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The observed proportionality between the centripetal contribution of thedynamically insignificant HI gas in the discs of spiral galaxies and thedominant contribution of DM - the "Bosma effect" - has been repeatedlymentioned in the literature but largely ignored. We have re-examined theevidence for the Bosma effect by fitting Bosma effect models for 17 galaxies inthe THINGS data set, either by scaling the contribution of the HI gas alone orby using both the observed stellar disc and HI gas as proxies. The results arecompared with two models for exotic cold DM: internally consistent cosmologicalNFW models with constrained compactness parameters, and URC models using fullyunconstrained Burkert density profiles. The Bosma models that use the stellardiscs as additional proxies are statistically nearly as good as the URC modelsand clearly better than the NFW ones. We thus confirm the correlation betweenthe centripetal effects of DM and that of the interstellar medium of spiralgalaxies. The edificacy of "maximal disc" models is explained as the naturalconsequence of "classic" Bosma models which include the stellar disc as a proxyin regions of reduced atomic gas. The standard explanation - that the effectreflects a statistical correlation between the visible and exotic DM - seemshighly unlikely, given that the geometric forms and hence centripetalsignatures of spherical halo and disc components are so different. A literalinterpretation of the Bosma effect as being due to the presence of significantamounts of disc DM requires a median visible baryon to disc DM ratio of about40%.

The Bosma effect revisited - I. HI and stellar disc scaling models