Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5 < z < 6.5$ with deep JADES medium-band imaging
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2026
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| author | Trussler, James A. A. Eisenstein, Daniel J. Bunker, Andrew J. Cameron, Alex J. Carniani, Stefano Charlot, Stéphane Chevallard, Jacopo Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma D'Eugenio, Francesco Egami, Eiichi Hainline, Kevin Hausen, Ryan Helton, Jakob M. Hsiao, Tiger Yu-Yang Ji, Zhiyuan Johnson, Benjamin D. Looser, Tobias J. Maiolino, Roberto Puskás, Dávid Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Übler, Hannah Williams, Christina C. Willmer, Christopher N. A. Witstok, Joris Wu, Zihao |
| author_facet | Trussler, James A. A. Eisenstein, Daniel J. Bunker, Andrew J. Cameron, Alex J. Carniani, Stefano Charlot, Stéphane Chevallard, Jacopo Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma D'Eugenio, Francesco Egami, Eiichi Hainline, Kevin Hausen, Ryan Helton, Jakob M. Hsiao, Tiger Yu-Yang Ji, Zhiyuan Johnson, Benjamin D. Looser, Tobias J. Maiolino, Roberto Puskás, Dávid Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Übler, Hannah Williams, Christina C. Willmer, Christopher N. A. Witstok, Joris Wu, Zihao |
| contents | JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, $Z < 1\%~\mathrm{Z}_\odot$) at $z > 3$, mostly through serendipitous NIRSpec discoveries and blind slitless spectroscopy. To accelerate our understanding of pristine star formation, we further develop a methodology to identify EMPG candidates from photometry, using the extensive deep medium-band imaging from JADES. Our EMPG candidates at $2.5 < z < 6.5$ exhibit strong photometric boosts by H$α$, yet correspondingly weak boosts by [O III] + H$β$, likely indicating extremely low metallicity to explain their lack of [O III] emission. We further demand our EMPG candidates to have strong Balmer jumps, as revealed by medium-band imaging, to ensure that they are young starbursts, as opposed to broad-line AGN/LRDs, though contamination by dusty/dense-gas starbursts and highly-obscured AGN remains a concern. SED-fitting with near-pristine models (${\sim}0.1$-$1\%~\mathrm{Z}_\odot$) indicates that our 22 EMPG candidates are low-mass (median $M_* \approx 10^{6.7}~\mathrm{M}_\odot$), faint dwarf galaxies ($M_\mathrm{UV} \approx -16.6$), with high ionizing photon production efficiencies ($\log\, (ξ_\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) \approx 26.0$). Hence these are plausible sites of near-pristine star formation, comprising ${\sim}0.04$-$0.6\%$ of $2.5 < z < 6.5$ galaxies at $-19 < M_\mathrm{UV} < -16$. We discuss this extremely metal-poor extension to the mass-metallicity relation. We forecast that deep (${\sim}28$ h) NIRCam slitless spectroscopy can identify bright EMPGs through strong H$β$ but lack of [O III] emission, or secure the redshifts of fainter systems through H$α$ detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to $z=10$, without requiring deep medium-band/MIRI imaging to identify secure candidates. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_15761 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5 < z < 6.5$ with deep JADES medium-band imaging Trussler, James A. A. Eisenstein, Daniel J. Bunker, Andrew J. Cameron, Alex J. Carniani, Stefano Charlot, Stéphane Chevallard, Jacopo Conselice, Christopher J. Curti, Mirko Curtis-Lake, Emma D'Eugenio, Francesco Egami, Eiichi Hainline, Kevin Hausen, Ryan Helton, Jakob M. Hsiao, Tiger Yu-Yang Ji, Zhiyuan Johnson, Benjamin D. Looser, Tobias J. Maiolino, Roberto Puskás, Dávid Rinaldi, Pierluigi Robertson, Brant Sun, Fengwu Tacchella, Sandro Übler, Hannah Williams, Christina C. Willmer, Christopher N. A. Witstok, Joris Wu, Zihao Astrophysics of Galaxies JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, $Z < 1\%~\mathrm{Z}_\odot$) at $z > 3$, mostly through serendipitous NIRSpec discoveries and blind slitless spectroscopy. To accelerate our understanding of pristine star formation, we further develop a methodology to identify EMPG candidates from photometry, using the extensive deep medium-band imaging from JADES. Our EMPG candidates at $2.5 < z < 6.5$ exhibit strong photometric boosts by H$α$, yet correspondingly weak boosts by [O III] + H$β$, likely indicating extremely low metallicity to explain their lack of [O III] emission. We further demand our EMPG candidates to have strong Balmer jumps, as revealed by medium-band imaging, to ensure that they are young starbursts, as opposed to broad-line AGN/LRDs, though contamination by dusty/dense-gas starbursts and highly-obscured AGN remains a concern. SED-fitting with near-pristine models (${\sim}0.1$-$1\%~\mathrm{Z}_\odot$) indicates that our 22 EMPG candidates are low-mass (median $M_* \approx 10^{6.7}~\mathrm{M}_\odot$), faint dwarf galaxies ($M_\mathrm{UV} \approx -16.6$), with high ionizing photon production efficiencies ($\log\, (ξ_\mathrm{ion, obs}/\mathrm{(Hz\ erg^{-1})}) \approx 26.0$). Hence these are plausible sites of near-pristine star formation, comprising ${\sim}0.04$-$0.6\%$ of $2.5 < z < 6.5$ galaxies at $-19 < M_\mathrm{UV} < -16$. We discuss this extremely metal-poor extension to the mass-metallicity relation. We forecast that deep (${\sim}28$ h) NIRCam slitless spectroscopy can identify bright EMPGs through strong H$β$ but lack of [O III] emission, or secure the redshifts of fainter systems through H$α$ detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to $z=10$, without requiring deep medium-band/MIRI imaging to identify secure candidates. |
| title | Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5 < z < 6.5$ with deep JADES medium-band imaging |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2603.15761 |