Rotation curve fitting gives astronomers a way to characterize the amount of dark matter in a galaxy. Unfortunately, it is sensitive to assumptions made about the disk mass-to-light ratio. A given rotation curve can often be fitted by a variety of models, some with small disk mass and large halo density and others with a massive disk and very little dark matter. This "parameter degeneracy" complicates the study of dark matter halos.
Keeping this degeneracy problem in mind, it is still interesting to ask how the properties of the dark matter halo depend on the kind of galaxy being considered. Do Sa galaxies have more dark matter than Sc galaxies? Do bright galaxies have more dark matter than faint ones?
Certainly the rotation curves of galaxies do vary from one to another. Some galaxies show rotation curves which continue to rise out as far as can be observed; other show a bit of a drop (though not as extreme as a Keplerian
r-½ falloff). Rotation curve fits can be used to constrain the total mass-to-light ratio in these galaxies.
By fitting rotation curves from a wide variety of galaxies, several tentative conclusions can be made. First, there is no obvious correlation between the total mass-to-light ratio and Hubble type -- Sa galaxies and Sc galaxies have roughly the same mass per unit luminosity. Second, there does seem to be a correlation between
(M/L)tot and the luminosity density of galaxy disks (known as the "surface brightness"). In high surface brightness disk galaxies (like our Milky Way),(M/L)tot is about 10 or so, while for low surface brightness disk galaxies it is much higher(M/L)tot~ 50. Differences in
(M/L)tot between galaxies is interesting because it tells us something about dark matter and how galaxies form. These results have important cosmological significance.
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