First Sample of Hα+[O III]λ5007 Line Emitters at z > 6 Through JWST/NIRCam Slitless Spectroscopy: Physical Properties and Line-luminosity Functions
Koekemoer, Anton M.; Pirzkal, Nor; Gennaro, Mario; Bunker, Andrew J.; Maiolino, Roberto; Perna, Michele; Cameron, Alex J.; Kumari, Nimisha; Rest, Armin; Smit, Renske; Curti, Mirko; Egami, Eiichi; Eisenstein, Daniel J.; Rieke, Marcia; Robertson, Brant E.; Sun, Fengwu; Tacchella, Sandro; Williams, Christina C.; Willmer, Christopher N. A.; Baum, Stefi; Boyett, Kristan; Maseda, Michael; Sunnquist, Ben; Greene, Thomas P.; Schlawin, Everett; Stansberry, John; Boyer, Martha; Kelly, Doug; Jaffe, Daniel
United States, Canada, Australia, United Kingdom, Spain
Abstract
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] λ5007 and Hα lines with other fainter lines, which were tentatively detected in some cases (e.g., [O II] λ3727, [O III] λ4959). In the [O III]/Hβ-[N II]/Hα Baldwin-Phillips-Terlevich diagram, these galaxies occupy the same parameter space as that of z ~ 2 star-forming galaxies, indicating that they have been enriched rapidly to subsolar metallicities (~0.4 Z ⊙), similar to galaxies with comparable stellar masses at much lower redshifts. The detection of strong Hα lines suggests a higher ionizing photon production efficiency within galaxies in the early universe. We find brightening of the [O III] λ5007 line-luminosity function (LF) from z = 3 to 6, and weak or no redshift evolution of the Hα line LF from z = 2 to 6. Both LFs are underpredicted at z ~ 6 by a factor of ~10 in certain cosmological simulations. This further indicates a global Lyα photon escape fraction of 7%-10% at z ~ 6, which is slightly lower than previous estimates through the comparison of the UV-derived star formation rate density and Lyα 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 × 108 M ⊙, suggesting the ubiquity of strong Hα and [O III] line emitters in the Epoch of Reionization, which will be further uncovered in the era of JWST.