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Abstract

The Hydrogen-alpha (Ha) 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 Ha in galaxies at high redshifts (z > 3) and thus measure their star-formation rates (SFRs). However, using "classical" SFR(Ha) 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 Ha luminosity and the SFR. In this work, we develop a set of new SFR(Ha) calibrations that allow us to predict the SFRs of Ha-emitters at z > 3 with minimal error. We use the SPHINX cosmological simulations to select a sample of star-forming galaxies representative of the Ha-emitter population observed with JWST. We then derive linear corrections to the classical SFR(Ha) calibrations, taking into account variations in the physical properties (e.g., stellar metallicities) among individual galaxies. We obtain two new SFR(Ha) calibrations that, compared to the classical calibrations, reduce the root mean squared error (RMSE) in the predicted SFRs by $\Delta$RMSE $\approx$ 0.04 dex and $\Delta$RMSE $\approx$ 0.06 dex, respectively. Using the recent JWST NIRCam/grism observations of Ha-emitters at z ~ 6, we show that the new calibrations affect the high-redshift galaxy population statistics: (i) the estimated cosmic star-formation density decreases by $\Delta\rho$(SFR) $\approx$ 12%, and (ii) the observed slope of the star-formation main sequence increases by $\Delta$ $\partial$log SFR / $\partial$log M* = 0.08 $\pm$ 0.02.