Massive black holes or stars first: the key is the residual cosmic electron fraction

Recent JWST observations have unveiled that the first supermassive black holes (SMBHs) were in place at z ≥10, a few hundred Myrs after the Big Bang. These disco v eries are pro viding strong constraints on the seeding of BHs and the nature of the first objects in the Universe. Here, we study the impact of the freeze-out electron fractions ( f e ) at the end of the epoch of cosmic recombination on the formation of the first structures in the Universe. At f e below the current fiducial cosmic values of ∼10-4 , the baryonic collapse is delayed due to the lack of molecular hydrogen cooling until the host halo masses are increased by 1 to 2 orders of magnitude compared to the standard case and reach the atomic cooling limit. This results in an enhanced enclosed gas mass by more than an order of magnitude and higher inflow rates of up to 0 . 1 M⊙yr-1 . Such conditions are conducive to the formation of massive seed BHs with ∼10-4 M⊙. Our results reveal a new pathway for the formation of massive BH seeds which may naturally arise from freeze-out conditions in the early Universe.