The THESAN-ZOOM project: Population III star formation continues until the end of reionization

TL;DR

This study uses 14 high-resolution THESAN-ZOOM zoom-in simulations to test whether Population III stars can continue forming until the end of reionization. By employing on-the-fly radiative transfer that includes Lyman-Werner radiation, a multiphase ISM, dust physics, and non-equilibrium H$_2$ chemistry, the authors find that pristine gas fueling Pop III star formation persists in minihaloes and satellite galaxies down to about $z\approx 5$, after which photoevaporation halts further formation in those hosts. Pop III remnants predominantly reside in satellites of larger haloes at lower redshift, while Pop II/I star formation dominates in central galaxies. The results imply lingering primordial star formation could imprint observable signatures on high-redshift galaxies and motivate future work to include explicit Pop III feedback and IMF modeling to sharpen JWST-era constraints. Overall, the THESAN-ZOOM framework demonstrates that while Pop III activity wanes toward the end of reionization, it leaves a detectable footprint in the metal-poor stellar populations of assembling galaxies.

Abstract

Population III (Pop III) stars are the first stars in the Universe, forming from pristine, metal-free gas and marking the end of the cosmic dark ages. Their formation rate is expected to sharply decline after redshift $z \approx 15$ due to metal enrichment from previous generations of stars. In this paper, we analyze 14 zoom-in simulations from the THESAN-ZOOM project, which evolves different haloes from the THESAN-1 cosmological box down to redshift $z=3$. The high mass resolution of up to $142 M_\odot$ per cell in the gas phase combined with a multiphase model of the interstellar medium (ISM), radiative transfer including Lyman-Werner radiation, dust physics, and a non-equilibrium chemistry network that tracks molecular hydrogen, allows for a realistic but still approximate description of Pop III star formation in pristine gas. Our results show that Pop III stars continue to form in low-mass haloes ranging from $10^6 M_\odot$ to $10^9 M_\odot$ until the end of reionization at around $z=5$. At this stage, photoevaporation suppresses further star formation in these minihaloes, which subsequently merge into larger central haloes. Hence, the remnants of Pop III stars primarily reside in the satellite galaxies of larger haloes at lower redshifts. While direct detection of Pop III stars remains elusive, these results hint that lingering primordial star formation could leave observable imprints or indirectly affect the properties of high-redshift galaxies. Explicit Pop III feedback and specialized initial mass function modelling within the THESAN-ZOOM framework would further help interpreting emerging constraints from the James Webb Space Telescope.

Figures (17)

• Figure 1: The dark matter density (left), gas density (middle), and gas metallicity (right) centred around the target halo m10.8_8x at redshift $z=7$. We show volume averaged quantities in a slice in z-direction of depth $\rm 50\,ckpc$ and the comoving virial radius at $z=7$ (circle with solid line) and for comparison the comoving virial radius at $z=3$ (circle with dashed line). The star symbols represent Pop III star particles that formed between $z=7$ and $z=8$. All Pop III stars formed outside of the central halo but can merge later with it. The gas around the central halo almost reaches solar metallicity, but there are still pockets of metal-free gas outside of the halo.
• Figure 2: The stellar mass (within twice the stellar half mass radius) - stellar metallicity (mass-weighted mean within the stellar half mass radius) relation for the central galaxies of all target haloes at different redshifts. We always choose the highest resolution run for each halo and additionally highlight the three simulations we focus on in the main part of the paper by increasing their symbol. We compare our results with the following cosmological simulations: FiBYfiby, SIMBAdave2019simba, FLARESwilkins2023first, FIREma2016origin, the simulation E001 from Jeong2024, MilleniumTNG pakmor2023millenniumtngkannan2023millenniumtng, and thesan-1Thesan1ThesanDR. We also add observational results from the VANDELS survey cullen2019vandels, photometry-only data from JWST Furtak2023Robertson2023 and spectroscopically confirmed data Wang2023Tacchella2023Curti2024Carniani2024CurtisLake2023 as well as data from dwarf galaxies within the local group Kirby2013. Our results compare well with FiBY and the VANDELS survey. The observational results from JWST show a larger scatter than our results, though especially the spectroscopically confirmed objects agree on average with our simulation. However, they lie rather on the lower end of the metallicity. This could be explained by the typically higher redshift of the observations compared to our data points at the same stellar mass. Jeong2024 finds a higher metallicity than our results in their simulation, especially when considering their higher redshift. This could be explained by their different feedback model, especially their specific Pop III star formation model with higher metal yields. We find consistently higher metallicities than FIRE for $M_\star / \text{M}_\odot > 10^5$. Nevertheless, the slope between FIRE and our simulations is very similar.
• Figure 3: We show a histogram of the birth mass of all stellar particles that can be found in three high-resolution haloes at redshift $z=3$ as a function of their birth redshift and metallicity normalized by the solar metallicity. All haloes contain Pop III stars ($\rm Z < 10^{-6}\,\text{Z}_\odot$) born at the end of the epoch of reionization ($z \approx 5 - 5.5$), which mostly form in satellite galaxies or ex-situ. On average, metal-rich stars can be found in larger haloes and are born at lower redshift. \ref{['fig:age_metallicity_stars']} contains the data for the remaining 11 target haloes.
• Figure 4: The birth mass fraction of different stellar populations as a function of the birth redshift. We averaged the results over $100\,\rm Myr$ and used the birth mass of the star particles. We only consider stars in the target halo at $z=3$, but they can be born outside of it. Pop III star formation dominates at high redshift, and only in the most massive halo do we find a transition to Pop I star formation before redshift $z=3$. Nevertheless, we can find Pop III stars formed at the end of the EoR at $z = 5-6$ in all haloes. \ref{['fig:ratio_star_formation_populations_per_halo']} contains the data for the remaining 11 target haloes.
• Figure 5: The fraction of different stellar populations (represented by three different solid lines) found in satellites of the target halo at different times averaged over $100\, \rm Myr$. Only a neglectable fraction of stars within the halo is not bound to any subhalo. Pop I stars can be mostly found in the central galaxy, while Pop III stars are found more often in satellites. The satellite fraction can significantly increase during major mergers when the halo finder identifies the two merging haloes for the first time as a single FOF group. If the haloes do not directly merge the satellite fraction can also drop again when the halo finder identifies the two separate haloes again. This can be seen e.g. for Pop I stars for m12.6_4x. We only consider star particles that are at redshift $z$ in the target halo and use the initial birth mass of stars to calculate mass fractions to reduce the impact of a Pop III IMF.