Tidally Compressed Gas in Centers of Early Type and Ultraluminous Galaxies

Abstract

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.