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

article Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization published yes M. Llerena author L. Pentericci author R. Amorín author A. Ferrara author M. Dickinson author F. Arevalo-Gonzalez author A. Calabrò author L. Napolitano author Sara Mascia author edaf889c-c7cd-11ef-ab1b-bb28c431bd29 P. Arrabal Haro author R. Begley author N. J. Cleri author K. Davis author W. Hu author J. S. Kartaltepe author A. M. Koekemoer author R. A. Lucas author E. Mcgrath author D. J. Mcleod author C. Papovich author T. M. Stanton author A. J. Taylor author R. Tripodi author X. Wang author L. Y.A. Yung author JoMa department 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(ξion [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. https://research-explorer.ista.ac.at/download/23049/23100/2026_AstronomyAstrophysics_Llerena.pdf application/pdfno EDP Sciences2026 eng Astronomy and Astrophysics 0004-6361 1432-0746 2510.2564710.1051/0004-6361/202557897 708 Llerena, M., et al. “Extreme Equivalent-Width-Selected Low-Mass Starbursts at z = 4 − 9: Insights into Their Role in Cosmic Reionization.” Astronomy and Astrophysics, vol. 708, A152, EDP Sciences, 2026, doi:<a href="https://doi.org/10.1051/0004-6361/202557897">10.1051/0004-6361/202557897</a>. Llerena, M., L. Pentericci, R. Amorín, A. Ferrara, M. Dickinson, F. Arevalo-Gonzalez, A. Calabrò, et al. “Extreme Equivalent-Width-Selected Low-Mass Starbursts at z = 4 − 9: Insights into Their Role in Cosmic Reionization.” Astronomy and Astrophysics. EDP Sciences, 2026. <a href="https://doi.org/10.1051/0004-6361/202557897">https://doi.org/10.1051/0004-6361/202557897</a>. M. Llerena et al., “Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization,” Astronomy and Astrophysics, vol. 708. EDP Sciences, 2026. Llerena, M., Pentericci, L., Amorín, R., Ferrara, A., Dickinson, M., Arevalo-Gonzalez, F., … Yung, L. Y. A. (2026). Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization. Astronomy and Astrophysics. EDP Sciences. <a href="https://doi.org/10.1051/0004-6361/202557897">https://doi.org/10.1051/0004-6361/202557897</a> Llerena M, Pentericci L, Amorín R, et al. Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization. Astronomy and Astrophysics. 2026;708. doi:<a href="https://doi.org/10.1051/0004-6361/202557897">10.1051/0004-6361/202557897</a> Llerena M, Pentericci L, Amorín R, Ferrara A, Dickinson M, Arevalo-Gonzalez F, Calabrò A, Napolitano L, Mascia S, Arrabal Haro P, Begley R, Cleri NJ, Davis K, Hu W, Kartaltepe JS, Koekemoer AM, Lucas RA, Mcgrath E, Mcleod DJ, Papovich C, Stanton TM, Taylor AJ, Tripodi R, Wang X, Yung LYA. 2026. Extreme equivalent-width-selected low-mass starbursts at z = 4 − 9: Insights into their role in cosmic reionization. Astronomy and Astrophysics. 708, A152. M. Llerena, L. Pentericci, R. Amorín, A. Ferrara, M. Dickinson, F. Arevalo-Gonzalez, A. Calabrò, L. Napolitano, S. Mascia, P. Arrabal Haro, R. Begley, N.J. Cleri, K. Davis, W. Hu, J.S. Kartaltepe, A.M. Koekemoer, R.A. Lucas, E. Mcgrath, D.J. Mcleod, C. Papovich, T.M. Stanton, A.J. Taylor, R. Tripodi, X. Wang, L.Y.A. Yung, Astronomy and Astrophysics 708 (2026). 230492026-10-04T22:01:55Z2026-10-12T07:20:30Z