Population III star formation near high-redshift active galactic nuclei

Ethan M. Fisk, Madeline A. Marshall, Phoebe R. Upton Sanderbeck, Jarrett L. Johnson

First listed 2025-08-14 | Last updated 2026-04-12

Abstract

Using cosmological radiation-hydrodynamical simulations, we study the effect of accreting supermassive black holes (SMBHs) on nearby dark-matter (DM) haloes in the very early universe. We find that an SMBH with a spectral energy distribution (SED) extending from the near-ultraviolet to hard X-rays, can produce a radiation background sufficient to delay gravitational collapse in surrounding DM haloes until up to $10^7$ M$_\odot$ of zero-metallicity gas is available for the formation of Population III (Pop III) stars or direct-collapse black holes (DCBHs). We model three scenarios, corresponding to an SMBH located at physical distances of 10, 100, and 1000 kpc from the Pop III host DM halo. Using these three scenarios, we use the SED to compute self-consistent photoionization, photoheating, and photodissociation rates. We include the effects of Compton scattering and gas self-shielding. The X-ray portion of the spectrum maintains an elevated free-electron fraction as the gas collapses to high density. This stimulates H2 formation, allowing the gas to cool further while counteracting the dissociation of H2 by Lyman-Werner radiation. As a result, a large cluster of Pop III stars is expected to form, except in the case with the most intense radiation in which a DCBH may instead form. Our simulated Pop III clusters have comparable HeII 1640 luminosities to the recently discovered Pop III host candidate near GN-z11, observed by the James Webb Space Telescope. In two of the scenarios we consider, the resulting clusters could be detectable using the telescope's NIRSpec instrument out to z ~ 15.

Short digest

Cosmological radiation‑hydrodynamical simulations test quasar SEDs (near‑UV to hard X‑ray) impinging from 10–1000 kpc onto a neighboring halo and quantify how AGN radiation regulates primordial collapse. X‑rays keep the free‑electron fraction high, catalyzing H2 despite LW dissociation, so collapse is delayed until ≲10^7 Msun of pristine gas accumulates—yielding a massive Pop III cluster for the weaker two fields and a DCBH for the strongest. Runaway collapse occurs at z ≃ 24.99, 15.46, and 12.63 (A→C), directly tying radiation intensity to later collapse and larger available gas reservoirs. Predicted He II 1640 luminosities match the GN‑z11 Pop III candidate and should be detectable with NIRSpec to z ≈ 15, outlining an AGN‑proximity pathway to luminous Pop III clusters or early DCBH seeds.

Key figures to inspect

• Figure 1: Read off the collapse redshifts (z = 24.99, 15.46, 12.63 for A, B, C) versus halo virial mass/temperature to see how stronger backgrounds push collapse to later times and higher virial temperatures.
• Figure 2: Compare radial profiles of xe and H2 among scenarios to see X‑ray–sustained ionization boosting H2 toward the center while LW limits it—key to why cooling and collapse differ between A and C.
• Figure 3: Inspect the density/temperature slices to confirm HD‑driven cooling to the CMB floor in the lowest background (A) and LW‑limited H2 cooling with warmer cores in the higher backgrounds (B, C).
• Figure 4: Use infall time versus enclosed mass with the 3 Myr and 30 Myr guides to estimate how much gas can assemble within massive‑star lifetimes, distinguishing the massive Pop III cluster regimes from the DCBH case.