The first "seeds" of supermassive black holes (BH) can range from $\sim10^2-10^6~M_{ \odot}$. However, the lowest mass seeds ($\lesssim10^3 M_{ \odot}$) are inaccessible to most cosmological simulations due to resolution limitations. We present our new BRAHMA suite of cosmological simulations that uses a novel flexible seeding approach to represent low mass seeds. Our suite consists of two types of boxes that model $\sim10^3~M_{ \odot}$ seeds using two distinct but mutually consistent seeding prescriptions at different simulation resolutions. First, we have the highest resolution $[9~\mathrm{ Mpc}]^3$ (BRAHMA-9-D3) boxes that directly resolve $\sim10^3~M_{ \odot}$ seeds and place them within halos with dense and metal poor gas. Second, we have lower-resolution and larger-volume $[18~\mathrm{ Mpc}]^3$ (BRAHMA-18-E4) and $\sim[36~\mathrm{ Mpc}]^3$ (BRAHMA-36-E5) boxes that seed their smallest resolvable $\sim10^4~\&~10^5~\mathrm{ M_{ \odot}}$ BH descendants using new stochastic seeding prescriptions calibrated using the BRAHMA-9-D3 results. The three boxes together probe BHs between $\sim10^3-10^7 M_{ \odot}$ at $z>7$ and we predict their key observables. The variation in the AGN luminosity functions is small (factors of $\sim2-3$) at the anticipated detection limits of potential future X-ray facilities ($\sim10^{ 43} \mathrm{ ergs~s^{ -1}}$ at $z\sim7$). Our simulations predict BHs $\sim10-100$ times heavier than expectations from local $M_*$ vs $M_{ bh}$ relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge BH binaries at $\sim1-15~\mathrm{ kpc}$, with rates of $\sim200-2000$ per year for $\gtrsim10^3 M_{ \odot}$ BHs, $\sim6-60$ per year for $\gtrsim10^4~M_{ \odot}$ BHs, and up to $\sim10$ per year amongst $\gtrsim10^5 M_{ \odot}$ BHs. These results suggest that the LISA mission has promising prospects for constraining seed models.
Categories: astro-ph.GA