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"Looser, Thomas D"
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Cloudy with a chance of starshine: 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 top-heavy star formation 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 \\(_ion\\) 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 \\(\\, (_ion, obs/(Hz\\ erg^-1)) > 25.60\\) to power the strong nebular continuum, together with a relatively non-blue UV slope indicating a lack of stellar continuum emission. Our nebular-dominated candidates, constituting \\(\\)10% of galaxies at \\(z 6\\) (decreasing to \\(\\)3% at \\(z 2\\), not completeness-corrected) are faint in the rest-frame optical (median \\(M_opt = -17.95\\)) with extreme line emission (median \\(EW_H rest = 1567\\) \\(EW_[O\\ III] + H rest = 2244\\) ). 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 \\(_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 AGN.
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