Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Yoshinobu Fudamoto, Jakob M. Helton, Xiaojing Lin, Fengwu Sun, Peter Behroozi
| Summary:
We report the discovery of a galaxy proto-cluster candidate (dubbed
MACS0416-OD-z8p5) at a spectroscopic redshift of $zsim8.47$, dating back to
$sim550$Myr after the Big Bang. The observations are part of the JWST Cycle-3
treasury program, Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey
(SAPPHIRES) with NIRCam-grism. Using wide field slitless spectroscopy (WFSS)
obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at
$z_{rm spec}sim8.47$ via emission line detections of [OIII]5008A (with
$>5,sigma$) and tentatively confirm one additional galaxy (at
$sim3,sigma$). This discovery represents the highest-redshift,
spectroscopically confirmed galaxy over-density known to date, which is
$sim6$–$8$ times more dense than the average volume density of galaxies at
the same redshift. Furthermore, a galaxy hosting a low-mass active galactic
nucleus (“Little-Red-Dot”) is found as a member, suggesting an early
emergence of active, massive black holes and feedback between these black holes
and their surrounding environments. We also discuss the spatial structures
connecting the galaxy over-density to nearby massive star-forming galaxies
(separated by $sim 5$pMpc, including MACS0416-Y1 and MACS0416-JD. This finding
of a massive dark matter halo hosting a galaxy over-density at $zsim8.5$ is
surprising given that our survey covered only a small, random field
($16.5,{rm arcmin^2}$) as part of a pure parallel observation. The comparison
with cosmological simulations shows that the likelihood of finding such a
large-scale structure is $<5,%$ under the current galaxy formation scenario
and the observed survey volume. Our results demonstrate the power of WFSS
observations to build a complete line-emitter sample and suggest an important
role for over-densities in enhancing galaxy formation by funneling large-scale
gas supplies into small cosmological volumes.
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