Embedded Files
Impact of initial mass function on the chemical evolution of high-redshift galaxies (arXiv:2506.06139)
Impact of initial mass function on the chemical evolution of high-redshift galaxies (arXiv:2506.06139)
Star formation and metal enrichment in galaxies are regulated by supernova (SN) explosions and metal yields from massive stars, which are sensitive to the high-mass end of the initial mass function (IMF). Recent JWST observations have reached a consensus on an invariant relation between stellar mass, metallicity, and star formation rate up to z~8 and its breakdown at higher redshifts. It is crucial to understand the underlying physics, especially the role played by the IMF. We explore the impact of IMF on the chemical evolution of high-redshift galaxies and the interplay between IMF and galactic outflow parameters. The ultimate goal is to constrain the high-mass end of the IMF by the cosmic star formation history and stellar mass-metallicity-star formation rate relation (MZSFR) inferred from observations at z~4-10. Using the semi-analytical galaxy evolution code A-SLOTH, we follow galactic baryon cycles along merger trees built from a high-resolution cosmological simulation. Stellar feedback is modeled with up-to-date stellar evolution tracks covering the full metallicity range Z~10^-11-0.03) and a broad stellar mass range (2-600 Msun) including the metal yields from stellar winds, core-collapse SNe, (pulsational) pair-instability SNe, and Type Ia SNe. Assuming that the IMF follows a Kroupa-like shape with a varying upper mass limit m_max, we find m_max≳200 Msun is required to reproduce the observed MZSFR. Observational data at z≳6 favor a galactic outflow model where the outflow rate is proportional to the supernova energy injection rate divided by the halo binding energy. We conclude that very massive (≳200 Msun) stars can play important roles in the star formation and chemical enrichment histories of high-z galaxies. We also discuss the implications of our best-fit model for reionization and transient sources of both electromagnetic waves and gravitational waves.
Metallity-stellar mass relation (MZR) at z~4-10. Simulated galaxies from our best-fit model are shown as dots/circles, color-coded by SFR. A linear fit in log-log space is derived from the simulated galaxies with log(SFR [Msun/yr])>-0.5 that are detectable in current JWST spectroscopy surveys as the long-dashed line, i.e., the predicted MZR. The other lines show the MZRs obtained from observations, which are corrected for the bias of observations towards high-SFR galaxies. The diamonds with errorbars show the original average values (without corrections) and 1 sigma scatters of the stellar mass, metallicity, and SFR of the galaxies observed by JWST from Nakajima et al. (2023). The small dots with errorbars denote the extremely metal-poor galaxy (EMPG, [O/H]<-1.7) candidates observed by JWST.
Rest-frame UV luminosity per unit star formation rate η_UV for a top-hat star formation history within a timescale of 100 Myr as a function of the minimum initial mass of CHE stars given the Salpeter IMF slope α = 2.35. For the solid curves, the minimum mass of non-rotating stars is fixed to 1.7 M⊙, and the mass fraction of CHE stars f_CHE increases for lighter colors. The dotted curve with errorbars shows the maximum η_UV in an alternative population model with a fixed overall IMF. We also show the reference value ηUV ≃ 8.70 × 10^27 from Madau & Dickinson (2014) with the dashed line, which is very close to our result for f_CHE = 0. The shaded regions show the results for Pop III stars with extremely top-heavy IMFs from Inayoshi et al. (2022).
Signatures of rapidly rotating stars with chemically homogeneous evolution in the first galaxies (ApJ, 980, L30)
Signatures of rapidly rotating stars with chemically homogeneous evolution in the first galaxies (ApJ, 980, L30)
The James Webb Space Telescope (JWST) has revealed an unexpectedly high abundance of UV luminous galaxies at redshifts z ≳ 10, challenging ‘standard’ galaxy formation models. We investigate the role of rapidly rotating (massive) stars undergoing chemically homogeneous evolution (CHE) in reconciling this potential tension. These stars are more compact, hotter, and exhibit enhanced UV emission. We find that the rest-frame UV luminosity of star-forming galaxies can be significantly enhanced by a factor of ∼ 3 − 6 when CHE stars above a minimum initial mass of ∼ 2 − 10 M⊙ account for more than half of the total stellar mass following a Salpeter initial mass function (IMF). As a result, the UV luminosity functions observed at z ∼ 12 − 16 can be reproduced with less extreme values of star formation efficiency and UV luminosity stochastic variability. Moreover, in the CHE scenario, the feedback of supernova explosions is less enhanced compared with the alternative scenario of top-heavy IMFs. The UV spectra also become harder under CHE, which can possibly explain the bluer galaxies at higher redshifts in observations.
Our results highlight the potential of CHE in explaining the UV-bright galaxy populations detected by JWST and call for future work to explore the broader astrophysical implications of CHE, stellar rotation in general, and the associated phenomena in the early universe, such as Wolf-Rayet stars, gamma-ray bursts, compact object binaries, and metal enrichment (see, e.g., Tsiatsiou et al. 2024; Boco et al. 2025)
Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization (arXiv:2504.00535)
Effects of chemically homogeneous evolution of the first stars on the 21-cm signal and reionization (arXiv:2504.00535)
The UV radiation from massive stars is sensitive to stellar evolution and regulates the thermal and ionization histories at Cosmic Dawn, which are captured by the 21-cm signal. It is shown in our earlier work Sibony et al. (2022) that the UV emission from Pop III stars can be significantly boosted by chemically homogeneous evolution (CHE) due to fast rotation, which may facilitate early cosmic reionization. In the hope of constraining Pop III stellar evolution with observations of Cosmic Dawn, we investigate the effects of Pop III CHE by combining spectra of Pop III stars derived by the atmosphere model TLUSTY (Hubeny et al., 2017) with semi-numerical simulations of Cosmic Dawn (Gessey-Jones et al., 2023). Our results show that CHE boosts the ionizing power of Pop III stars by a factor of 2, which reduces the Pop III stellar mass density required to reproduce the observed Thomson scattering optical depth proportionally (Fig. 1). Meanwhile, the maximum 21-cm global absorption signal is shallower by up to 15 mK (11%), partly due to the reduced Lyman-band emission from CHE, and the large-scale (k ~ 0.2/cMpc) power drops by a factor of a few at z>25 (Fig. 2). In general, the effects of CHE can be comparable to those of Pop~III star formation parameters. Our results highlight the importance of metal-free/poor stellar evolution in understanding the early Universe and suggest that future studies should consider joint constraints on the physics of star/galaxy formation and stellar evolution.
