Exploring the active galactic nucleus population in protoclusters: Results from the TNG300 simulation and comparison with observations

Present-day galaxy clusters evolved from high-redshift overdensities (z > 2). Galaxy and supermassive black hole (SMBH) formation and evolution in these environments are expected to be accelerated by environmental effects, such as higher merger rates and large gas reservoirs. However, current observational studies aimed at characterizing this enhancement are limited by heterogeneous selection criteria for both protoclusters and active galactic nuclei (AGNs) as well as by the difficulty in confirming them spectroscopically. As a consequence, a robust and unbiased characterization of galaxy growth and AGN activity in dense environments is still lacking. In this work, we investigate the physical properties and cosmic evolution of galaxies and AGNs in a sample of 280 protoclusters identified in the TNG300 simulation selected to end up in z = 0 clusters with M_ 200, ̊m c > 10^ 14 , ̊m M_⊙ . Our goal is to provide the first statistical view of AGN activity enhancement in a uniformly defined sample of overdense environments and to identify the physical mechanisms driving it. We identified protoclusters as the progenitors of present-day galaxy clusters through merger-tree reconstruction and compared their galaxy and AGN populations with a control sample of field galaxies across the redshift range 0 łeq z łeq 6. We investigated galaxy and SMBH demographics, AGN fractions, bolometric luminosity functions, and the SMBH accretion rate density while consistently applying homogeneous selection criteria in all environments. We find that TNG300 protoclusters host systematically more massive galaxies and SMBHs than the field by up to 1 , ̊m dex at all redshifts, with signatures of accelerated galaxy evolution already visible at z ∼ 3-4. The AGN fraction increases with stellar mass in both environments, and at a fixed host galaxy stellar mass, it is broadly consistent between protoclusters and field galaxies, indicating no strong environmental triggering of SMBH accretion. However, when analyzed as a function of redshift, protoclusters exhibit a significant enhancement of AGN activity (by a factor $> 2$), particularly at high luminosities and early cosmic times. We show that this enhancement primarily arises from differences in the stellar mass distributions of galaxies in overdense regions, where massive systems assemble earlier than in the field. Protoclusters consistently dominate the bright end of the AGN bolometric luminosity function and contribute up to ∼ 50% of the total SMBH accretion rate density at z ∼ 6.