“Cosmic noon” refers to the epoch in the history of the Universe when star formation and black hole growth peaked. It occurred roughly 2 to 3 billion years after the big bang (corresponding to redshifts of z=2~3). Over the last decade, a growing number of studies have identified star-forming (or star-bursting) protoclusters from this time. The Spiderweb protocluster at z=2.16 is a spectacular example of such a high-redshift structure, which is centred on the radio galaxy MRC 1138–262.

Pérez-Martínez et al. have obtained CO(1-0) molecular gas measurements with the Australia Telescope Compact Array on a sample of 43 spectroscopically confirmed H-alpha emitters (HAEs) in the Spiderweb protocluster and investigated the relation between their star formation activities and cold gas reservoirs as a function of environment. Their project is called COALAS: the CO ATCA Legacy Archive of Star-Forming Galaxies (COALAS)! Their environmental analyses suggest that galaxies residing in the outskirts of the protocluster have have larger molecular-to-stellar mass ratios and lower star formation efficiencies than galaxies residing in the core.

The image above shows the Spiderweb protocluster field. Blue circles display the full sample of candidate HAEs. Red circles show those HAEs with measured spectroscopic redshift. Orange crosses and contours respectively show the CO(1-0) emitters detected by ATCA and the limits of the COALAS ATCA footprint. Empty squares depict the subsample of sub-millimetre galaxies in this field. Black stars show the Lyma-alpha Emitters, while X-ray sources are shown by black crosses. The Spiderweb galaxy is marked with a yellow star. A bar with a 1 Mpc physical scale at z=2.16 is shown in the lower-left corner of the diagram for reference.