Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Katja Fahrion, Torsten Böker, Michele Perna, Tracy L. Beck, Roberto Maiolino
| Summary:
We present a detailed study of the centre of NGC4654, 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 and
Hubble Space Telescope imaging of the inner 330 $times$ 330 pc, we find that
the nucleus harbours in fact three massive star clusters. Maps of infrared
emission lines from NIRSpec show different morphologies for the ionised and
molecular gas components. The emission from molecular hydrogen gas is
concentrated at the 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 use surface brightness modelling to measure their
fluxes from ultraviolet to mid-infrared wavelengths and fit their spectral
energy distributions (SEDs). Two of the clusters are UV-bright and well
described by single stellar populations with young ages ($sim$ 3 and 5 Myr)
and low masses ($M_ast sim 4 times 10^{4} – 10^{5} M_odot$), whereas the
central cluster is much more massive ($3 times 10^7 M_odot$), and cannot be
fitted by a single stellar population. Instead, we find that a minor young
population ($sim$ 1 Myr) embedded in a dominant old population ($sim$ 8 Gyr)
is needed 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 NGC4654 a unique laboratory to study NSC
growth from both in-situ star formation and the infall of star clusters.
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