The SEEDZ Simulations: Methodology and First Results on Massive Black Hole Seeding and Early Galaxy Growth

TL;DR

SEEDZ addresses how the first massive black holes form and grow in the early Universe by implementing self-consistent light and heavy seed channels within a cosmological, high-resolution hydrodynamic framework. The methodology combines a moving-mesh code with detailed subgrid prescriptions for PopIII/PopII star formation, SN feedback, metal enrichment, and Eddington and super-Eddington BH accretion, including vorticity adjustments and dynamical-friction corrections. The first results at redshift z=15 show MBHs growing much faster than their host galaxies, with heavy seeds reaching $\sim10^6\,M_{\odot}$ and an over-massive BH population, while PopII stellar mass rapidly dominates and heavy-seed formation tends to occur in low-metallicity environments. These findings imply that the canonical local M_BH–M_* relation is not yet established at high redshift and that accretion-driven growth, rather than mergers, dominates MBH evolution in these early epochs. The work provides a framework for comparing MBH demographics with JWST observations and motivates higher-resolution follow-ups to capture light-seed growth and MBH merger dynamics.

Abstract

Here we introduce the SEEDZ simulations, a suite of cosmological hydrodynamic simulations exploring the formation and growth of the first massive black holes in the Universe. SEEDZ includes models for Population III star formation, supernovae explosions and the resulting formation of light seed black holes, metal enrichment and subsequent Population II star formation, heavy seed black hole formation, Eddington and super-Eddington accretion schemes as well as black hole feedback. In this paper, we cover the overall methodologies employed and present our current results at $z=15$. Our main result so far is that black holes initially grow faster than their host galaxy, and hence over-massive black holes are a feature of the high-redshift Universe. The fundamental black hole-galaxy relationships we observe at $z = 0$ (especially the M$_{\rm BH}$ - M$_*$ relationship) likely only emerge in more mature galaxies. At high-redshift, that relationship has not yet been established. We find that even at these high redshifts, MBHs can grow from their initial heavy seed mass of $\sim$10$^4$ M$_\odot$ up to 10$^6$ M$_\odot$. At the high end of our MBH masses, our simulated galaxy M$_{\rm BH}$ - M$_*$ relations match the observed high redshift trends i.e. over-massive BHs with M$_{\rm BH}$/M$_{\rm star} \sim 10^{-2}$. This initial set of simulations will continue to run down to $z=10$, where we will perform a comprehensive comparison of simulated MBH number densities and M$_{\rm BH}$ - M$_*$ relations with JWST observations. Further simulations with higher resolution will then follow.

Figures (12)

• Figure 1: The Parent Box region (see Table \ref{['table:BoxSize']}) shown as a projection of the dark matter density field. The Parent Box is 40 comoving Mpc h$^{-1}$, run to $z = 10$ with only dark matter. From this box, three regions were selected - the Rarepeak region contains the most massive halo in the box at $z = 10$ (M$_{\rm Halo} = 1.8 \times 10^{11}$$\rm{M_{\odot}}$). Both of the normal regions (Normal1 & Normal2) contain halos with masses between $1.5 \times 10^{10}$$\rm{M_{\odot}~}$ and $2.5 \times 10^{10}$$\rm{M_{\odot}}$. The scale line in the bottom right shows 5 comoving Mpc h$^{-1}$.
• Figure 2: The growth of the most massive halo in each of the target regions selected. The most massive halos are coloured and identified in the legend. Other halos from the Parent box simulation are shown in greyscale colour. Outputs from these dark matter only simulations were set at $\delta z = 1.0$ and hence there is some discrete noise effects in halo growth trajectories due to this cadence.
• Figure 3: Halo mass functions for the Rarepeak, Normal1 and Normal2 regions at $z=15$. The halos were identified using the Subfind halo finder. Overlaid (dashed red) is the predicted halo mass function at $z=15$ from Sheth2001 -- labelled "SMT". As expected the Rarepeak region shows a deviation from the analytical prediction at $z = 15$ at the higher mass end, reflective of the overdense environment found in that region. The Normal1 and Normal2 regions show closer agreement to the expected halo distribution.
• Figure 4: Redshift evolution of SmartStar particle masses for the Rarepeak (top) and Normal1 (middle) and Normal2 (bottom) regions, showing PopIII stars (blue), PopII stellar cluster particles (orange) and BHs (black). The left panel of each figure shows the redshift evolution while the right hand panel shows the resulting demographics at $z = 15$. The number of PopII stars is approximately the same in each region, while the number of heavy seed black holes is largest in the Rarepeak region, as expected.
• Figure 5: For each of the simulation boxes, we show the total mass in PopIII stars (blue), BHs (black) and PopII clusters (orange) as a function of redshift. While PopIII stars form first in all cases (see Figure \ref{['fig:growth']}), PopII star formation quickly dominates over the PopIII population. The mass in BHs is approximately 2 orders of magnitude below the mass in stars, with the mass growth in PopII stars exceeding that in BH by approximately 2 orders of magnitude as well.