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

The first "seeds" of supermassive black holes (BHs) continue to be an outstanding puzzle, and it is currently unclear whether the imprints of early seed formation survive today. Here 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 $1.5\times10^5~M_{\odot}$ BHs using different seeding models. The first four boxes initialize BHs as heavy seeds using criteria that depend on dense & metal-poor gas, Lyman-Werner radiation, gas spin, and environmental richness. The fifth box initializes BHs as descendants of lower mass seeds ($\sim10^3~M_{\odot}$) using a new stochastic seed model built in our previous work. We find that strong signatures of seeding survive in $\sim10^5-10^6~M_{\odot}$ local BHs hosted in $M_*\lesssim10^{9}~M_{\odot}$ dwarf galaxies. The signatures survive due to two reasons: 1) there is a substantial population of local $\sim10^5~M_{\odot}$ BHs that are ungrown relics of early seeds from $z\sim5-10$; 2) BH growth up to $\sim10^6~M_{\odot}$ is dominated by mergers all the way down to $z\sim0$. As the contribution from gas accretion increases, the signatures of seeding start to weaken in more massive $\gtrsim10^6~M_{\odot}$ BHs, and they eventually disappear for $\gtrsim10^7~M_{\odot}$ BHs. This is in contrast to high-z ($z\gtrsim5$) BH populations wherein the BH growth is fully merger dominated, which causes the seeding signatures to persist at least up to $\sim10^8~M_{\odot}$. The different seed models predict abundances of local $\sim10^6~M_{\odot}$ BHs ranging from $\sim0.01-0.05~\mathrm{Mpc}^{-3}$ with occupation fractions of $\sim20-100\%$ in $M_*\sim10^{9}~M_{\odot}$ galaxies. Our results highlight the potential for local $\sim10^5-10^6~M_{\odot}$ BH populations in dwarf galaxies to serve as a promising probe for BH seeding models.