H α as a tracer of star formation in the SPHINX cosmological simulations

The H α emission line in galaxies is a powerful tracer of their recent star formation activity. With the advent of JWST , we are now able to routinely observe H α in galaxies at high redshift ( z ≳ 3) and thus measure their star formation rates (SFRs). However, using classical SFR(H α ) calibrations to derive the SFRs leads to biased results because high-redshift galaxies are commonly characterized by low metallicities and bursty star formation histories, affecting the conversion factor between the H α luminosity ( L H α ) and the SFR. We developed a set of new SFR(H α ) calibrations that allowed us to predict the SFRs of H α -emitters at z ≳ 3 with very little error. We used the SPHINX cosmological simulations to select a sample of star-forming galaxies representative of the H α -emitter population observed with JWST . We then derived linear corrections to the classical SFR(H α ) calibrations that took variations in the physical properties (e.g., stellar metallicities) among individual galaxies into account. We obtained two new SFR(H α ) calibrations that compared to the classical calibrations reduce the root mean squared error (RMSE) in the predicted SFRs by ΔRMSE ≈ 0.04 dex and ΔRMSE ≈ 0.06 dex, respectively. Using the recent JWST NIRCam/grism observations of H α -emitters at z ∼ 6, we show that the new calibrations affect the high-redshift galaxy population statistics: (i) the estimated cosmic SFR density decreases by Δ ρ SFR ≈ 12%, and (ii) the observed slope of the star formation main sequence increases by Δ∂logSFR/∂logM ★ = 0.08 ± 0.02.