The ionizing photon production efficiency of star-forming galaxies at z ∼ 4–10

¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
² Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro, 2, 00185 Roma, Italy
³ Institute of Science and Technology Austria (ISTA), Am Campus 1, A-3400 Klosterneuburg, Austria
⁴ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁵ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
⁶ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
⁷ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁸ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Institute of Physics, Laboratory of Galaxy Evolution, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹¹ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218 Hawthorn, VIC 3122, Australia
¹² Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
¹³ Black Hole Initiative, Harvard University, Cambridge, MA 02138, USA
¹⁴ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁵ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
¹⁶ Institute of Space Sciences and Astronomy, University of Malta, Msida, MSD 2080, Malta
¹⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁸ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁹ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
²⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
²¹ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
²² Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
²³ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
²⁴ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
²⁵ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA
²⁶ ESA/AURA Space Telescope Science Institute, Baltimore, USA

^⋆ Corresponding author: mario.llerenaona@inaf.it

Received: 2 December 2024
Accepted: 23 May 2025

Abstract

Context. Investigating the ionizing emission of star-forming galaxies and the escape fraction of ionizing photons is critical to understanding their contribution to reionization and their impact on the surrounding environment. The number of ionizing photons available to reionize the intergalactic medium (IGM) depends on not only the abundance of galaxies but also their efficiency in producing ionizing photons (ξ_ion). This quantity is thus fundamental to quantify the role of faint versus bright sources in driving this process, as we must assess their relative contribution to the total ionizing emissivity.

Aims. Our goal is to estimate the ξ_ion using Balmer lines (Hα or Hβ) in a sample of 761 galaxies at 4 ≤ z ≤ 10 selected from different JWST spectroscopic surveys. We aim to determine the redshift evolution of ξ_ion and the relation of ξ_ion with the physical properties of the galaxies.

Methods. We used the available HST and JWST photometry to perform a spectral energy distribution (SED) fitting in the sample to determine their physical properties and relate them with ξ_ion. We used the BAGPIPES code for the SED fitting and assumed a delayed exponential model for the star formation history. We used the NIRSpec spectra from prism or grating configurations to estimate Balmer luminosities, and then constrained ξ_ion values after dust correction.

Results. We find a mean value of 10^25.22 Hz erg⁻¹ for ξ_ion in the sample with an observed scatter of 0.42 dex. We find an increase in the median values of ξ_ion with redshift from 10^25.09 Hz erg⁻¹ at z ∼ 4.18 to 10^25.28 Hz erg⁻¹ at z ∼ 7.14, confirming the redshift evolution of ξ_ion found in other studies. Regarding the relation between ξ_ion and physical properties, we find a decrease in ξ_ion with increasing stellar mass, indicating that low-mass galaxies are efficient producers of ionizing photons. We also find an increase in ξ_ion with increasing specific star formation rate (sSFR) and increasing UV absolute magnitude. This indicates that faint galaxies and galaxies with high sSFR are also efficient producers. We also investigated the relation of ξ_ion with the rest-frame equivalent width (EW) of [OIII]λ5007 and find that galaxies with the higher EW([OIII]λ5007) are more efficient producers of ionizing photons, with the best fit leading to the relation log(ξ_ion)  =  0.43 × log(EW[OIII])+23.99. Similarly, we find that galaxies with higher O32 = [OIII]λ5007/[OII]λλ3727,3729 and lower gas-phase metallicities (based on the R23 = ([OIII]λλ4959,5007+[OII]λλ3727,3729)/Hβ calibration) show higher ξ_ion values.