[Paper Review] Revealing the formation histories of the first stars with the cosmic near-infrared background

Guochao Sun, Jordan Mirocha+2 more|arXiv (Cornell University)|Jul 20, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy179 references27 citations

TL;DR

This paper proposes that the cosmic near-infrared background (NIRB) can reveal the formation histories of high-redshift Population III stars through their unique spectral imprints, particularly strong Ly𝛼 emission redward of 1 μm. Using a semi-empirical model calibrated to high-redshift galaxy data, it shows that efficient Pop III star formation could produce NIRB fluctuations detectable by upcoming missions like SPHEREx, offering a powerful probe of early star formation despite direct detection challenges.

ABSTRACT

The cosmic near-infrared background (NIRB) offers a powerful integral probe of radiative processes at different cosmic epochs, including the pre-reionization era when metal-free, Population III (Pop III) stars first formed. While the radiation from metal-enriched, Population II (Pop II) stars likely dominates the contribution to the observed NIRB from the reionization era, Pop III stars -- if formed efficiently -- might leave characteristic imprints on the NIRB thanks to their strong Ly$\alpha$ emission. Using a physically-motivated model of first star formation, we provide an analysis of the NIRB mean spectrum and anisotropy contributed by stellar populations at $z>5$. We find that in circumstances where massive Pop III stars persistently form in molecular cooling haloes at a rate of a few times $10^{-3}\,M_\odot \ \mathrm{yr}^{-1}$, before being suppressed towards the epoch of reionization (EoR) by the accumulated Lyman-Werner background, a unique spectral signature shows up redward of $1\,\mu$m in the observed NIRB spectrum sourced by galaxies at $z>5$. While the detailed shape and amplitude of the spectral signature depend on various factors including the star formation histories, IMF, LyC escape fraction and so forth, the most interesting scenarios with efficient Pop III star formation are within the reach of forthcoming facilities such as the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx). As a result, new constraints on the abundance and formation history of Pop III stars at high redshifts will be available through precise measurements of the NIRB in the next few years.

Motivation & Objective

• To model the contribution of high-redshift (z > 5) Population II and III star-forming galaxies to the cosmic near-infrared background (NIRB).
• To investigate how the formation histories of Population III stars—particularly their star formation rates and initial mass functions—leave detectable imprints on the NIRB's spectral and spatial properties.
• To assess the feasibility of constraining Pop III star formation using future NIRB measurements from missions like SPHEREx and CDIM.
• To explore component separation techniques for isolating the high-redshift NIRB contribution from foregrounds and distinguishing Pop II from Pop III signals.

Proposed method

• Develops a semi-empirical model of galaxy evolution and star formation, calibrated to observed high-redshift galaxy luminosity functions and star formation rate densities.
• Incorporates physical feedback mechanisms, including the Lyman-Werner background, to regulate Pop III star formation in minihaloes.
• Uses synthetic galaxy populations to compute the NIRB mean spectrum and angular power spectrum, accounting for Ly𝛼 emission, dust attenuation, and Lyman continuum escape fractions.
• Applies full-covariance angular power spectrum analysis to separate NIRB components based on their distinct spatial and spectral structures.
• Performs joint analysis with 3D tracers like 21-cm maps and photometric galaxy catalogs to improve separation of Pop II and Pop III contributions.
• Evaluates detectability of Pop III signatures using sensitivity forecasts for SPHEREx and CDIM, considering varying star formation histories and initial mass functions.

Experimental results

Research questions

• RQ1 Can the cosmic near-infrared background (NIRB) reveal the formation history of Population III stars at z > 5 through their spectral and spatial signatures?
• RQ2 What specific spectral features in the NIRB are expected to arise from strong Ly𝛼 emission in massive, metal-free Pop III stars?
• RQ3 How do uncertainties in the Lyman-Werner feedback, initial mass function, and LyC escape fraction affect the detectability of Pop III signals in the NIRB?
• RQ4 To what extent can future missions like SPHEREx and CDIM separate the Pop III contribution from the dominant Pop II and low-redshift foregrounds in the NIRB?
• RQ5 How can cross-correlations with 21-cm and photometric galaxy surveys improve the isolation of high-redshift Pop III signals in the NIRB?

Key findings

• A unique spectral signature redward of 1 μm in the observed NIRB spectrum arises from strong Ly𝛼 emission in massive Pop III stars, particularly in scenarios with efficient formation in molecular cooling haloes.
• In optimistic models with Pop III star formation rates of ~10−3 M⊙ yr−1 Mpc−3, the Pop III contribution to NIRB fluctuations can reach up to a few tens of percent of the Pop II signal, making it potentially detectable.
• The shape and amplitude of the NIRB spectral signature are highly sensitive to the Pop III initial mass function, LyC escape fraction, and feedback from both Pop III and Pop II stars.
• Component separation via full-covariance angular power spectrum analysis can isolate the high-redshift NIRB contribution with S/N ≳5, even without external data, enabling constraints on Pop III formation histories.
• Forthcoming missions such as SPHEREx and CDIM are expected to place meaningful constraints on Pop III star formation, especially by ruling out or disfavoring extreme scenarios with high-mass, efficient Pop III formation.
• Joint analyses of NIRB with 21-cm and photometric galaxy surveys will enhance the ability to disentangle Pop II and Pop III contributions and overcome systematics like foreground contamination.