On the apparent coupling of neutral hydrogen and dark matter in spiral galaxies

We have studied a mass model for spiral galaxies in which the dark matter s urface density is a scaled version of the observed H I surface density. App lying this mass model to a sample of 24 spiral galaxies with reliable rotat ion curves, one obtains good fits for most galaxies. The scaling factors cl uster around 7, after correction for the presence of primordial helium. For several cases, however, different, often larger, values are found. For gal axies that cannot be fitted well, the discrepancy occurs at large radii and results from a fairly rapid decline of the Hr surface density in the outer most regions. Because of such imperfections and in view of possible selecti on effects, it is not possible to conclude here that there is a real coupli ng between HI and dark matter in spiral galaxies.