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

We measure Hα 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 Hα emitters based on the grism spectra and photometric redshifts from combined Hubble Space Telescope and JWST imaging data. The numbers of the Hα emitters show a large field-to-field variation, highlighting the necessity of multiple fields to mitigate cosmic variance. We calculate observed and dust-corrected Hα LFs at the two redshifts. Thanks to the gravitational lensing, the measured Hα LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches L_Hα ∼ 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 Hα 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 Hα 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 Hα 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.