Tidally Compressed Gas in Centers of Early‐Type and Ultraluminous Galaxies

Abstract

In this paper we propose that the compressive tidal Ðeld in the centers of Ñat-core early-type galaxies and ultraluminous galaxies compresses molecular clouds producing dense gas observed in the centers of these galaxies. The e †ect of galactic tidal Ðelds is usually considered disruptive in the literature. However, for some galaxies, the mass proÐle Ñattens toward the center and the resulting galactic tidal Ðeld is not disruptive, but instead it is compressive within the Ñat-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 Ñat-core, early-type galaxies and ultralumin-ous galaxies. For early-type galaxies with a core-type luminosity proÐle, the tidal Ðeld of the galaxy is compressive within half the core radius. We have calculated the mean gas densities for molecular gas in a sample of early-type galaxies which have already been detected in CO emission, and we obtain mean densities of SnT D 103È106 cm~3 within the central 100 pc radius. We also use our model to calculate the molecular cloud densities in the inner few hundred parsecs of a sample of ultraluminous galaxies. From the observed rotation curves of these galaxies we show that they have a compressive core within their nuclear region. Our model predicts minimum molecular gas densities in the range 102È104 cm~3 in the nuclear gas disks ; the smaller values are applicable typically for galaxies with larger core radii. The resulting density values agree well with the observed range. Also, for large core radii, even fairly low-density gas (D102 cm~3) can remain bound and stable close to the galactic center.