The Young and the Wild: What Happens to Protoclusters Forming at Redshift z ≈ 4?

Abstract

Using one of the largest volumes of the hydrodynamical cosmological simulation suit Magneticum, we study the evolution of protoclusters identified at redshift ≈ 4, with properties similar to the well-observed protocluster SPT2349-56. We identify 42 protoclusters in the simulation as massive and equally rich in substructures as observed, confirming that these observed structures can already be virialized. The dynamics of the internally fast-rotating member galaxies within these protoclusters resemble observations, merging rapidly to form the cores of the brightest cluster galaxies of the assembling clusters. Half of the gas reservoir of these structures is in a hot phase, with the metal enrichment at a very early stage. These systems show a good agreement with the observed amount of cold star-forming gas, largely enriched to solar values. We predict that some of the member galaxies are already quenched at z ≈ 4, rendering them undetectable through measurements of their gas reservoirs. Tracing the evolution of protoclusters reveals that none of the typical mass indicators at high redshift are good tracers to predict the present-day mass of the system. We find that none of the simulated protoclusters at z = 4.3 are among the top ten most massive clusters at redshift z = 0.2, with some barely reaching masses of M ≈ 2 × 1014 M ⊙. Although the average star formation and mass growth rates in the simulated galaxies match observations at high redshift reasonably well, the simulation fails to reproduce the extremely high total star formation rates within the observed protoclusters, indicating that the subgrid models are lacking the ability to reproduce a higher star formation efficiency (or lower depletion timescales).