The JWST Emission Line Survey (JELS): An untargeted search for H α emission line galaxies at z > 6 and their physical properties

We present the first results of the JWST Emission Line Survey (JELS). Utilizing the first NIRCam narrow-band imaging at 4.7 μm, over 63 arcmin2 in the PRIMER/COSMOS field, we have identified 609 emission line galaxy candidates. From these, we robustly selected 35 H α star-forming galaxies at z ∼ 6.1, with H α star-formation rates (SFRH α) of ∼ 0.9 − 15 M yr−1. Combining our unique H α sample with the exquisite panchromatic data in the field, we explored their physical properties and star-formation histories, and compared these to a broad-band selected sample at z ∼ 6 which has offered vital new insights into the nature of high-redshift galaxies. UV-continuum slopes (β) were considerably redder for our H α sample (β ∼ −1.92) compared to the broad-band sample (β ∼ −2.35). This was not due to dust attenuation as our H α sample was relatively dustpoor (median AV = 0.23); instead, we argue that the reddened slopes could be due to nebular continuum. We compared SFRH α and the UV-continuum-derived SFRUV to SED-fitted measurements averaged over canonical time-scales of 10 and 100 Myr (SFR10 and SFR100). We found an increase in recent SFR for our sample of H α emitters, particularly at lower stellar masses (< 109 M). We also found that SFRH α strongly traces SFR averaged over 10 Myr time-scales, whereas the UV-continuum overpredicts SFR on 100 Myr time-scales at low stellar masses. These results point to our H α sample undergoing ‘bursty’ star formation. Our F356W z ∼ 6 sample showed a larger scatter in SFR10/SFR100 across all stellar masses, which has highlighted how narrow-band photometric selections of H α emitters are key to quantifying the burstiness of star-formation activity.