Clarify the stellar-population calibration of Hα star-formation rates

Determine the true stellar-population properties of high-redshift galaxies, including their metallicities, binary-star fractions, and related ionizing-photon production efficiencies, in order to establish the appropriate Hα star-formation-rate calibration κ_Hα and thereby obtain reliable cosmic star-formation-rate densities at z ≃ 6.

Background

The conversion from Hα luminosity to star-formation rate depends on the initial mass function, stellar metallicity, and the relative abundance of single and binary stellar populations. Applying BPASS models with low metallicity produces an Hα-derived star-formation-rate density approximately 0.43 dex lower than the standard calibration, while producing little change in the UV calibration.

The discrepancy between Hα- and UV-based star-formation-rate densities may instead reflect dust corrections, selection biases, or cosmic variance. Directly constraining the stellar populations and their luminosity-dependent calibration is therefore necessary to resolve the systematic uncertainty in the inferred star-formation-rate density.

References

In addition, the true nature of stellar populations in the high-redshift Universe is still not well understood and could therefore impact our inference of ρ_SFR.

— The JWST Emission Line Survey (JELS): A narrow-band determination of the H$α$ Luminosity Function and Cosmic Star Formation into the Epoch of Reionization  (2609.01586 - Pirie et al., 1 Sep 2026) in Section 4.2, “The star-formation rate density: κ_Hα as a function of L_Hα”; Section 7, Conclusions