Constraining the role of star cluster mergers in nuclear cluster formation: Simulations confront integral-field data

TL;DR

This study combines observations, dynamical models, and simulations to investigate whether star cluster mergers alone can explain the formation of nuclear clusters, concluding that gas dissipation is essential for their development.

Contribution

The paper provides a comprehensive comparison between simulations and observations, demonstrating the limitations of pure stellar mergers and highlighting the need for gas dissipation in nuclear cluster formation.

Findings

Star cluster mergers can produce diverse nuclear cluster properties.

Pure stellar mergers cannot fully reproduce observed kinematic profiles.

Gas dissipation is necessary for forming at least 50% of nuclear cluster mass.

Abstract

We present observations and dynamical models of the stellar nuclear clusters (NCs) at the centres of NGC 4244 and M33. We then compare these to an extensive set of simulations testing the importance of purely stellar dynamical mergers on the formation and growth of NCs. Mergers of star clusters are able to produce a wide variety of observed properties, including densities, structural scaling relations, shapes (including the presence of young discs) and even rapid rotation. Nonetheless, difficulties remain, most notably that the second order kinematic moment V_rms = (V^2 + sigma^2)^(1/2) of the models is too centrally peaked to match observations. This can be remedied by the merger of star clusters onto a pre-existing nuclear disc, but the line-of-sight velocity V is still more slowly rising than in NGC 4244. Our results therefore suggest that purely stellar dynamical mergers cannot form NCs, and that gas dissipation is a necessary ingredient for at least ~50% of a NC's mass. The negative vertical anisotropy found in NGC 4244 however requires at least 10% of the mass to be accreted as stars, since gas dissipation and in situ star formation leads to positive vertical anisotropy.