  Starting from Bosma' (1981) paper, it was demonstrated by different authors
that the observed shape of rotation curves of many spiral galaxies can be
explained if to assume that the radial density distribution of the dark matter
is correlated with the distribution of $HI$: the column densities of the dark
matter and $HI$ are proportional. We show that this correlation is obviously to
be an artifact and can be explained by assuming that the gas surface density is
approximately equal or in general proportional to the critical density for the
local gravitational stability of gaseous layer.
