SEEDZ: Rapid Galaxy Assembly as a Pathway to Supermassive Stars, Dense Stellar Environments and Massive Black Hole Seeds

Lewis R. Prole, John A. Regan, Daxal Mehta, Devesh Nandal, Rüdiger Pakmor, Ricarda S. Beckmann, Michael Tremmel, Martin G. Haehnelt, Simon C. O. Glover, Ralf S. Klessen, John H. Wise, Sophie Koudmani, Martin A. Bourne, Debora Sijacki, John Brennan, Pelle van de Bor, Paul C. Clark

First listed 2026-06-29 | Last updated 2026-06-25

Abstract

We investigate the assembly history of early galaxies in the SEEDZ hydrodynamic simulations, to investigate the high inflow rates believed to be required for the formation of supermassive stars (SMSs), dense stellar clusters and subsequently heavy seed black holes. Using a heavy seed formation criteria of $>$1 M$_\odot$ yr$^{-1}$ flowing into 10 pc regions, we find that heavy seeds form in halos that grow rapidly compared to those halos that never meet the criteria. Halos with growth rates of $\gtrsim$1 M$_\odot$ yr$^{-1}$ at their virial radius (scales of a few hundred pc) are able to sustain a flow rate of 0.1 M$_\odot$ yr$^{-1}$ into the inner 1 pc of the halo, maintaining higher density environments within the central 10 - 100~pc. These halos continue to grow rapidly after their initial collapse, typically forming heavy seeds $\sim$100 Myr after forming their first stars and stellar mass black holes. By $z=10$, most heavy seeds form in regions of near-solar metallicity, although a minority of heavy seeds do continue to form in low metallicity (10$^{-2}$ Z$_\odot$) regions. Under the assumption that a SMS forms as the progenitor to a heavy seed if it forms in a region of low (10$^{-2}$ Z$_\odot$) metallicity, and can sustain high accretion rates above 0.02 M$_\odot$ yr$^{-1}$ throughout the SMS lifetime of 2 Myr, we find a number density of SMSs of 0.1 cMpc$^{-3}$, meaning that only a fraction of 10$^{-4}$ of these SMSs would need to be visible to JWST to account for the observed population of Little Red Dot galaxies.

Short digest

Using the SEEDZ hydrodynamic simulations, this paper argues that the key pathway to heavy black-hole seed formation is not pristine isolation but unusually rapid galaxy assembly: halos with virial-scale growth rates of at least about 1 M_sun/yr are the ones that drive the >1 M_sun/yr inflows into 10 pc required by the model, while also sustaining ~0.1 M_sun/yr into the inner 1 pc and keeping the central 10-100 pc dense. In these systems, heavy seeds generally appear about 100 Myr after the first stars and stellar-mass black holes, linking seed formation to continued post-collapse growth rather than the very first episode of star formation. By z=10, most seeds in the simulation form in near-solar-metallicity environments, with only a minority in low-metallicity gas, so the paper shifts the emphasis from classic metal-free channels toward rapidly assembled, already enriched galaxies. Under the paper's explicit assumption that only low-metallicity seeds with sustained >0.02 M_sun/yr accretion for 2 Myr become supermassive stars, the inferred SMS number density is 0.1 cMpc^-3, high enough that only 10^-4 would need to be observable to explain the Little Red Dot population.

Key figures to inspect

• Figure 1. Use this as the setup figure because it makes the paper's main claim visually obvious: heavy-seed-forming halos have systematically faster assembly histories, higher growth rates, and more merger-driven mass build-up than halos that never cross the inflow threshold. It directly connects large-scale halo growth to the small-scale heavy-seed criterion that drives the rest of the paper.
• Figure 3. This is the core physical-diagnostic figure showing that heavy-seed-forming halos differ structurally, not just historically. The radial profiles demonstrate the enhanced inflow, higher central densities, and larger enclosed gas masses that distinguish seed-forming systems within the central 10-100 pc and support the claim that virial-scale accretion feeds dense inner regions.
• Figure 5. Include this figure to document whether newly formed seeds remain in the high-accretion regime relevant for a supermassive-star progenitor. The accretion histories over the first 2 Myr and the rate distribution at 2 Myr are the paper's direct bridge between the heavy-seed prescription and the sustained >0.02 M_sun/yr condition invoked for SMS survival and growth.
• Figure 6. This is the most efficient figure for the metallicity-dependent interpretation. It shows how formation metallicity correlates with early growth, surrounding gas supply, and formation redshift, while explicitly marking the metallicity-accretion region the authors identify as compatible with SMS formation, which is central to their LRD relevance argument.
• Figure 8. Use this as the synthesis figure because it converts the simulation results into population-level implications. By separating all heavy seeds, metal-enriched cluster-like channels, and the low-metallicity plus sustained-inflow SMS subset, it delivers the bottom-line number-density comparison that motivates the connection to observed Little Red Dots.