When we look at a galaxy in visible light we obviously see the most luminous objects. However, some of the mass may not be luminous. It could be there but hard to detect. The only sure way to trace out the total mass, whether it is bright or dark, is to study its gravitational effects. In a galaxy, the easiest way to study the gravitational forces is to measure the rotation curve. We have already discussed the rotation curve in our galaxy in Chapter 16.
that v(r) stays roughly constant out as far as we see luminous material. This immediately tells us that the mass doesn't fall off as fast as the luminosity. The masses that are found are as high as 2 X 1012M0. In many galaxies, no edge has yet been found. The rotation curves are still flat out to radii where the interstellar medium can no longer be detected, even using 21 cm observations, which show material farther out than the Ha emission.
Where can this matter reside? One possibility is that it is part of the disk. However, theoretical models show that such a large mass would gather the disk into a bar. The disks that we see would
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