 33, although this galaxy has no obvious satellites which can be held responsible for the distortion.

Another phenomenon which is possibly connected with the interaction between the Galaxy and its satellites is the existence of HIGH-VELOCITY CLOUDS of neutral hydrogen. These are found in many different places in the sky, measure a few degrees across, and move very rapidly: some towards the Sun with maximum velocity of about 200 kms-1, some away with half that speed. As we have-seen, typical velocities far away from the Galaxy do not nearly span a range of 300km s-1 ! It appears likely, therefore, that the high- velocity clouds are not very distant from the Sun, and that they arise from an interaction between intergalactic gas and the reaches of the galactic atmosphere. The origin of the infalling gas could be a tidal interaction between the Galaxy and the Magellanic Clouds, which are very rich in gas.

The observation of satellite motions makes it possible to estimate the masses of the central galaxies, just as the mass of the Sun can be derived from planetary motions. Likewise, the average positions and velocities of globular clusters can be used to measure galactic masses. In general the masses thus obtained agree well with those found from the analysis of the rotation curves.

The satellite galaxies have almost no influence on the large-scale motions in the Local Group because of their small masses. The only really important galaxies are M 31, the Galaxy and M 33. Our Galaxy moves with respect to the centre of mass of the Local Group with a velocity of about 170kms-1 along a line through 1=10°, b = 0°. The radial velocities of M 31 and M 33, after correction for this motion, are —68 and — 11kms-1, respectively. The absolute average velocity of the dominant members of the Local Group is therefore 60kms-1 or more, because motions at right angles to the line of sight cannot as yet be measured. The escape velocity at 350kpc from the Galaxy is only 60kms-1, so that one might conclude that the three most massive galaxies move too rapidly to be bound to each other by gravitation. Then the existence of the Local Group would be merely due to chance. However, there is evidence that the Group is a bound unit. Firstly, there are no comparable massive galaxies nearby for at least 1000kpc around. Secondly small groups of galaxies like ours are very common in the it appears unlikely that these are together by chance only than being bound by mutual gravitational attraction.

This is a real poser: if the Local Group is gravitationally bo how can it be that its members move so rapidly ? The galaxies see around us do not contribute enough mass to prevent the d’ solution of the Local Group that these velocities would imply The only solution appears to be, that there must be more matter in the Group than we can readily observe. By itself, this is not very difficult to accept, because only the matter which is concentrated in stars, or in highly energetic gas streams like supernova remnants, emits a substantial amount of radiation for a given amount of mass. In other words, the mass-to-luminosity ratio of stars and the like is probably very much smaller than is that ratio for matter in general. As we have seen in the discussion of rotation curves, the attraction which a particle in an orbit experiences is proportional to the mass encompassed by that orbit. Classical mechanics tells us that the escape velocity is proportional to the square root of the mass within the orbit. Therefore, in order to make the escape velocity exceed 120 km s-1, which is about what is needed to bind the Local Group together, the total mass of the Group must be at least four times the mass observed in the galaxies.

If this is the solution of the problem, in what form could the material be ? The number of possibilities is practically infinity Almost any dense object which is not of itself luminous would escape detection. For example, the HIDDEN MASS (also called MISS¬ING MASS) might be in the form of football-size rocks, but given the enormous amount of equally possible alternatives (e.g. golfball size rocks) it is not very fruitful to pursue the point. It is of more interest to determine the distribution in space of the hidden matter. There are two alternatives. Either the mass is present somewhere in the galaxies, more specifically in an extended halo, or it pervades the whole of the Local Group as an INTERGALACTIC MEDIUM. The second possibility appears slightly more probable than does the first, because it is somewhat difficult to reconcile the presence of a massive halo with the observations of the motions of globular clusters and satellite galaxies. But it is not at present known with certainty which solution is correct.

The galaxies of the Local Group individually, or, if the Group is gravitationally bound, as a unit, move under the influence of more distant galaxies and groups of galaxies. The nearest of these are a few million parsecs away from our Galaxy. On a cosmic scale, this is not very far: only a hundred galactic diameters. Some of the problems we have encountered here do also occur on this larger scale of organization.

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