First light - II. Emission line extinction, population III stars, and X-ray binaries

Summary

Abstract

Barrow and collaborators investigate how metal-free Population III stars and high-mass X-ray binaries could appear in observations of galaxies at z = 15. Using galaxies from the Renaissance Simulations, they model how radiation from stars and X-ray binaries is absorbed, scattered, and re-emitted by gas, metals, and dust. They predict several important emission-line signatures, including Lyman-alpha, C IV, H-alpha, and the Ca II infrared triplet, and explore whether these signatures could help JWST identify galaxies containing some of the first stars.

1. Introduction and Background

• Population III stars formed from nearly metal-free gas and were likely hotter and more massive than many stars forming in the present-day universe.
• Massive Population III stars can leave behind black holes or neutron stars that may become high-mass X-ray binaries when they accrete material from a companion star.
• Radiation from these stars and X-ray binaries interacts with surrounding gas and metals, producing emission lines that could provide indirect evidence for otherwise difficult-to-detect Population III populations.
• The study extends earlier synthetic-observation work by modeling absorption, scattering, and emission throughout the three-dimensional interstellar and circumgalactic medium.

2. Research Methods

• The authors analyze galaxies from the Rarepeak region of the Renaissance Simulations at z = 15, focusing on 146 halos that contain metal-free stellar populations.
• Stellar spectra are generated for both metal-enriched and Population III stars, while a separate model is used to represent radiation from high-mass X-ray binaries.
• The Caius post-processing pipeline combines stellar spectra with three-dimensional gas, metal, and dust distributions to calculate absorption, scattering, and emission-line production.
• Monte Carlo radiative transfer is used to propagate radiation from hard X-rays through the infrared and generate synthetic spectra and telescope observations.
• The processed spectra are passed through JWST and Hubble filter responses to predict observable colors and fluxes.

3. Results

• The most common emission features are Lyman-alpha, the C IV doublet, H-alpha, and the Ca II infrared triplet.
• Lyman-alpha appears in about 90% of the Population III sample and is particularly strong in young, actively star-forming systems.
• H-alpha appears much more frequently in galaxies containing Population III stars and X-ray binaries than in the comparison sample containing only metal-enriched stars.
• Galaxies with larger fractions of metal-free stars tend to show stronger Lyman-alpha equivalent widths, although their small stellar populations also make them intrinsically faint.
• Radiative transfer can substantially change a galaxy’s predicted JWST colors, meaning that the intrinsic stellar spectrum alone does not reliably determine what the galaxy will look like to an observer.

4. Discussion

• Individual Population III galaxies at z = 15 are generally too faint for straightforward photometric detection with JWST.
• A more promising direct-detection scenario is a merger involving Population III galaxies, where a larger stellar population can produce enough luminosity to become observable.
• Strong Lyman-alpha emission can increase the JWST J200W − J277W color, potentially producing a recognizable observational signature of young metal-free stellar populations.
• C IV and Ca II are useful indicators of energetic environments but are not unique signatures of Population III stars or X-ray binaries because they also appear in metal-enriched galaxies.
• H-alpha may provide a more useful diagnostic because its occurrence increases significantly in halos containing Population III stars and becomes stronger as the metal-free stellar fraction increases.

5. Conclusions

• Only a small number of simulated halos would be detectable by JWST at z = 15 even with very long exposures and gravitational magnification.
• The best opportunity for directly observing a Population III population may be a rare merger between galaxies containing large fractions of metal-free stars.
• Young Population III populations generate strong Lyman-alpha emission, which can alter JWST colors and provide an indirect clue to their presence.
• Population III stars and high-mass X-ray binaries increase the prevalence of H-alpha emission relative to otherwise similar metal-enriched galaxies.
• Strong Lyman-alpha emission can also produce the Ca II infrared triplet, which experiences less extinction and may provide an additional observational signature.
• Overall, galaxies containing Population III stars and X-ray binaries tend to appear bluer than the metal-enriched control population.

Best Figures

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