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

ABSTRACT 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 $\xi _\mathrm{ion}$ 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 ($1.5 \lt z \lt 8.5$), through the deficit in rest-frame optical continuum level. As Balmer jumps are a general recombination feature of young starbursts ($\lesssim 3$ Myr), we further demand a high observed $\log \, (\xi _\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) \gt 25.60$ to power the strong nebular continuum, together with a relatively non-blue UV slope ($m_\mathrm{F115W}-m_\mathrm{F200W} \gt -0.4$ at $z=6$) indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting ${\sim }11$ per cent of galaxies at $z \sim 6$ (decreasing to ${\sim }2$ per cent at $z \sim 2$, not completeness-corrected) are faint in the rest-frame optical (median $M_\mathrm{opt} = -17.95$) with extreme line emission (median $\mathrm{EW}_\mathrm{H\alpha , rest} = 1567$ Å, $\mathrm{EW}_\mathrm{[O\ III] + H\beta ,rest} = 2292$ 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 $\xi _\mathrm{ion, obs}$ 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 active galactic nucleus.