First Sample of H plus [O III]5007 Line Emitters at z > 6 Through JWST/NIRCam Slitless Spectroscopy: Physical Properties and Line-luminosity Functions

We present a sample of four emission-line galaxies at z = 6.11-6.35 that were serendipitously discovered using the commissioning data for the James Webb Space Telescope (JWST)/NIRCam wide-field slitless spectroscopy mode. One of them (at z = 6.11) has been reported previously, while the others are new discoveries. These sources are selected by the secure detections of both [O III] lambda 5007 and Ha lines with other fainter lines, which were tentatively detected in some cases (e.g., [O II]lambda 3727, [O III] lambda 4959). In the [O III]/H beta-[N II]/Ha Baldwin-Phillips-Terlevich diagram, these galaxies occupy the same parameter space as that of z similar to 2 star-forming galaxies, indicating that they have been enriched rapidly to subsolar metallicities (similar to 0.4 Z(circle dot)), similar to galaxies with comparable stellar masses at much lower redshifts. The detection of strong Ha lines suggests a higher ionizing photon production efficiency within galaxies in the early universe. We find brightening of the [O III] lambda 5007 line-luminosity function (LF) from z = 3 to 6, and weak or no redshift evolution of the Ha line LF from z = 2 to 6. Both LFs are underpredicted at z similar to 6 by a factor of similar to 10 in certain cosmological simulations. This further indicates a global Lya photon escape fraction of 7%-10% at z similar to 6, which is slightly lower than previous estimates through the comparison of the UV-derived star formation rate density and Lya luminosity density. Our sample recovers 66(-44)(+128)% of z = 6.0-6.6 galaxies in the survey volume with stellar masses greater than 5 x 10(8) M-circle dot, suggesting the ubiquity of strong H alpha and [O III] line emitters in the Epoch of Reionization, which will be further uncovered in the era of JWST.