Introducing the BRAHMA Simulation Suite: Signatures of Low Mass Black Hole Seeding Models in Cosmological Simulations

While the first "seeds" of supermassive black holes (BH) can range from 10(2)-10(6) M-circle dot, the lowest mass seeds (less than or similar to 10(3) M-circle dot) are inaccessible to most cosmological simulations due to resolution limitations. We present our new BRAHMA simulations that use a novel flexible seeding approach to predict the z >= 7 BH populations for low-mass seeds. We ran two types of boxes that model similar to 10(3)M(circle dot) seeds using two distinct but mutually consistent seeding prescriptions at different simulation resolutions. First, we have the highest resolution [9Mpc](3) (BRAHMA-9-D3) boxes that directly resolve similar to 10(3) M-circle dot seeds and place them within haloes with dense, metal-poor gas. Second, we have lower resolution, larger volume [18Mpc](3) (BRAHMA-18-E4), and similar to [36Mpc](3) (BRAHMA-36-E5) boxes that seed their smallest resolvable similar to 10(4) & 10(5) M-circle dot BH descendants using new stochastic seeding prescriptions calibrated using BRAHMA-9-D3. The three boxes together probe key BH observables between similar to 10(3) and 10(7) M-circle dot. The active galactic nuclei (AGN) luminosity function variations are small (factors of similar to 2-3) at the anticipated detection limits of potential future X-ray facilities (similar to 10(43) ergs s(-1) z similar to 7). Our simulations predict BHs similar to 10-100 times heavier than the local M-* versus M-bh relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge binaries at similar to 1-15kpc, with rates of similar to 200-2000 yr(-1) for greater than or similar to 10(3) M-circle dot BHs, similar to 6-60 yr(-1) for greater than or similar to 10(4) M-circle dot BHs, and up to similar to 10 yr(-1) amongst greater than or similar to 10(5) M-circle dot BHs. These results suggest that Laser Interferometer Space Antenna mission has promising prospects for constraining seed models.