EELG1002: A Record-Breaking [OIII]+H${\hat{I}}^2$ EW $\sim 3700$Ã ~Galaxy at $z\sim 0.8$ -- Analog of Early Galaxies?

Abstract

We present a detailed analysis of EELG1002: a $z=0.8275$ EELG identified within archival Gemini/GMOS spectroscopy as part of the on-going COSMOS Spectroscopic Archive. We find EELG1002 is a low-mass ($\sim 10^8$ M${}_{\odot }$), compact ($\sim 530$ pc), bursty star-forming galaxy with a $\sim 15-35$ Myr mass doubling timescale. EELG1002 has record-breaking rest-frame [O{\sc iii}]+H${\hat{I}}^2$~EW $\sim 3100-3700$Ã ; $\sim 32-36\times$ higher than typical $z\sim 0.8$ [O{\sc iii}]~emitters with similar stellar mass and higher than typical $z>5$ galaxies. We find no clear evidence of an AGN suggesting the emission lines are star formation driven. EELG1002 is chemically unevolved (direct $T_e$; $12+{\log }_{10}(\text{O/H})\sim 7.52$ 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 ISM ([O{\sc iii}]/[O{\sc ii}]~$\sim 9$) and hard ionizing radiation field (elevated [Ne{\sc iii}]/[O{\sc ii}]~at fixed [O{\sc iii}]/[O{\sc ii}]). Coupled with its compact, metal-poor, and actively star-forming nature, EELG1002 is found to efficiently produce ionizing photons ($\hat{I}{¾}_{\mathrm{ion}}\sim 10^{25.74}$erg${}^{-1}$ Hz) and may have $\sim 10-20\%$LyC escape suggesting such sources may be important analogs of galaxies responsible for reionization. We find dynamical mass of $\sim 10^9$M${}_{\odot }$~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 datasets can support high-$z$ science and next-generation surveys planned with \textit{Euclid} and \textit{Roman}.

27 pages, 9 Figures, 8 Tables. Accepted to ApJ (09/04/2025). Abstract is abridged. All code can be accessed via our github repository (https://github.com/akhostov/EELG1002)