Medium-band Astrophysics with the Grism of NIRCam In Frontier fields (MAGNIF): Spectroscopic Census of H$\hat{I}\pm$ Luminosity Functions and Cosmic Star Formation at $z\sim 4.5$ and 6.3

Abstract

We measure H$\hat{I}\pm$ luminosity functions (LFs) at redshifts $z\sim 4.5$ and 6.3 using the JWST MAGNIF (Medium-band Astrophysics with the Grism of NIRCam In Frontier fields) survey. MAGNIF obtained NIRCam grism spectra with the F360M and F480M filters in four Frontier Fields. We identify 248 H$\hat{I}\pm$ emitters based on the grism spectra and photometric redshifts from combined HST and JWST imaging data. The numbers of the H$\hat{I}\pm$ emitters show a large field-to-field variation, highlighting the necessity of multiple fields to mitigate cosmic variance. We calculate both observed and dust-corrected H$\hat{I}\pm$ LFs in the two redshift bins. Thanks to the gravitational lensing, the measured H$\hat{I}\pm$ LFs span three orders of magnitude in luminosity, and the faint-end luminosity reaches $L_{\mathrm{H}\hat{I}\pm }\sim 10^{40.3}\mathrm{erg}{\mathrm{s}}^{-1}$ at $z\sim 4.5$ and $10^{41.5}\mathrm{erg}{\mathrm{s}}^{-1}$ at $z\sim 6.3$, corresponding to star-formation rates (SFRs) of $\sim$ 0.1 and 1.7 ${\mathrm{M}}_{\odot }{\mathrm{yr}}^{-1}$. We conclude no or weak redshift evolution of the faint-end slope of H$\hat{I}\pm$ LF across $z\simeq 0.4-6.3$, and the comparison with the faint-end slopes of UV LF indicates stochastic star formation history among low-mass H$\hat{I}\pm$ emitters. The derived cosmic SFR densities are $0.058_{-0.006}^{+0.008}{M}_{\odot }{\mathrm{yr}}^{-1}{\mathrm{Mpc}}^{-3}$ at $z\sim 4.5$ and $0.025_{-0.007}^{+0.009}{M}_{\odot }{\mathrm{yr}}^{-1}{\mathrm{Mpc}}^{-3}$ at $z\sim 6.3$. 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$\hat{I}\pm$ LF measurements, including those propagate from the lens models, cosmic variance, and AGN contribution.

25 pages, 14 figures, submitted to ApJ