2025ApJ...994...34K

Extreme emission line galaxies (EELGs) are powerful low-z analogs of high-z galaxies that can provide us valuable insights of early Universe conditions. We present a detailed analysis of EELG1002: a z = 0.8275 EELG identified within archival Gemini/GMOS spectroscopy as part of the ongoing COSMOS Spectroscopic Archive. We find EELG1002 is a low-mass (∼10⁸M_⊙), compact (∼530 pc), bursty star-forming galaxy with a ∼15-35 Myr mass doubling timescale. EELG1002 has record-breaking rest-frame [O III]+Hβ EW ∼3100-3700 Å; ∼32-36× higher than typical z ∼ 0.8 [O III] emitters with similar stellar mass; and higher than typical z > 5 galaxies. We find no clear evidence of an active galactic nucleus suggesting the emission lines are star formation driven. EELG1002 is chemically unevolved (direct T_e; $12+{\mathrm{log}}_{10}({\rm{O/H}})\sim 7.52$ consistent with z > 5 galaxies at fixed stellar mass) and may be undergoing a first intense, bursty star formation phase analogous to conditions expected of galaxies in the early Universe. We find evidence for a highly energetic interstellar medium ([O III]/[O II] ∼ 9) and hard ionizing radiation field (elevated [Ne III]/[O II] at fixed [O III]/[O II]). Coupled with its compact, metal-poor, and actively star-forming nature, EELG1002 is found to efficiently produce ionizing photons (ξ_ion ∼ 10^25.74 erg^–1 Hz) and may have ∼10%-20% Lyman Continuum (LyC) escape suggesting such sources may be important analogs of galaxies responsible for reionization. We find a dynamical mass of ∼10⁹M_⊙ suggesting copious amounts of gas to support intense star formation as also suggested by identified Illustris-TNG analogs. EELG1002 may be an ideal low-z laboratory of galaxies in the early Universe and demonstrates how archival data sets can support high-z science and next-generation surveys planned with Euclid and Roman.