Out of oxygen: Extremely metal-poor galaxy candidates identified at $2.5<z<6.5$ with deep JADES medium-band imaging

Abstract

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$\alpha$, yet correspondingly weak boosts by [O III] + H$\beta$, 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 $M_{\ast }\approx 10^{6.7}{\mathrm{M}}_{\odot }$), faint dwarf galaxies ($M_{\mathrm{UV}}\approx -16.6$), with high ionizing photon production efficiencies ($\log ({\xi }_{\mathrm{ion},\mathrm{obs}}/(\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1}))\approx 26.0$). 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$\beta$ but lack of [O III] emission, or secure the redshifts of fainter systems through H$\alpha$ 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.

26 pages, 14 figures, 2 tables. Updated author list

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