Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5 < z < 8.5$ JADES galaxies
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| Formato: | Preprint |
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2025
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| author | Trussler, James A. A. Cameron, Alex J. Eisenstein, Daniel J. Katz, Harley Adams, Nathan J. Austin, Duncan Bunker, Andrew J. Carniani, Stefano Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma Hainline, Kevin Harvey, Thomas Johnson, Benjamin D. Li, Qiong Looser, Tobias J. Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Williams, Christina C. Willmer, Christopher N. A. Willott, Chris Wu, Zihao |
| author_facet | Trussler, James A. A. Cameron, Alex J. Eisenstein, Daniel J. Katz, Harley Adams, Nathan J. Austin, Duncan Bunker, Andrew J. Carniani, Stefano Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma Hainline, Kevin Harvey, Thomas Johnson, Benjamin D. Li, Qiong Looser, Tobias J. Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Williams, Christina C. Willmer, Christopher N. A. Willott, Chris Wu, Zihao |
| contents | 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 $ξ_\mathrm{ion}$ 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 ($\lesssim 3$~Myr), we further demand a high observed $\log\, (ξ_\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) > 25.60$ 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 ${\sim}$10% of galaxies at $z \sim 6$ (decreasing to ${\sim}$3% 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 $\mathrm{EW}_\mathrm{Hα,rest} = 1567$ Å, $\mathrm{EW}_\mathrm{[O\ III] + Hβ,rest} = 2244$ 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 $ξ_\mathrm{ion, obs}$ 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 AGN. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_12622 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5 < z < 8.5$ JADES galaxies Trussler, James A. A. Cameron, Alex J. Eisenstein, Daniel J. Katz, Harley Adams, Nathan J. Austin, Duncan Bunker, Andrew J. Carniani, Stefano Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma Hainline, Kevin Harvey, Thomas Johnson, Benjamin D. Li, Qiong Looser, Tobias J. Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Williams, Christina C. Willmer, Christopher N. A. Willott, Chris Wu, Zihao Astrophysics of 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 $ξ_\mathrm{ion}$ 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 ($\lesssim 3$~Myr), we further demand a high observed $\log\, (ξ_\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) > 25.60$ 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 ${\sim}$10% of galaxies at $z \sim 6$ (decreasing to ${\sim}$3% 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 $\mathrm{EW}_\mathrm{Hα,rest} = 1567$ Å, $\mathrm{EW}_\mathrm{[O\ III] + Hβ,rest} = 2244$ 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 $ξ_\mathrm{ion, obs}$ 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 AGN. |
| title | Cloudy with a chance of starshine: Possible photometric signatures of nebular-dominated emission in $1.5 < z < 8.5$ JADES galaxies |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2510.12622 |