Introducing the BRAHMA simulation suite: signatures of low-mass black hole seeding models in cosmological simulations

ABSTRACT While the first “seeds” of supermassive black holes (BH) can range from 10²-10⁶ ~ M, the lowest mass seeds (10³~ M) 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 10³~ M seeds using two distinct but mutually consistent seeding prescriptions at different simulation resolutions. First, we have the highest resolution 9~Mpc³ (BRAHMA-9-D3) boxes that directly resolve 10³~ M seeds and place them within haloes with dense, metal-poor gas. Second, we have lower resolution, larger volume 18~Mpc³ (BRAHMA-18-E4), and 36~Mpc³ (BRAHMA-36-E5) boxes that seed their smallest resolvable 10⁴~\&~10⁵~ M BH descendants using new stochastic seeding prescriptions calibrated using BRAHMA-9-D3. The three boxes together probe key BH observables between 10³\, and\, 10⁷~ M. The active galactic nuclei (AGN) luminosity function variations are small (factors of 2-3) at the anticipated detection limits of potential future X-ray facilities (10^43~ ergs~s^-1 at z 7). Our simulations predict BHs 10-100 times heavier than the local M_* versus M ₁₇ relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge binaries at 1-15~kpc, with rates of 200-2000 yr−1 for 10³~ M BHs, 6-60 yr−1 for 10⁴~ M BHs, and up to 10 yr−1 amongst 10⁵~ M BHs. These results suggest that Laser Interferometer Space Antenna mission has promising prospects for constraining seed models.