James Trussler, Alex J. Cameron, Daniel J. Eisenstein, Harley Katz, Nathan Adams, Duncan Austin, Andrew J. Bunker, Stefano Carniani, Christopher J. Conselice, Mirko Curti, Emma Curtis-Lake, Kevin Hainline, Thomas Harvey, Benjamin D. Johnson, Qiong Li, Tobias J. Looser, Pierluigi Rinaldi, Brant Robertson, Fengwu Sun, Sandro Tacchella, Christina C. Williams, Christopher N. A. Willmer, Chris J. Willott, Zihao Wu
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.