TNG300 Finds Protocluster AGN Boost at z~6 Driven by Early Massive Galaxy Assembly, Not Environmental Triggering
TNG300 protoclusters show accelerated SMBH growth at early times solely because massive galaxies form earlier, not because the dense environment directly fuels black holes. The result reframes how we interpret AGN overdensities observed in high-redshift clusters and supplies a concrete prediction for next-generation surveys.
The study reconstructs merger trees to identify protocluster progenitors and applies uniform selection to galaxies and SMBHs across 0 ≤ z ≤ 6. At fixed stellar mass the AGN fraction matches the field, ruling out strong environmental triggering, yet the overall AGN luminosity function shows protoclusters dominating the bright end and supplying up to 50% of the cosmic SMBH accretion rate density by z6. This pattern emerges because the stellar-mass function in protoclusters is already skewed toward systems above 10^11 solar masses at z3–4.
Comparison with earlier TNG100 and Illustris results reveals the same mass-assembly bias but now quantified with 280 independent volumes, reducing cosmic variance. The simulation’s subgrid AGN feedback produces realistic bolometric luminosities without over-quenching, allowing direct comparison to Chandra and JWST protocluster surveys that currently suffer from small-number statistics.
Future wide-field X-ray and infrared surveys will test whether the predicted factor-of-two enhancement persists when selection functions are matched exactly. Discrepancies would point to missing physics such as merger-driven accretion or different SMBH seeding in dense environments.
The key limitation remains reliance on a single subgrid model; varying AGN feedback efficiency or seeding prescriptions could shift the redshift at which the enhancement appears.
Traina et al.: JWST NIRSpec and Chandra observations of spectroscopically confirmed z>4 protoclusters will detect an AGN fraction at least 1.8 times higher than field controls above Lbol=10^44 erg/s by 2028.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2609.09297)
- [2]Supporting Source(https://arxiv.org/abs/1707.03395)
- [3]Supporting Source(https://arxiv.org/abs/1812.00043)