Questioning Planck-selected star-forming high-redshift galaxy protoclusters and their fate

Abstract

About 2100 star-forming galaxy protocluster candidates at z ∼ 1 − 4 were identified at sub-millimetre wavelengths in the Planck all-sky survey. Follow-up spectroscopic observations of a few candidates have confirmed the presence of actual galaxy overdensities with large star formation rates (SFRs). In this work, we use state-of-the-art hydrodynamical simulations to investigate whether the Planck high-z sub-millimetre sources (PHz) are progenitors of massive clusters at z = 0. To match the PHz sources with simulated halos, we select the most star-forming (SF) halos in 19 redshift bins from z = 3 to z = 1.3 in the TNG300 simulation of the IllustrisTNG project. At each redshift, the total SFR of the simulated protocluster candidates is computed from the SFR of all the galaxies within an aperture corresponding to the Planck beam size, including those along the line of sight (LOS). The simulations reproduce the Planck-derived SFRs as the sum of both the SFR of at least one of the most SF high-z halos and the average contribution from SF sources along the LOS. Focusing on the spectroscopically confirmed z ∼ 2 PHz protoclusters, we compare the observed properties of their galaxy members with those in the most SF simulated halos. We find a good agreement in the stellar mass and SFR distributions, and in the galaxy number counts, but the SFR-stellar mass relation of the simulated galaxies tends to be shifted to lower SFRs with respect to the observed galaxies. Based on the estimated final masses of the simulated halos, we infer that between 63% and 72% of the Planck-selected protoclusters will evolve into massive galaxy clusters by z = 0. Despite contamination from star-forming galaxies along the LOS, we thus confirm the efficiency of Planck in selecting star-forming protoclusters at cosmic noon with the simulations, and provide a new criterion for selecting the most massive cluster progenitors at high-z, using observables such as the number of galaxy members and their SFR distribution.