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 surface density is a scaled version of the observed H I surface density. Applying this mass model to a sample of 24 spiral galaxies with reliable rotation curves, one obtains good fits for most galaxies. The scaling factors cluster around 7, after correction for the presence of primordial helium. For several cases, however, different, often larger, values are found. For galaxies 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 outermost regions. Because of such imperfections and in view of possible selection effects, it is not possible to conclude here that there is a real coupling between HI and dark matter in spiral galaxies.