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, o v er 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 ( SFR_Hα) 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 dust- poor (median A V = 0 . 23); instead, we argue that the reddened slopes could be due to nebular continuum. We compared SFR_Hα and the UV-continuum-derived SFR_UV to SED-fitted measurements av eraged o v er canonical time-scales of 10 and 100 Myr ( SFR 10 and SFR 100 ). 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 SFR_Hα strongly traces SFR averaged over 10 Myr time-scales, whereas the UV-continuum o v erpredicts 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 ho w narro w-band photometric selections of H α emitters are key to quantifying the burstiness of star-formation activity.