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

TL;DR

EELG1002 is a low-mass, compact galaxy at $z\sim0.83$ with a record rest-frame $\mathrm{EW}_{\mathrm{[OIII]+H\beta}}$ of $\sim3100$–$3700$ Å, indicating a recent intense starburst in an chemically unevolved ISM. Through deep GMOS spectroscopy and comprehensive SED fitting (Cigale and Bagpipes) calibrated to direct $T_e$ abundances, the study reveals $12+\log(\mathrm{O/H}) \approx 7.52$ and a high ionization parameter ($\log U \sim -2.2$ to $-1.96$) with a hard radiation field ($\mathrm{O32} \approx 9$; $n_e \sim 7.8\times10^2$ cm$^{-3}$). The galaxy shows elevated $\xi_{ion} \approx 10^{25.7}$ erg$^{-1}$ Hz and a likely LyC escape fraction of order $f_{esc}\sim0.1$–$0.2$, making it a compelling local analog of reionization-era systems and a valuable testbed for ISM physics, star-formation histories, and feedback-driven LyC leakage. Comparisons with Illustris-TNG analogs support a realistic, burst-driven SFH and indicate substantial gas reservoirs and potential evolutionary paths toward massive, chemically enriched galaxies or compact quiescent descendants. Overall, EELG1002 demonstrates the power of archival datasets to illuminate high-$z$ galaxy processes and informs planning for Euclid/Roman surveys targeting early-universe analogs.

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$β$~EW $\sim3100-3700$Å; $\sim32-36\times$ 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 ($ξ_\mathrm{ion}\sim10^{25.74}~$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}.

Figures (9)

• Figure 1: GMOS slit position (blue) for EELG1002 with a $10" \times 10"$HST/ACS F814W cutout ($0.03"$ pix$^{-1}$ resolution; background). Subaru/HSC $i$ detection is shown in red contours highlighting the lower resolution of ground-based observations where we find $\sim 50$% of EELG1002's total flux is observed along the $0.5"$ GMOS slit. EELG1002 has GALEX/NUV (purple contours) and FUV detections which would suggest strong Ly$\alpha$ emission and possible LyC escape, respectively; however, GALEX spatial resolution is quite poor. The proximity of two nearby sources along line-of-sight, shown with their COSMOS2020/Classic IDs and best-fit photometric redshifts, suggests the GALEX detections suffer blending issues. We therefore ignore GALEX, as well as Spitzer, photometry in this analysis due to blending issues.
• Figure 2: Top: Reduced GMOS spectra of EELG1002 highlighting multiple emission line detections securing the spectroscopic redshift with the fitted emission line spectrum (black dashed line). Strong [O iii]5007Å relative to [O ii]3727Å emission along with comparable [Ne iii]3869Å and [O ii] emission highlight the highly energetic ISM. [O ii]4363Å detection allows for direct-$T_e$ abundance measurements confirming the low-metallicity nature. Bottom: Multiwavelength photometry and best-fit SEDs for EELG1002 with Cigale (green) and Bagpipes (red). Strong [O iii]+H$\beta$ color excess is clearly observed in Subaru/HSC and SCam $i$ indicative of high EWs. HST F814W also shows an excess although not as sensitive given the wider wavelength coverage. Subaru intermediate and narrowbands are also affected by strong nebular emission line features. Cigale and Bagpipes are in relative agreement with slight differences. We note stellar population, IMF, and SFH modeling assumptions may be driving these minor differences.
• Figure 3: The rest-frame [O iii]+H$\beta$ EW -- stellar mass anti-correlation. EELG1002 is shown as a star with dark blue using Bagpipes and light blue for Cigale for the continuum flux density in measuring [O iii]+H$\beta$ EW. We find EELG1002 is a uniquely 'extreme' ELG with [O iii]+H$\beta$ EW $\sim 32$ -- $36\times$ higher compared to the typical [O iii]$+$H$\beta$ EW at $z \sim 0.8$ (blue line; Khostovan2016) and somewhat higher than the typical EW at $z \sim 3$ -- $9$Khostovan2016Matthee2023Boyett2024. EELG1002 also has EW higher than Blueberries Yang2017_BB, Green Peas Cardamone2009, the $z \sim 2.5$ intense starburst BOSS-EUVLG1 Marques-Chaves2020, and the $z \sim 3.2$ LyC emitter Ion2deBarros2016. EELG1002 is consistent with local LyC leakers Izotov2016Izotov2018Izotov2021 and we do discuss the potential of LyC escape further in this study. Overall, EELG1002 is a uniquely rare and 'extreme' ELG with EWs somewhat more extreme than EoR-era galaxies.
• Figure 4: Mass Excitation (MEx) diagram for SFG/AGN classification where EELG1002 is found to have [O iii]/H$\beta$ ratios and stellar mass consistent with star-forming galaxies. Bagpipes-derived stellar mass is near the boundary; however, this can also be due to the different IMF assumed. [O iii]/H$\beta$ ratios in conjunction with its low direct $T_e$ measured metallicity is consistent with star-forming galaxies with low [N ii]/H$\alpha$ in the BPT diagram Pettini2004Marino2013. EELG1002 also has [O iii]/H$\beta$ ratios consistent with Blueberries Yang2017_BB and Green Peas Yang2017_GP. Based on this diagnostic, EELG1002 shows no evidence of an AGN component.
• Figure 5: Mass -- Metallicity Relation (MZR) with measured relations at $z \sim 0$Andrews2013, $z \sim 0.5 - 1$Ly2016, $z\sim 2.2$ -- $3.3$Sanders2021, and $z \sim 8$Langeroodi2023. EELG1002 is found to have $12+\log_{10}$ (O/H) below $z < 1$ EELGs Amorin2015, Blueberries Yang2017_BB, and Green Peas Yang2017_GP at fixed stellar mass. Both Cigale and Bagpipes stellar mass measurement places EELG1002 highly consistent with $z \sim 5$ -- $9$ galaxies at similar stellar mass Matthee2023Langeroodi2023Boyett2024. Similar to EELG1002, Local LyC leakers Izotov2016Izotov2018, Ion2deBarros2016, and BOSS-EUVLG1 Marques-Chaves2020 are consistent with high-$z$ MZRs but with higher stellar mass and $12 + \log_{10}$ (O/H) suggesting past star formation and chemical enrichment periods. EELG1002 is found to have both low-mass and low metallicity consistent with high-$z$ MZR suggesting a chemically-unevolved system that could be undergoing a first bursty phase of star-formation activity as expected of high-$z$ galaxies (e.g., Cohn2018).