Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Robert G. Pascalau, Francesco D’Eugenio, Sandro Tacchella, Roberto Maiolino, Michele Cappellari
| Summary:
JWST observations uncovered a large number of massive quiescent galaxies
(MQGs) at z>3, which theoretical models struggle to reproduce. Explaining the
number density of such objects requires extremely high conversion efficiency of
baryons into stars in early dark matter halos. Using stellar kinematics, we can
investigate the processes shaping the mass assembly histories of MQGs. We
present high-resolution JWST/NIRSpec integral field spectroscopy of GS-9209, a
massive, compact quiescent galaxy at z=4.66 ($log left (M_{ast}/M_{odot}
right) = 10.52 pm 0.06 $, $R_{eff} = 220 pm 20$ pc). Full spectral fitting
of the spatially resolved stellar continuum reveals a clear rotational pattern,
yielding a spin parameter of $lambda_{R_{eff}} = 0.65 pm 0.12$. With its high
degree of rotational support, this galaxy challenges the scenario of MQGs
growing mainly by dry major mergers. This study suggests that at least a
fraction of the earliest quiescent galaxies were fast rotators and that
quenching was dynamically gentle process, preserving the stellar disc even in
highly compact objects. Using Jeans anisotropic modelling (JAM) and a NFW
profile, we measure a dark matter fraction of $f_{rm DM} left (< R_{eff}
right ) = 6.3^{+2.8}_{-1.7}%$, which is plausible given that this galaxy is
extremely compact. Our findings use kinematics to independently confirm the
massive nature of early quiescent galaxies, previously inferred from stellar
population modelling. We suggest that GS-9209 has a similar structure to
low-redshift ‘relic’ galaxies. However, unlike relic galaxies which have
bottom-heavy initial mass functions (IMF), the dynamically inferred stellar
mass-to-light ratio of GS-9209 is consistent with a Milky-Way like IMF. The
kinematical properties of GS-9209 are different from those of z<1 early-type
galaxies and more similar to those of recently quenched post-starburst galaxies
at z>2.
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