  In this paper we propose that the compressive tidal field in the centers of
flat-core early type galaxies and ultraluminous galaxies compresses molecular
clouds producing dense gas obseved in the centers of these galaxies. The effect
of galactic tidal fields is usually considered disruptive in the literature.
However, for some galaxies, the mass profile flattens towards the center and
the resulting galactic tidal field is not disruptive but instead it is
compressive within the flat-core region. We have used the virial theorem to
determine the minimum density of a molecular cloud to be stable and
gravitationally bound within the tidally compressive region of a galaxy. We
have applied the mechanism to determine the mean molecular cloud densities in
the centers of a sample of flat-core, early-type galaxies and ultraluminous
galaxies.
