Overmassive black holes and little red dots naturally form in simulations

Abstract

The origin of supermassive black holes (SMBHs) remains a long-standing problem in astrophysics. Recent JWST observations reveal an unexpectedly abundant population of overmassive black holes at $z>4$--$6$, where the BH masses lie far above local scaling relations and are not reproduced by current cosmological models. How such overmassive black holes form and rapidly grow within young galaxies has remained unclear. Here we present fully cosmological radiation-hydrodynamic simulations that self-consistently follow the birth, early growth, and emergent observable signatures of SMBHs in proto-cluster environments. We find that heavy seeds of order $10^{6}\,M_\odot$ naturally form, exceeding typical theoretical expectations by an order of magnitude. These seeds rapidly develop dense, optically thick disks whose strong electron scattering produces broad H$α$ emission comparable to that seen in little red dots (LRDs). Sustained super-Eddington accretion then drives fast growth to $\sim3\times10^{7}\,M_\odot$ by $z\simeq 8$. This is the first demonstration that unifies LRDs to a short-lived, enshrouded phase of heavy-seed formation, which naturally evolve into the overmassive quasars detected by JWST and ultimately the progenitors of today's SMBHs.