Growing a nuclear star cluster from star formation and cluster mergers: The JWST NIRSpec view of NGC 4654^⋆

¹ European Space Agency, European Space Research and Technology Centre, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
e-mail: katja.fahrion@esa.int
² European Space Agency, c/o STScI, 3700 San Martin Drive, Baltimore, MD 21218, USA
³ Centro de Astrobiología (CAB), CSIC–INTA, Departamento de Astrofísica, Cra. de Ajalvir Km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁷ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁸ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 Bis bd Arago, 75014 Paris, France
⁹ Dipartimento di Fisica e Astronomia, Universitá di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
¹⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ University of Trento, Via Sommarive 14, 38123 Trento, Italy

Received: 16 February 2024
Accepted: 12 April 2024

Abstract

We present a detailed study of the centre of NGC 4654, a Milky Way-like spiral galaxy in the Virgo cluster that has been reported to host a double stellar nucleus, thus promising a rare view of ongoing star cluster infall into a galaxy nucleus. Analysing JWST NIRSpec integral-field spectroscopic data in combination with Hubble Space Telescope Wide Field Camera 3 imaging of the inner 330 × 330 pc, we find that the NGC 4645 nucleus is in fact more complex than previously thought, harbouring three massive star clusters within 32 pc of the centre. Maps of infrared emission lines in the NIRSpec spectra show different morphologies for the ionised and molecular gas components. The emission from molecular hydrogen gas is concentrated at the nuclear star cluster (NSC) location, while emission from hydrogen recombination lines is more extended beyond the central cluster. The velocity fields of both gas and stars indicate that the three clusters are part of a complicated dynamical system, with the NSC having an elevated velocity dispersion in line with its high stellar mass. To investigate the stellar populations of the three clusters in more detail, we used surface brightness modelling to measure their fluxes from UV to mid-infrared wavelengths. This information, together with spectroscopically derived extinction values, are then used to fit the spectral energy distributions (SEDs) of the clusters. Two of the clusters are UV-bright and well described by single stellar populations with young ages (∼3 and 5 Myr) and relatively low masses (M_* ∼ 4 × 10⁴ M_⊙ and M_* ∼ 10⁵ M_⊙, respectively), whereas the central cluster is much more massive (M_* = 3 × 10⁷ M_⊙), and cannot be fitted by a single stellar population. Instead, we find that the presence of a minor young population (∼1 Myr, M_* ∼ 3 × 10⁴ M_⊙) embedded in a dominant old population (∼8 Gyr) is required to explain its SED. Given its complex composition and the close proximity of two young star clusters that are likely to merge with it within a few hundred million years, we consider the nucleus of NGC 4654 a unique laboratory to study NSC growth from both in situ star formation and the infall of star clusters.