EELG1002: A Record-breaking [O <scp>iii</scp> ]+H <i>β</i> EW ∼ 3700 Å Galaxy at <i>z</i>  ∼ 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 &gt; 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 ; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>12</mml:mn> <mml:mo>+</mml:mo> <mml:msub> <mml:mrow> <mml:mi>log</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> </mml:msub> <mml:mo stretchy="false">(</mml:mo> <mml:mi mathvariant="normal">O/H</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>∼</mml:mo> <mml:mn>7.52</mml:mn> </mml:math> consistent with z &gt; 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.