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 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 IMF 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 ionising photon production efficiencies ξion associated with massive star formation. We utilise the extensive medium-band imaging from JADES, which enables the identification of Balmer jumps across a wide range of redshifts (1.5 < z < 8.5), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts (≲ 3 Myr), we further demand a high observed log (ξion, obs/(Hz erg−1)) > 25.60 to power the strong nebular continuum, together with a relatively non-blue UV slope (mF115W − mF200W > −0.4 at z = 6) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting ∼11 per cent of galaxies at z ∼ 6 (decreasing to ∼2 per cent at z ∼ 2, not completeness-corrected) are faint in the rest-frame optical (median Mopt = −17.95) with extreme line emission (median EWHα, rest = 1567 Å, EW[O III] + Hβ, rest = 2292 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 ξion, obs 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 AGN.