Kavli Affiliate: Debora Sijacki
| First 5 Authors: Callum Witten, Callum Witten, , ,
| Summary:
Protoclusters represent the most extreme environments in the very early
Universe. They form from large-scale dark matter overdensities, harbouring an
overabundance of galaxies fed by large gas reservoirs. Their early and
accelerated evolution results in a distinct difference in the properties of
galaxies resident in protoclusters versus the field, which is known to be in
place by $zsim 5-6$. We utilise JWST NIRCam observations of the A2744-z7p9OD
protocluster at $z=7.88$ to constrain the properties of resident galaxies. We
identify seven new protocluster members, bringing the total number to 23 and
the total stellar mass of the protocluster to in excess of $10^10
rmM_odot$. These galaxies are remarkably evolved just 650 Myr after the
Big Bang, preferentially showing redder UV-slopes and stronger Balmer breaks
than is typical of field galaxies. We use the PROSPECTOR spectral energy
distribution fitting code to derive key galaxy properties, finding distinct
populations in the core versus the outskirts of the protocluster. The core is
largely composed of dusty, massive galaxies which can be characterised as
undergoing a synchronised (mini)-quenched phase, while galaxies in the
protocluster outskirts are undergoing recent bursts of star formation. Finally,
a strong suppression of the continuum around the Ly$alpha$-break evidences
extreme neutral hydrogen column densities in many resident galaxies ($N_rm
HIgtrsim10^23 rm cm^-2$). The A2744-z7p9OD system is the most
extreme, evolved overdensity yet observed at $z>7$, with higher stellar masses,
gas densities, and dust attenuation, revealing the intersection of local
environment and high-redshift galaxy formation at their extremes.
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