Possible photometric signatures of nebular-dominated emission in 1.5 < z < 8.5 JADES galaxies

The discovery of high-redshift galaxies exhibiting a steep spectral ultraviolet (UV) downturn potentially indicative of two-photon continuum emission marks a turning point in our search for signatures of star formation following a top-heavy initial mass function in the early Universe. We develop a photometric search method for identifying further nebular-dominated galaxy candidates, whose nebular continuum dominates over the starlight, due to the high ionizing photon production efficiencies associated with massive star formation. We utilize the extensive medium-band imaging from the JWST Advanced Deep Extragalactic Survey (JADES), which enables the identification of Balmer jumps across a wide range of redshifts (), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts ( Myr), we further demand a high observed to power the strong nebular continuum, together with a relatively non-blue UV slope ( at ) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting per cent of galaxies at (decreasing to per cent at , not completeness-corrected) are faint in the rest-frame optical (median ) with extreme line emission (median Å, Å). However, hot H ii region temperatures, collisionally enhanced two-photon continuum emission, and strong UV lines are expected to accompany top-heavy star formation. Thus nebular-dominated galaxies do not necessarily exhibit the biggest Balmer jumps, nor the largest or reddest UV slopes. Hence continuum spectroscopy is ultimately required to establish the presence of a two-photon downturn in our candidates, thus advancing our understanding of primordial star formation and active galactic nucleus.