Medium-band Astrophysics with the Grism of NIRCam In Frontier Fields (MAGNIF): Spectroscopic Census of Hα Luminosity Functions and Cosmic Star Formation at z ∼ 4.5 and 6.3

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We measure H α luminosity functions (LFs) at redshifts z ∼ 4.5 and 6.3 using the JWST Medium-band Astrophysics with the Grism of NIRCam In Frontier fields (MAGNIF) survey. MAGNIF obtained NIRCam grism spectra with the F360M and F480M filters in four Frontier Fields. We identify 248 H α emitters based on the grism spectra and photometric redshifts from combined Hubble Space Telescope and JWST imaging data. The numbers of the H α emitters show a large field-to-field variation, highlighting the necessity of multiple fields to mitigate cosmic variance. We calculate observed and dust-corrected H α LFs at the two redshifts. Thanks to the gravitational lensing, the measured H α LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches L H α ∼ 10 40.3 erg s −1 at z ∼ 4.5 and 10 41.5 erg s −1 at z ∼ 6.3. They correspond to star formation rates of ∼0.1 and 1.7 M ⊙ yr −1 , respectively. We conclude no or weak redshift evolution of the faint-end slope of H α LF across z ≃ 0.4–6.3, and the comparison with the faint-end slopes of UV LF indicates stochastic star formation history among low-mass H α emitters. The derived cosmic star formation rate densities are 0.05 8 − 0.006 + 0.008 M ⊙ yr − 1 Mpc − 3 at z ∼ 4.5 and 0.02 5 − 0.007 + 0.009 M ⊙ yr − 1 Mpc − 3 at z ∼ 6.3. These are approximately 2.2 times higher than previous estimates based on dust-corrected UV LFs, but consistent with recent measurements from infrared surveys. We discuss uncertainties in the H α LF measurements, including those propagated from the lens models, cosmic variance, and active galactic nucleus contribution, and we find that they have a negligible impact on the above results.