Abstract
We report the spectroscopic confirmation of a massive quiescent galaxy,
GS-9209 at a new redshift record of $z=4.658$, just 1.25 Gyr after the Big
Bang, using new deep continuum observations from JWST NIRSpec. From our
full-spectral-fitting analysis, we find that this galaxy formed its stellar
population over a $\simeq200$ Myr period, approximately $600-800$ Myr after the
Big Bang ($z_form=7.3\pm0.2$), before quenching at
$z_quench=6.7\pm0.3$. GS-9209 demonstrates unambiguously that massive
galaxy formation was already well underway within the first billion years of
cosmic history, with this object having reached a stellar mass of
log$_10(M_*/M_ødot) > 10.3$ by $z=7$. This galaxy also clearly
demonstrates that the earliest onset of galaxy quenching was no later than
$\simeq800$ Myr after the Big Bang. We estimate the iron abundance and
$\alpha$-enhancement, finding Fe/H $= -0.97^+0.06_-0.07$ and
$\alpha$/Fe $= 0.67^+0.25_-0.15$, suggesting the stellar mass vs iron
abundance relation at $z\simeq7$, when this object formed most of its stars,
was $\simeq0.4$ dex lower than at $z\simeq3.5$. Whilst its spectrum is
dominated by stellar emission, GS-9209 also exhibits broad H$\alpha$ emission,
indicating that it hosts an active galactic nucleus (AGN), for which we measure
a black-hole mass of log$_10(M_\bullet/M_ødot) = 8.7\pm0.1$. Although
large-scale star formation in GS-9209 has been quenched for almost half a
billion years, the significant integrated quantity of accretion implied by this
large black-hole mass suggests AGN feedback plausibly played a significant role
in quenching star formation in this galaxy. GS-9209 is also extremely compact,
with an effective radius of just $215\pm20$ parsecs. This intriguing object
offers perhaps our deepest insight yet into massive galaxy formation and
quenching during the first billion years of cosmic history.
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