Protoclusters as drivers of stellar mass growth in the early Universe, a case study: Taralay – a massive protocluster at z ∼ 4.57

Simulations predict that the galaxy populations inhabiting protoclusters may contribute considerably to the total amount of stellar mass growth of galaxies in the early universe. In this study, we test these predictions observationally, using the Taralay protocluster (formerly PCl J1001+0220) at z similar to 4.57 in the COSMOS field. With the Charting Cluster Construction with VUDS and ORELSE (C3VO) survey, we spectroscopically confirmed 44 galaxies within the adopted redshift range of the protocluster (4.48 < z < 4.64) and incorporate an additional 18 galaxies from ancillary spectroscopic surveys. Using a density mapping technique, we estimate the total mass of Taralay to be similar to 1.7 x 10(15) M-circle dot, sufficient to form a massive cluster by the present day. By comparing the star formation rate density (SFRD) within the protocluster (SFRDpc) to that of the coeval field (SFRDfield), we find that SFRDpc surpasses the SFRDfield by Delta log (SFRD/M(circle dot)yr(-1) Mpc(-3)) = 1.08 +/- 0.32 (or similar to 12 x). The observed contribution fraction of protoclusters to the cosmic SFRD adopting Taralay as a proxy for typical protoclusters is 33.5 per cent-4.3 per cent+8.0 per cent, a value similar to 2 sigma higher than the predictions from simulations. Taralay contains three peaks that are 5 sigma above the average density at these redshifts. Their SFRD is similar to 0.5 dex higher than the value derived for the overall protocluster. We show that 68 per cent of all star formation in the protocluster takes place within these peaks, and that the innermost regions of the peaks encase similar to 50 per cent of the total star formation in the protocluster. This study strongly suggests that protoclusters drive stellar mass growth in the early universe and that this growth may proceed in an inside-out manner.