2008ApJ...674..653E

For a density that is not too sharply peaked toward the center, the local tidal field becomes compressive in all three directions. Available gas can then collapse and form a cluster of stars in the center, including or even being dominated by a central black hole. We show that for a wide range of (deprojected) Sérsic profiles in a spherical potential, the tidal forces are compressive within a region that encloses most of the corresponding light of observed nuclear clusters in both late- and early-type galaxies. In such models, tidal forces become disruptive nearly everywhere for relatively large Sérsic indices n≳3.5. We also show that the mass of a central massive object (CMO) required to remove all radial compressive tidal forces scales linearly with the mass of the host galaxy. If CMOs formed in (progenitor) galaxies with n∼1, we predict a mass fraction of ∼0.1% to 0.5%, consistent with observations of nuclear clusters and supermassive black holes. While we find that tidal compression possibly drives the formation of CMOs in galaxies, beyond the central regions and on larger scales in clusters disruptive tidal forces might contribute to prevent gas from cooling.