Population III.1 Scenario Overview

• Population III.1 stars are the first generation of stars formed from pristine, metal-free gas in isolated halos cooling primarily via molecular hydrogen.
• The scenario predicts a top-heavy initial mass function with the majority of stellar mass in massive stars, often accompanied by dynamic fragmentation and mergers.
• Radiative, mechanical, and chemical feedback from these stars regulate early galaxy evolution and trigger the transition to Population II star formation.

Population III.1 (Pop III.1) refers to the first generation of stars forming from truly pristine, metal-free gas, in the absence of radiative and chemical feedback from earlier generations. This scenario describes the initial conditions for primordial star and galaxy formation, as well as the associated chemical, radiative, and mechanical feedback that ultimately regulates the transition to subsequent stellar populations and the emergence of the first galaxies.

1. Definition and Formation Environment

Pop III.1 stars are defined as those forming from cosmological initial conditions—namely, neutral, metal-free gas that cools primarily via molecular hydrogen (H₂) in dark matter minihalos with masses of order $10^6$ M$_\odot$ at redshifts $z\sim 30$–15 (Norman, 2010, Crosby et al., 2013). The critical requirement is that these halos remain isolated from previous star formation and external ionizing or dissociating radiation fields. The gas cools quasi-statically to $T\sim200$ K (set by H₂ cooling limits), allowing collapse to proceed until densities of $\sim10^8$ cm$^{-3}$ trigger three-body H₂ formation and a dynamically unstable phase that leads to protostar formation (Stacy et al., 2016).

The Pop III.1 scenario is distinguished from the Pop III.2 pathway (which occurs in gas pre-processed by Lyman–Werner or ionizing backgrounds), as well as from chemically enriched star formation (Pop II and later) (Norman, 2010, Park et al., 2021).

2. Stellar Initial Mass Function and Multiplicity

Multiple lines of evidence indicate that Pop III.1 stars are very massive, with a top-heavy initial mass function (IMF) that can be represented as:

$$f(\log M)\,dM = M^{-1.3} \exp \left[ - \left( \frac{M_{\rm char}}{M} \right)^{1.6} \right] dM$$

with $M_{\rm char} \sim 40$–100 M$_\odot$ as a typical characteristic mass (Wise et al., 2010, Xu et al., 2013). Simulations initialized from cosmological conditions yield clusters containing tens of protostellar fragments, but the mass distribution is top-heavy: most of the stellar mass is contained in a few massive members ($\gtrsim10$–20 M$_\odot$), while the majority of individual objects are predicted to have lower masses ($<$ 1 M$_\odot$) (Stacy et al., 2016, Hirano et al., 2016).

The fragmentation process is highly dynamic: sink particle or stiff equation-of-state simulations show frequent mergers and ejections, with rapid inward migration of secondary fragments on the viscous timescale ($\alpha \lesssim 1$), leading to a single dominant remnant or a small multiple system (Hirano et al., 2016). Ejected low-mass protostars—if their final mass is below 0.8 M$_\odot$—may survive to the present day as main sequence stars (Dutta et al., 2017, Johnson, 2014).

The inferred IMF slope for Pop III supernova progenitors, as probed by EMP star abundances, is $\alpha \simeq 2.35_{-0.24}^{+0.29}$ (Salpeter value), with an upper progenitor mass limit of $M_{\rm max} \sim 87_{-33}^{+13}$ M$_\odot$ and no evidence for regular contributions above $120$ M$_\odot$ (Fraser et al., 2015).

3. Feedback Processes and Metal Enrichment

Pop III.1 stars strongly affect their environment via radiative, mechanical, and chemical feedback. The most massive stars ($\sim140$–260 M$_\odot$) end their lives as pair-instability supernovae (PISNe), which eject $\sim10^{-3}$ Z$_\odot$ worth of metals and can raise the ambient metallicity in their host halos and surroundings to the observed DLA metallicity floor ($Z \sim 10^{-3}$ Z$_\odot$) with just a single event (Wise et al., 2010). The explosion energy of a PISN is given by:

$$E_{\rm PISN} = 10^{51} \Bigl[5.0 + 1.304 \Bigl( \frac{M_{\rm He}}{M_\odot} - 64 \Bigr) \Bigr]\,\mathrm{erg}$$

with the helium core mass $$M_{\rm He} = \frac{13}{24}\,(M_\star - 20)\,M_\odot$$.

The dispersal of heavy elements enables metal- and dust-cooling to become efficient, shifting the characteristic mass scale downward and triggering the transition to Pop II star formation. In halos above $M_{\rm vir} \sim 10^7$ M$_\odot$, post-supernova gas recovers the cosmic baryon fraction rapidly (Wise et al., 2010, Xu et al., 2013).

Mixing-fallback in faint Pop III SNe can provide a source of carbonaceous dust with high C-to-silicate ratios, potentially matching high-$z$ UV bump features (Chiaki et al., 24 Apr 2025).

