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

A. Giménez-Alcázar, R. Amorín, J. M. Vílchez, A. Hernán-Caballero, M. González-Otero, A. Arroyo-Polonio, J. Iglesias-Páramo, A. Lumbreras-Calle, J. A. Fernández-Ontiveros, L. Bonatto, R. M. González Delgado, C. Kehrig, A. Torralba, P. T. Rahna, Y. Jiménez-Teja, I. Márquez, I. Breda, A. Álvarez-Candal, R. Abramo, J. Alcaniz, N. Benitez, S. Bonoli, S. Carneiro, J. Cenarro, D. Cristóbal-Hornillos, R. Dupke, A. Ederoclite, C. Hernández-Monteagudo, A. Marín-Franch, C. Mendes de Oliveira, M. Moles, L. Sodré, K. Taylor, J. Varela, H. Vázquez Ramió

TL;DR

This work develops a robust photometric method to identify Extreme Emission Line Galaxies (EELGs) in the J-PAS survey by combining a narrow-band EW threshold with a neural-network filter to remove AGN and cosmetic defects. Applying the method to a 30 deg$^2$ subset (i$<22.5$) yields 917 EELGs up to $z\approx 0.8$, with a purity of $\sim95\%$ and completeness of $\sim96\%$, and cross-matches with DESI confirm redshifts and emission-line measurements. SED fitting with CIGALE and extended photometry provide physical properties (median $\log(M_*/M_\odot) \approx 8.66$) and show that the ionizing photon production efficiency $\xi_{\mathrm{ion}}$ correlates positively with $EW_0$([O III]) with a slope of about 0.69, and that many sources exceed the reionization threshold $\log_{10}(\xi_{\mathrm{ion}}) \gtrsim 25.3$. The results reinforce EELGs as local analogues of early-universe galaxies and demonstrate a scalable pathway to build large EELG samples with J-PAS, enabling investigations of ionizing budgets and photon escape in low-redshift systems.

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 Å. 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 $ξ_\mathrm{ion}$ 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.

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

TL;DR

This work develops a robust photometric method to identify Extreme Emission Line Galaxies (EELGs) in the J-PAS survey by combining a narrow-band EW threshold with a neural-network filter to remove AGN and cosmetic defects. Applying the method to a 30 deg subset (i) yields 917 EELGs up to , with a purity of and completeness of , and cross-matches with DESI confirm redshifts and emission-line measurements. SED fitting with CIGALE and extended photometry provide physical properties (median ) and show that the ionizing photon production efficiency correlates positively with ([O III]) with a slope of about 0.69, and that many sources exceed the reionization threshold . The results reinforce EELGs as local analogues of early-universe galaxies and demonstrate a scalable pathway to build large EELG samples with J-PAS, enabling investigations of ionizing budgets and photon escape in low-redshift systems.

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 Å. The method achieves 95% purity and 96% completeness for mag. We identify 917 EELGs up to ; spectroscopic cross-matching with DESI/DR1 confirms the reliability of our redshifts and emission-line measurements. The selected galaxies show strong correlations between 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.

Paper Structure

This paper contains 25 sections, 6 equations, 18 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data
  3. Methodology
  4. Step I: Classical search
  5. Step II: AGN/Quasar and cosmetic defects
  6. Neural Network: input and output
  7. Sample selection and characterization
  8. Purity and AGN contribution
  9. Redshift estimation
  10. SED fitting
  11. Emission line flux extraction
  12. Line flux equivalent widths
  13. Discussion
  14. The ionizing rate of EELGs
  15. Ionizing rate vs EW (OIII])
  16. ...and 10 more sections

Figures (18)

  • Figure 1: J-PAS IDR202406 observed footprint with all the filters showing the positions of the seed fields. The coordinates (RA, DEC) in degrees are: CODEX (126.1125, 40.1053), miniJPAS (214.4500, 52.7261), JPSV (244.00, 43.00), and StephQuint (339.00, 22.50).
  • Figure 2: Data products from J-PAS for the EELG candidates. Left: The black line represents the J-PAS photometric spectrum, while the red line shows the corresponding DESI spectrum. The shaded gray region marks the wavelength range selected for integration. Right: Image cutouts resulting from integrating the data cube over the selected spectral region. The horizontal white bar corresponds to 2 arcseconds in length. Highlight that with J-PAS we are able to detect the continuum, whereas DESI cannot.
  • Figure 3: BPT diagram (BPT1981PASP...93....5B). Blue point represent the data points that have a counterpart in DESI with the H$\alpha$. The data points classified as AGNs are plotted in orange. The solid red line corresponds with the Kewley relationship kewley2001ApJ...556..121K.
  • Figure 4: A 1:1 correlation is observed between the spectroscopic redshifts and the best-fit values from LePhare. The inset quantifies the relative differences between the two redshift estimates.
  • Figure 5: Main sequence of star-forming galaxies. The plot shows the logarithm of the star formation rate, $\log(\mathrm{SFR}_{10})$, versus the logarithm of the stellar mass, $\log(M_\star)$. The SFR$_{10}$ refers to the average star formation rate over the past 10 Myr. Red points indicate galaxies with spectroscopic observations from DESI. The dotted black line shows the relation from cole2025ApJ...979..193C at redshift 4.5–5. The solid black line corresponds to the relation from (curtis2021MNRAS.503.4855C; mock photometric samples of galaxies at z $\approx$ 5), the dot-dashed line represents the relation from speagle2014ApJS..214...15S (64 measurements of the star-forming "Main Sequence" from literature out to z $\approx$ 6) and the pink line shows the results from the Millennium Simulation millenium2005Natur.435..629S. The dashed lines draw regions of constant specific star formation rate (sSFR) at values of -7 and -9.
  • ...and 13 more figures