Fig. 1. Impact of Pop III CHE on the optical depth of Thomson scattering (left) and cosmic ionization history (right) with varying Pop III star formation efficiency (SFE). The dotted curves show the `standard' case for non-rotating (NR) Pop III stars, while the solid curves show the CHE case. The curves are color-coded by the Pop III SFE. The special models with SFE values required to produce the best-fitting value and the 3-sigma upper limit of the optical depth for Thomson scattering measured by Planck are shown by the lower and upper thick curves, respectively.
Fig. 2. Impact of Pop III CHE on the global 21-cm signal (left) and the power spectrum of the 21-cm signal (right) with varying Pop III SFE. The annotations are identical to those in Fig. 1.
Distributions of self-enriched minihalos (blue pixels) on the iron abundance and carbon enhancement diagram, from our fiducial model of Pop III star clusters and winds, with and without faint SNe and accretion from the ISM. Observed CEMP -no and CEMO-s/rs stars are shown with red and green data points, respectively.
Stellar winds from fast-rotating Population III (Pop III) stars have long been suspected to make important contributions to early metal enrichment, as features in the nucleosynthesis of such `spinstars' are consistent with the chemical abundance patterns of some metal-poor stars in the local Universe. Particularly, stellar winds rich in light elements can provide another pathway towards explaining the carbon enhancement in carbon-enhanced metal-poor (CEMP) stars.
In this work, we focus on the feedback of Pop III stellar winds combined with supernovae (SNe), and derive the resulting chemical signatures in the enriched medium. We explore a large parameter space of Pop III star formation and feedback with a semi-analytical model. The predicted pattern of carbon and iron abundances of second-generation stars agrees well with observations of CEMP-no stars under the optimistic assumption of significant mass loss by winds and proper amounts of iron gained by accretion from the interstellar medium.
The first generation of stars, the so-called Population III (Pop III), started to form 100-200 Myr after the Big Bang, in metal-poor/free primordial gas. It remains an open question when this unique mode of star formation ended in cosmic history.
In this work, we construct a theoretical framework for late-time (z<6) Pop III star formation, by combining simulation data and semi-analytical models. Althoug we cannot provide a definitive answer to the question, we are able to identify metal mixing as the key process and the total mass of active Pop III stars per halo as the key parameter for potential observation of Pop III stars at late times. If metal mixing is inefficient, Pop III star formation can still occur in parcels of dense pristine gas along cold accretion flows during the post-reionization era, leading to promising detection rates at low redshifts (z<0.5) for planned instruments such as WFIRST, although space-based UV telescopes are required to distinguish Pop III stars from their metal-enriched stars. Improved theoretical and observational efforts in these aspects will shed light on this fundamental question in the near future.
Locations of active Pop III stars with ages <3 (orange triangles) and ~3-10 (red filled circles) Myr, as well as their host haloes (empty circles), during the formation of simulated cosmic web down to z=4. Note that by the end of the simulation, 50% of the haloes with recent Pop III star formation are massive (>10^10 Msun), where metal mixing plays the most important role for regulating Pop III star formation.
Evolution of halo star formation efficiency (ratio of stellar to halo mass) with redshift, inferred from the age distribution of stars in XLSSC, under different baryon-dark matter streaming velocities (shown with normal curves of different line styles). The gray shaded region shows an exponential fit to our results.
Lessons on early structure formation from a mature galaxy cluster observed at cosmic noon (2020, MNRAS, 495, 1700)
Lessons on early structure formation from a mature galaxy cluster observed at cosmic noon (2020, MNRAS, 495, 1700)
A grand challenge for modern astrophysics is to constrain star and galaxy formation in the first billion years of cosmic history.
In this work, we demonstrate a new approach of indirectly constraining both early star and structure formation via mature galaxy clusters at cosmic noon (z~2) with the standard extended Press Schechter (EPS) formalism, using the cluster XLSSC 122 as an example. We infer a rapid evolution of the star formation efficiency (SFE) at z~20-13, and place new constraints on fuzzy dark matter models, based on the age distribution of stars in XLSSC 122. Our results are generally consistent with previous constraints on high-z SFE.
Similar inference can also be conducted based on the 21 cm absorption signal, as shown in e.g., Schauer et al. (2019, SA19).
Star formation in general crucially depends on the cooling processes active in the host cloud. The standard model of first star formation identify H2 and HD as the main molecular coolants in primordial gas. However, it has long been suspected that lithium hydride (LiH) may also be an important coolant in primordial gas with its strong cooling efficiency, as shown
In thsi work, we explore the possible impact of LiH cooling during non-linear collapse of primordial gas. We find that at redshift z~5, the cooling by LiH has no effect on the thermal evolution of shocked primordial gas, and of collapsing primordial gas into minihaloes or relic H II region due to the low-abundance of LiH.
Cooling functions of H2, HD and LiH, in terms of cooling rate per molecule.
Page updated
Google Sites
Report abuse