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 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.