Signatures of black hole seeding in the local Universe: predictions from the <tt>BRAHMA</tt> cosmological simulations

ABSTRACT 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 $[18~\mathrm{Mpc}]^3$BRAHMA simulation boxes run to $z=0$. They initialize $1.5\times 10^5~\rm {M}_{\odot }$ 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 ($\sim 10^3~\rm {M}_{\odot }$) using a new stochastic seed model built in our previous work. In our simulations, we find that the abundances and properties of $\sim 10^5-10^6~\rm {M}_{\odot }$ local BHs hosted in $M_*\lesssim 10^{9}~\rm {M}_{\odot }$ dwarf galaxies, are sensitive to the assumed seeding criteria. This is for two reasons: (1) there is a substantial population of local $\sim 10^5~\rm {M}_{\odot }$ BHs that are ungrown relics of early seeds from $z\sim 5-10$; (2) BH growth up to $\sim 10^6~\rm {M}_{\odot }$ is dominated by mergers in our simulations all the way down to $z\sim 0$. As the contribution from gas accretion increases, the signatures of seeding start to weaken in more massive $\gtrsim 10^6~\rm {M}_{\odot }$ BHs, and they are erased for $\gtrsim 10^7~\rm {M}_{\odot }$ BHs. The different seed models explored here predict abundances of local $\sim 10^6~\rm {M}_{\odot }$ BHs ranging from $\sim 0.01-0.05~\mathrm{Mpc}^{-3}$ with occupation fractions of $\sim 20-100~{{\ \rm per\ cent}}$ for $M_*\sim 10^{9}~\rm {M}_{\odot }$ galaxies. These results highlight the potential for placing constraints on seeding models using local $\sim 10^5-10^6~\rm {M}_{\odot }$ 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.