Formation of central massive objects via tidal compression

TL;DR

This paper explores how tidal compression in galaxies can lead to the formation of central massive objects like black holes and nuclear clusters, with the process depending on galaxy profiles and potentially explaining observed mass fractions.

Contribution

It demonstrates that tidal forces are compressive in galaxy centers for a range of profiles and predicts a linear relation between CMO mass and galaxy mass, aligning with observations.

Findings

Tidal forces are compressive within regions enclosing most nuclear cluster light.

Disruptive tidal forces dominate for Sersic index n >= 3.5.

CMO mass scales linearly with host galaxy mass, matching observed mass fractions.

Abstract

For a density that is not too sharply peaked towards 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) Sersic profiles in a spherical potential, the tidal forces are compressive within a region which encloses most of the corresponding light of observed nuclear clusters in both late-type and early-type galaxies. In such models, tidal forces become disruptive nearly everywhere for relatively large Sersic 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 ~ 0.1-0.5%, consistent with observations of nuclear clusters and super-massive 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.