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The general picture we have of disk galaxy formation is of cool gas settling into a disk in a potential well dominated by dark matter. The dark matter is assumed to reside in a dissipationless, effectively spherical distribution, and to interact with the potentially luminous (``baryonic'') matter only gravitationally. To make predictions from this general picture, we need to specify something about both the halos and the baryons. I would like to emphasize the importance of making a priori predictions. If we simply tune our favorite model to fit to the data, we will certainly fit the data.

At minimum, two parameters are required to describe halos, such as a characteristic mass Mitalich and size Rh. Further parameters describing axial and kinematic isotropy, etc., could be invoked, but these distract from the basic point and are not fundamental. For the baryons, many parameters might in principle come into play: initial angular momentum and gas temperature, conversion of gas into stars and feedback into the ISM, and so on. It would be nice to study a population of galaxies for which the effects of the baryons were minimized. Low surface brightness (LSB) galaxies, with central surface brightnesses µ0 > 23 B mag.arcsec-2, turn out to be just such objects. They occupy a different region of parameter space than do high surface brightness (HSB) spirals, so they provide genuinely new tests of ideas contrived to explain observations of HSB disks.