4. Temporal Evolution, Environmental Context, and Spatial Distribution

Pop III.1 star formation is not confined to a single epoch but can occur as long as isolated, chemically pristine halos exist. Initially, both pristine and enriched star formation occurs in similar density environments (Crosby et al., 2013). As external feedback builds up, the population becomes restricted to spatially isolated, underdense regions—by $z \sim 10$, Pop III star formation is 4–6 orders of magnitude less frequent than chemically enriched star formation, and Pop III-forming halos are more likely to be adjacent to enriched than to other pristine halos (Crosby et al., 2013).

The local Lyman–Werner and X-ray backgrounds play key roles. LW photons suppress H₂ cooling, raising the minimum mass threshold for Pop III formation, while a moderate soft X-ray background can enhance H₂ formation and lower the critical halo mass, increasing the absolute number of Pop III sites at the expense of reducing the stellar mass per halo (Park et al., 2021).

Delayed collapse in ionized bubbles can shift Pop III formation to much higher mass halos by suppressing early cooling, but fragmentation remains limited, with a handful of massive stars likely forming per such halo (Kulkarni et al., 2019).

5. Transition to Population II and Star Formation History

A central prediction is that Pop II stars begin to form once the gas is enriched to the critical metallicity threshold, $Z_{\rm crit} \sim 10^{-6} - 10^{-3.5} Z_\odot$ (Wise et al., 2010). The transition can be sharp in regions experiencing prompt metal enrichment via PISNe or more gradual and complex in halos with extended accretion and merger histories. Violent mergers cause bimodal metallicity distributions, as earlier and later starbursts contribute distinct age–metallicity tracks; quiescent halos preserve a tighter age–metallicity correlation (Wise et al., 2010).

Empirically, the underlying star formation rate transitions from Pop III-dominated at $z\sim30–20$ to Pop II-dominated at lower redshift. Pop III star formation persists in residual, spatially isolated pockets to at least $z\sim10$ with rates $$\sim10^{-4}\,M_\odot\,{\rm yr}^{-1}\,{\rm Mpc}^{-3}$$ as traced by cosmological simulations (Xu et al., 2013).

6. Remnants and Black Hole Seeding

Pop III.1 remnants play a crucial role in early high-redshift galaxy evolution. Stars below $\sim140$ M$_\odot$ generally end as core-collapse supernovae or direct-collapse black holes. Above the PISN window, more massive stars may form intermediate-mass black holes. Star clusters formed from Pop III.1 can produce multiple remnants, whose mergers and gas accretion can seed the intermediate- and supermassive black holes found in high-$z$ galaxies (Xu et al., 2013, Singh et al., 2023, Cammelli et al., 13 Jul 2024).

Pop III.1 black hole merger rates are predicted to be small compared to those from more common channels, but their mass and spin distributions may be distinct (Kinugawa et al., 2016, Iwaya et al., 2023).

Supermassive black holes seeded in isolated Pop III.1 halos (characteristic seed mass $\sim10^5$ M$_\odot$) form at $z \gtrsim 20$ provided their host is separated by a physical isolation distance $d_{\rm iso}$ (parameter commonly explored in the range 50–100 kpc proper) from other sources (Singh et al., 2023, Cammelli et al., 13 Jul 2024). The value of $d_{\rm iso}$ regulates both the SMBH number density and spatial clustering, with best agreement to local calibration for $d_{\rm iso}<75$ kpc (Cammelli et al., 13 Jul 2024).

7. Observational Implications and Prospects

Direct detection of Pop III.1 stars is extremely challenging, but several indirect probes are accessible:

• Chemical signatures in EMP and CEMP stars: Low-mass Pop III.1 survivors may remain detectable in the Milky Way. Accretion of ISM gas (without dust) can uniquely alter their surface abundances, mimicking some carbon-enhanced metal-poor stars (Johnson, 2014).
• Abundance patterns in EMP stars: The lack of pair-instability supernova (PISN) yields in the EMP population suggests PISNe were rare or contributed only to limited environments (Fraser et al., 2015).
• Dust composition in high-$z$ galaxies: Early supernovae with efficient mixing-fallback can provide enough carbon dust to explain observed UV bumps in galaxies at $z>6$ (Chiaki et al., 24 Apr 2025).
• Gamma-ray bursts and gravitational wave sources: Pop III.1 stars may produce super-energetic GRBs (with $E_{\rm iso} \gtrsim 10^{55-57}$ erg) and contribute to the most massive BBH mergers observed via gravitational waves, though at a much lower intrinsic rate than Pop III.2 or Pop I/II origins (Souza et al., 2011, Kinugawa et al., 2016, Iwaya et al., 2023).
• Supermassive black hole demographics: The early seeding of SMBHs in isolated halos produces testable predictions regarding number densities, clustering, and AGN occupation fractions as functions of galaxy mass (Singh et al., 2023, Cammelli et al., 13 Jul 2024).
• Intensity mapping: He II/H$\alpha$ line-intensity mapping at $z>10$ can constrain the hardness of the Pop III.1 IMF, with ratios $>0.1$ indicating a dominant top-heavy population (Parsons et al., 2021).

These probes, when combined with next-generation wide-field surveys, deep galaxy counts, and gravitational wave data, will continue to test and refine the details of the Population III.1 scenario.