2024MNRAS.528.3679S

As a considerable investment of time from various telescope facilities was dedicated toward studying the Spiderweb protocluster at z = 2.2, it so far remains one of the most extensively studied protocluster. We report here the latest results in this field, adding a new dimension to previous research on cluster formation at high redshift. Previous studies have reported a significant overdensity (δ ∼ 10) of massive Hα (+ [N II])-emitting galaxies in 3700 comoving Mpc³. Many of these were previously considered to be dusty, actively star-forming galaxies, given their rest-frame optical and infrared features. However, this study argues that a third of them are more likely to be 'passively evolving' galaxies with low-luminosity active galactic nuclei (AGNs) rather than star-forming galaxies, given the multiwavelength spectral energy distribution (SED) fitting including an AGN component. For their SED-based star formation rates to be valid, bulk of their Hα + [N II] emission should come from the central AGNs. This difference in interpretation between this work and past studies, including ours, is particularly supported by the recent deep Chandra/X-ray observation. Furthermore, we have spectroscopically confirmed a quiescent nature for one of these AGNs, with its multiple stellar absorption lines but also low-ionization emission lines. This important update provides new insights into the role of AGNs in forming the cluster red sequence observed in the present-day universe.