Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization

¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
² Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 50126 Pisa, Italy
⁴ NSF NOIRLab, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁵ Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro, 2, 00185 Roma, Italy
⁶ Institute of Science and Technology Austria (ISTA), Am Campus 1, A-3400 Klosterneuburg, Austria
⁷ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
⁸ Armagh Observatory and Planetarium, College Hill, Armagh, BT61 9DG N. Ireland, UK
⁹ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
¹² Department of Physics, 196 Auditorium Road, Unit 3046, University of Connecticut, Storrs, CT 06269, USA
¹³ Los Alamos National Laboratory, Los Alamos, NM 87545, USA
¹⁴ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁵ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁶ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁸ Department of Physics and Astronomy, Colby College, Waterville, ME 04901, USA
¹⁹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
²⁰ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
²¹ Cosmic Frontier Center, The University of Texas at Austin, Austin, TX, USA
²² School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
²³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
²⁴ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

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

Received: 29 October 2025
Accepted: 4 March 2026

Abstract

We investigate the properties of extreme emission line galaxies (EELGs) at z = 4 − 9 and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compiled 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+Hβ) = 1616 Å and EW(Hα) = 763 Å. They are low-mass (median log(M_★/M_⊙) = 8.26) with high sSFRs (median 43 Gyr⁻¹), above the z ∼ 6 main sequence. UV slopes are diverse, with a median value of β = −2.0, and only 7% have extremely blue continua (β < −2.6). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with median log(ξ_ion [Hz erg⁻¹]) = 25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at z < 7 show escape fractions > 5% for both Lyα and LyC photons, while 82% lack detectable Lyα emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR > 25 Gyr⁻¹. These results indicate that EELGs contribute approximately 16 − 40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star formation burst. Our analysis indicates that sSFR and star formation rate surface density are the primary drivers of their extreme emission line strengths.