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

JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, Z < 1~\ per\ cent~Z_) at redshift 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, utilizing the extensive deep NIRCam medium-band imaging from JADES. Our EMPG candidates at 2.5 < z < 6.5 exhibit strong photometric boosts by Hα, yet correspondingly weak boosts by [O iii] + Hβ, 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 (~0.1–1~\ per\ cent~Z_) indicates that our 14 EMPG candidates are low-mass (median M* ≈ 106.7 M⊙), faint dwarf galaxies (MUV ≈ −16.9), with high ionizing photon production efficiencies (log (ξion, obs/(Hz erg−1)) ≈ 26.0). Hence these are plausible sites of near-pristine star formation, constituting ~0.02–0.4% of 2.5 < z < 6.5 galaxies at −19 < MUV < −16. We discuss this extremely metal-poor extension to the mass–metallicity relation. We forecast that deep (~28 h) NIRCam slitless spectroscopy can identify bright EMPGs through strong Hβ but lack of [O iii] emission, or secure the redshifts of fainter systems through Hα detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to z = 10, without requiring deep NIRCam medium-band and/or MIRI imaging to identify secure candidates.