Search for: All records

ABSTRACT A major open question in astrophysics is the mechanisms by which massive black holes (BHs) form in the early Universe, which pose constraints on seeding models. We study BH formation and evolution in a flexible model combining the cosmological IllustrisTNG (TNG) simulations with semi-analytic modelling in post-processing. We identify our TNG model hosts based on various criteria including a minimum gas mass of $10^7$$–$$10^9$${\rm M}_{\odot }$$, total host mass of $$10^{8.5}$$–$$10^{10.5}$${\rm M}_{\odot }$$, and a maximum gas metallicity of 0.01–0.1 $$\mathrm{Z}_{\odot }$$. Each potential host is assigned a BH seed with a probability of 0.01–1. The populations follow the TNG galaxy merger tree. This approach improves upon the predictive power of the simple TNG BH seeding prescription, narrowing down plausible seeding parameter spaces, and it is readily adaptable to other cosmological simulations. Several model realizations predict $$z\lesssim 4$$ BH mass densities that are consistent with empirical data as well as the TNG BHs. However, high-redshift BH number densities can differ by factors of $$\sim$$ 10 to $$\gtrsim$$ 100 between seeding parameters. In most model realizations, $$\lesssim 10^5$${\rm M}_{\odot }$$ BHs substantially outnumber heavier BHs at high redshifts. Mergers between such BHs are prime targets for gravitational-wave detection with Laser Interferometer Space Antenna. The $z=0$ BH mass densities in most realizations of the model agree well with observations, but our strictest seeding criteria fail at high redshift. Our findings strongly motivate the need for better empirical constraints on high-z BHs, and they underscore the significance of recent active galactic nucleus discoveries with JWST. 

ABSTRACT While the first “seeds” of supermassive black holes (BH) can range from $$\sim 10^2-10^6 \rm ~{\rm M}_{\odot }$$, the lowest mass seeds ($$\lesssim 10^3~\rm {\rm M}_{\odot }$$) 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 $$z\ge 7$$ BH populations for low-mass seeds. We ran two types of boxes that model $$\sim 10^3~\rm {\rm M}_{\odot }$$ 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 $$\sim 10^3~\rm {\rm M}_{\odot }$$ seeds and place them within haloes with dense, metal-poor gas. Second, we have lower resolution, larger volume $$[18~\mathrm{Mpc}]^3$$ (BRAHMA-18-E4), and $$\sim [36~\mathrm{Mpc}]^3$$ (BRAHMA-36-E5) boxes that seed their smallest resolvable $$\sim 10^4~\&~10^5~\mathrm{{\rm M}_{\odot }}$$ BH descendants using new stochastic seeding prescriptions calibrated using BRAHMA-9-D3. The three boxes together probe key BH observables between $$\sim 10^3\,\mathrm{ and}\,10^7~\rm {\rm M}_{\odot }$$. The active galactic nuclei (AGN) luminosity function variations are small (factors of $$\sim 2-3$$) at the anticipated detection limits of potential future X-ray facilities ($$\sim 10^{43}~ \mathrm{ergs~s^{-1}}$$ at $$z\sim 7$$). Our simulations predict BHs $$\sim 10-100$$ times heavier than the local $$M_*$$ versus $$M_{\mathrm{ bh}}$$ relations, consistent with several JWST-detected AGN. For different seed models, our simulations merge binaries at $$\sim 1-15~\mathrm{kpc}$$, with rates of $$\sim 200-2000$$ yr−1 for $$\gtrsim 10^3~\rm {\rm M}_{\odot }$$ BHs, $$\sim 6-60$$ yr−1 for $$\gtrsim 10^4~\rm {\rm M}_{\odot }$$ BHs, and up to $$\sim 10$$ yr−1 amongst $$\gtrsim 10^5~\rm {\rm M}_{\odot }$$ BHs. These results suggest that Laser Interferometer Space Antenna mission has promising prospects for constraining seed models.