J-PAS: First identification, physical properties, and ionization efficiency of extreme emission line galaxies

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC) Glorieta de la Astronomía s/n 18008 Granada, Spain
² Centro de Estudios de Física del Cosmos de Aragón (CEFCA) Plaza San Juan 1 44001 Teruel, Spain
³ Unidad Asociada CEFCA-IAA, CEFCA, Unidad Asociada al CSIC por el IAA y el IFCA Plaza San Juan 1 44001 Teruel, Spain
⁴ Vicerrectoría de Investigación y Postgrado, Universidad de La Serena 1700000, Chile
⁵ Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna Murata 605 00143 Rome, Italy
⁶ Observatório Nacional, Rua General José Cristino 77 São Cristóvão 20921-400 Rio de Janeiro, Brazil
⁷ Institute of Science and Technology Austria (ISTA) Am Campus 1 3400 Klosterneuburg, Austria
⁸ Dep. of Astrophysics, University of Vienna Türkenschanzstraße 17 1180 Vienna, Austria
⁹ Instituto de Física, Universidade de São Paulo (IF/USP) São Paulo, Brazil
¹⁰ Donostia International Physics Center (DIPC) Manuel Lardizabal Ibilbidea 4 San Sebastián, Spain
¹¹ Instituto de Astrofísica de Canarias (IAC), C/ Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain; Universidad de La Laguna (ULL) Avda Francisco Sánchez 38206 San Cristóbal de La Laguna Tenerife, Spain
¹² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo (IAG/USP) São Paulo, Brazil
¹³ Instruments 4121 Pembury Place La Canada Flintridge CA 91011, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 September 2025
Accepted: 3 December 2025

Abstract

Context. Extreme emission line galaxies (EELGs) are believed to significantly contribute to the star formation activity and mass assembly in galaxies. EELGs likely also play a leading role in the cosmic re-ionization as their interstellar medium may allow a significant fraction of their ionizing photons to escape (> 5%). Finding low-redshift analogues of these high-z galaxies is therefore essential to characterizing the physical conditions in the interstellar medium of these galaxies and understanding the processes that re-ionized the Universe.

Aims. We aimed to develop a robust and efficient method for the photometric identification of EELGs using the J-PAS survey. J-PAS will cover approximately 8500 deg² of the sky with 54 narrow-band filters in the optical range plus i-SDSS, enabling detailed studies of the physical properties of these galaxies. In this work we focused on an initial subset of the survey: a 30 square degree area with complete observations in all bands.

Methods. We combine equivalent width (EW) measurements from J-PAS narrow-band photometry with artificial intelligence techniques to identify galaxies with emission lines exceeding 300 Å. We validated our selection using spectroscopic data from DESI DR1 and characterized the selected sample through spectral energy distribution fitting with CIGALE.

Results. We identify 917 EELGs up to z = 0.8 over 30 deg², achieving a purity of 95% and a completeness of 96% for i-SDSS < 22.5 mag. Importantly, active galactic nucleus contamination was carefully considered and is estimated to be around 5%. Furthermore, a cross-match with DESI yielded 79 counterparts; their redshifts are in excellent agreement with our photometric estimates, thereby confirming the reliability of our redshift determination. In addition, the derived emission line fluxes are in good agreement with spectroscopic measurements. Moreover, the selected sample reveals strong correlations between the ionizing photon production efficiency (ξ_ion) and EW(Hβ), which are consistent with previous observational studies at low and high redshifts and theoretical expectations. Finally, most of the sources surpass the ionizing efficiency threshold required for re-ionization, highlighting their relevance as local analogues of early-Universe galaxies.