EELG1002: A Record-Breaking [OIII]+H$\beta$ EW $\sim 3700$\AA~Galaxy at $z \sim 0.8$ -- Analog of Early Galaxies?

TL;DR

EELG1002 is a low-mass, compact galaxy at z~0.8 with record-breaking [OIII]+Hβ EW, serving as an analog for early Universe galaxies and providing insights into star formation, ionizing photon production, and potential reionization contributors.

Contribution

This study identifies and characterizes EELG1002 as a local analog of early galaxies, highlighting its extreme emission, chemical properties, and ionizing capabilities, which were previously unobserved at this redshift.

Findings

EELG1002 has a record-breaking [OIII]+Hβ EW of 3700 Å.

It is chemically unevolved with low metallicity ($12+ ext{log}_{10}( ext{O/H}) extasciitilde7.52$).

EELG1002 exhibits high ionizing photon production and potential LyC escape (~10-20%).

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 ($\sim10^{8}$ M$_\odot$), compact ($\sim530$ pc), bursty star-forming galaxy with a $\sim15-35$ Myr mass doubling timescale. EELG1002 has record-breaking rest-frame [O{\sc iii}]+H$\beta$~EW $\sim3100-3700$\AA; $\sim32-36\times$ 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 $T_e$; $12+\log_{10}(\textrm{O/H})\sim7.52$ 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}]~$\sim9$) 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 ($\xi_\mathrm{ion}\sim10^{25.74}~$erg$^{-1}$ Hz) and may have $\sim10-20\%~$LyC escape suggesting such sources may be important analogs of galaxies responsible for reionization. We find dynamical mass of $\sim10^9~$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}.