Early results from GLASS-JWST. XIV: A spectroscopically confirmed protocluster 650 million years after the Big Bang

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We present the spectroscopic confirmation of a protocluster at $z=7.88$ behind the galaxy cluster Abell2744 (hereafter A2744-z7p9OD).

• Using JWST NIRSpec, we find seven galaxies within a projected radius of 60kpc.
• Although the galaxies reside in an overdensity around $>20\times$ greater than a random volume, they do not show strong Lyman-alpha emission.
• We place 2-$\sigma$ upper limits on the rest-frame equivalent width $<16$-$28$AA.