An X-ray-emitting protocluster at z ≈ 5.7 reveals rapid structure growth

Galaxy clusters are the most massive gravitationally bound structures in the universe and serve as tracers of the assembly of large-scale structure¹. Studying their progenitors, protoclusters, sheds light on the earliest stages of cluster formation. However, detecting protoclusters is demanding: their member galaxies are loosely bound and the emerging hot intracluster medium (ICM) may only be in the initial stages of virialization^2-4. Recent James Webb Space Telescope (JWST) observations located several protocluster candidates by identifying overdensities of z ≳ 5 galaxies^5-9. However, none of these candidates was detected by X-ray observations, which offer a powerful way to unveil the hot ICM. Here we report the combined Chandra and JWST detection of a protocluster, JADES-ID1, at z ≈ 5.68, merely one billion years after the Big Bang. We measure a bolometric X-ray luminosity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>L</mml:mi></mml:mrow> <mml:mrow> <mml:mrow><mml:mrow><mml:mi>bol</mml:mi></mml:mrow> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mn>1.</mml:mn> <mml:msubsup><mml:mrow><mml:mn>5</mml:mn></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>0.6</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn></mml:mrow> </mml:msubsup> </mml:mrow> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow> <mml:mrow><mml:mn>44</mml:mn></mml:mrow> </mml:msup> <mml:mspace></mml:mspace> <mml:mrow><mml:mrow><mml:mi>erg</mml:mi></mml:mrow> </mml:mrow> <mml:mspace></mml:mspace> <mml:msup> <mml:mrow> <mml:mrow><mml:mrow><mml:mi>s</mml:mi></mml:mrow> </mml:mrow> </mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>1</mml:mn></mml:mrow> </mml:msup> </mml:math> and infer a total gravitating mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow> <mml:mrow><mml:mn>500</mml:mn></mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mn>1.</mml:mn> <mml:msubsup><mml:mrow><mml:mn>8</mml:mn></mml:mrow> <mml:mrow><mml:mo>-</mml:mo> <mml:mn>0.7</mml:mn></mml:mrow> <mml:mrow><mml:mo>+</mml:mo> <mml:mn>0.6</mml:mn></mml:mrow> </mml:msubsup> </mml:mrow> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup><mml:mrow><mml:mn>0</mml:mn></mml:mrow> <mml:mrow><mml:mn>13</mml:mn></mml:mrow> </mml:msup> <mml:mspace></mml:mspace> <mml:msub><mml:mrow><mml:mi>M</mml:mi></mml:mrow> <mml:mrow><mml:mo>⊙</mml:mo></mml:mrow> </mml:msub> </mml:math> , making this system a progenitor of today's most massive galaxy clusters. The detection of extended, shock-heated gas indicates that substantial ICM heating can occur in massive halos as early as z ≈ 5.7. Also, given the limited survey volume, the discovery of such a massive cluster is statistically unlikely¹⁰, implying that the formation of the large-scale structure must have occurred more rapidly in some regions of the early universe than standard cosmological models predict.