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Arxiv: Growing a nuclear star cluster from star formation and cluster mergers: The JWST NIRSpec view of NGC 4654 Published: 4/13/2024 9:48:10 AM Updated: 4/13/2024 9:48:10 AM

Paper abstract: We present a detailed study of the centre of NGC4654, a Milky Way-like spiralgalaxy in the Virgo cluster that has been reported to host a double stellarnucleus, thus promising a rare view of ongoing star cluster infall into agalaxy nucleus. Analysing JWST NIRSpec integral-field spectroscopic data andHubble Space Telescope imaging of the inner 330 \times 330 pc, we find thatthe nucleus harbours in fact three massive star clusters. Maps of infraredemission lines from NIRSpec show different morphologies for the ionised andmolecular gas components. The emission from molecular hydrogen gas isconcentrated at the NSC location, while emission from hydrogen recombinationlines is more extended beyond the central cluster. The velocity fields of bothgas and stars indicate that the three clusters are part of a complicateddynamical system, with the NSC having an elevated velocity dispersion in linewith its high stellar mass. To investigate the stellar populations of the threeclusters in more detail, we use surface brightness modelling to measure theirfluxes from ultraviolet to mid-infrared wavelengths and fit their spectralenergy distributions (SEDs). Two of the clusters are UV-bright and welldescribed by single stellar populations with young ages (~ 3 and 5 Myr)and low masses (M_\ast ~ 4 \times 10^{4} - 10^{5} M_\odot), whereas thecentral cluster is much more massive (3 \times 10^7 M_\odot), and cannot befitted by a single stellar population. Instead, we find that a minor youngpopulation (~ 1 Myr) embedded in a dominant old population (~ 8 Gyr)is needed to explain its SED. Given its complex composition and the closeproximity of two young star clusters that are likely to merge with it within afew hundred million years, we consider NGC4654 a unique laboratory to study NSCgrowth from both in-situ star formation and the infall of star clusters.