First Sample of H$\alpha$+[O III] $\lambda$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 JWST/NIRCam wide-field slitless spectroscopy (WFSS) 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$5007 and H$\alpha$ lines with other fainter lines tentatively detected in some cases (e.g., [O II] $\lambda$3727, [O III] $\lambda$4959). In the [O III]/H$\beta$ - [N II]/H$\alpha$ Baldwin-Phillips-Terlevich diagram, these galaxies occupy the same parameter space as that of $z\sim2$ star-forming galaxies, indicating that they have been enriched rapidly to sub-solar metallicities ($\sim$0.4 $Z_{\odot}$), similar to galaxies with comparable stellar masses at much lower redshifts. The detection of strong H$\alpha$ 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 H$\alpha$ line LF from $z=2$ to 6. Both LFs are under-predicted at $z\sim6$ by a factor of $\sim$10 in certain cosmological simulations. This further indicates a global Ly$\alpha$ photon escape fraction of 7-10% at $z\sim6$, slightly lower than previous estimates through the comparison of the UV-derived star-formation rate density and Ly$\alpha$ luminosity density. Our sample recovers $66^{+128}_{-44}$% of $z=6.0-6.6$ galaxies in the survey volume with stellar masses greater than $5\times10^8$ $M_{\odot}$, 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.