Nuclear angular momentum of early-type galaxies hosting nuclear star clusters

Abstract

Nucleation is a common phenomenon in all types of galaxies and at least 70% of them host nuclear star clusters (NSCs) in their centres. Many of the NSCs co-habit with super-massive black holes and follow similar scaling relations with host galaxy properties. NSCs, unlike black holes, preserve the signature of their evolutionary path imprinted onto their kinematics and stellar populations. Thus their study provides us with important information about the formation of galactic nuclei. In this paper we explored the angular momentum of the nuclei of six intermediate mass (9.7 > log(Mdyn/M_sun) > 10.6) early-type galaxies in the Fornax cluster that host NSCs. Our goal was to derive a link between the nuclear angular momentum and the proposed formation scenarios of NSCs. We used Adaptive Optics assisted IFU observations with VLT/SINFONI to derive the spatially resolved stellar kinematics of the galaxy nuclei. We measured their specific stellar angular momenta ($\lambda_\mathrm{Re}$), and compared these with Milky Way globular clusters and N-body simulations of NSC formation. We found that all studied nuclei exhibit varied stellar kinematics. Their specific stellar angular momenta and ellipticities are similar to Milky Way globular clusters (GCs). Five out of six galaxy nuclei are consistent with the $\lambda_\mathrm{Re} - \epsilon_\mathrm{e}$ of simulated NSCs embedded in a contaminating nuclear bulge that have formed via the in-spiralling and merging of GCs. It has previously been suggested that the NSCs in higher mass galaxies, like the ones studied in this paper, form via dissipational sinking of gas onto the galactic nuclei with hints that some might also involve the merger of GCs. Here we showed that we cannot exclude the pure GC merging scenario as a viable path for the formation of NSCs.