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

This is the authors' abstract. We don't add key points for this paper.

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~Mpc]³ (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~Mpc]³ (BRAHMA-18-E4), and ~ [36~Mpc]³ (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 (~ 10⁴³~ ergs~s⁻¹ at z~ 7). Our simulations predict BHs ~ 10-100 times heavier than the local M_* versus Mbh 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.