EELG1002: A Record-breaking O iii +H β EW ∼ 3700 Å Galaxy at z  ∼ 0.8—Analog of Early Galaxies?

Abstract 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 (∼10 8 M ⊙ ), compact (∼530 pc), bursty star-forming galaxy with a ∼15–35 Myr mass doubling timescale. EELG1002 has record-breaking rest-frame O iii +H β 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 + log 10 ( O/H ) ∼ 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 ∼10 9 M ⊙ 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.