November
2025
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2025ApJ...994...34K
Authors
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Khostovan, Ali Ahmad
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Kartaltepe, Jeyhan S.
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Brinch, Malte
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Casey, Caitlin
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Faisst, Andreas
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Harish, Santosh
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Gozaliasl, Ghassem
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Onodera, Masato
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Yabe, Kiyoto
Abstract
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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 (∼108 M⊙), 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 Te; 12+log10(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 (ξion ∼ 1025.74 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 ∼109 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.
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