J-PAS: First Identification, Physical Properties and Ionization Efficiency of Extreme Emission Line Galaxies

TL;DR

This study develops a photometric method using machine learning to identify extreme emission line galaxies in the J-PAS survey, revealing their properties and potential role in cosmic reionization.

Contribution

It introduces a robust, high-purity, high-completeness technique for selecting EELGs in large photometric surveys, validated with spectroscopic data.

Findings

Identified 917 EELGs up to z=0.8 in 30 deg^2.

Most EELGs have ionizing efficiencies above reionization threshold.

Strong correlation between ionizing photon production and emission line strength.

Abstract

Extreme emission line galaxies (EELGs) are key tracers of intense star formation and potential analogues of the sources that reionized the early Universe. Their low-redshift counterparts offer a unique opportunity to study the physical conditions that enable high ionizing-photon escape fractions. We present a robust method to photometrically identify EELGs in the J-PAS survey, which provides 56 optical bands over 8500 deg^2. Using data from a fully observed 30 deg^2 region, we combine narrow-band equivalent widths with machine-learning techniques to select galaxies with emission lines above 300 {\AA}. The method achieves 95% purity and 96% completeness for $i_\mathrm{SDSS}<22.5$ mag. We identify 917 EELGs up to $z=0.8$; spectroscopic cross-matching with DESI/DR1 confirms the reliability of our redshifts and emission-line measurements. The selected galaxies show strong correlations between $\xi_\mathrm{ion}$ and EW([OIII]), consistent with previous low- and high-z studies. Most sources exceed the ionizing efficiency threshold required for reionization, reinforcing their role as local analogues of early-Universe galaxies.