Bosma (1978, 1981b) and Carignan et al. (1990) found a trend for
the gas distribution to have the same shape as DM distribution.
This correlation between gas and DM is puzzling and if real, has
no easy explanation in the light of present CDM models. Not only
is there a general trend, but several individual features found
in the rotation curve seems to correspond to features in gas
circular velocity. This can be observed in Fig. 2 for NGC 1560
and Fig. 8 for NGC 2460. This fact has inspired a theory
(commented in section 3.1), identifying the dark matter with an
as yet undetected dark gas. The magnetic hypothesis would
provide another explanation, as the rotation curve is due in
part to magnetic fields, which are generated by gas. A direct
relation is not obvious when very extended curves are obtained
(Corbelli and Salucci, 1999). Bosma (1998) himself states that
this relation may not be correct.