Signatures of black hole seeding in the local Universe: predictions from the BRAHMA cosmological simulations

The origin of the 'seeds' of supermassive black holes (BHs) continues to be a puzzle, as it is currently unclear if the imprints of early seed formation could survive to today. We examine the signatures of seeding in the local Universe using five [18 Mpc](3)BRAHMA simulation boxes run to z=0. They initialize 1.5 x 10(5) M-circle dot BHs using different seeding models. The first four boxes initialize BHs as heavy seeds using criteria that depend on dense and metal-poor gas, Lyman-Werner radiation, gas spin, and environmental richness. The fifth box initializes BHs as descendants of lower mass seeds (similar to 103 M-circle dot) using a new stochastic seed model built in our previous work. In our simulations, we find that the abundances and properties of similar to 10(5)-10(6) M-circle dot local BHs hosted in M & lowast; less than or similar to 109 M-circle dot dwarf galaxies, are sensitive to the assumed seeding criteria. This is for two reasons: (1) there is a substantial population of local similar to 10(5) M-circle dot BHs that are ungrown relics of early seeds from z similar to 5-10; (2) BH growth up to similar to 10(6) M-circle dot is dominated by mergers in our simulations all the way down to z similar to 0. As the contribution from gas accretion increases, the signatures of seeding start to weaken in more massive greater than or similar to 10(6) M-circle dot BHs, and they are erased for greater than or similar to 10(7) M-circle dot BHs. The different seed models explored here predict abundances of local similar to 10(6) M-circle dot BHs ranging from similar to 0.01-0.05 Mpc(-3) with occupation fractions of similar to 20-100 per cent for M-& lowast;similar to 10(9) M-circle dot galaxies. These results highlight the potential for placing constraints on seeding models using local similar to 10(5)-10(6) M-circle dot BHs hosted in dwarf galaxies. Since merger dynamics and accretion physics impact the persistence of seeding signatures, and both high and low mass seed models can produce similar local BH populations, disentangling their roles will require combining high and low redshift constraints.