Physical properties of extreme emission-line galaxies at z ∼ 4–9 from the JWST CEERS survey

¹ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monte Porzio Catone, Italy
² Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200, Norte, La Serena, Chile
³ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
⁴ ARAID Foundation, Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC, Plaza San Juan 1, E–44001 Teruel, Spain
⁵ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁶ Department of Physics, 196A Auditorium Road, Unit 3046, University of Connecticut, Storrs, CT 06269, USA
⁷ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
⁸ Department of Physics and Astronomy, Texas A&M University, College Station, 77843-4242 TX, USA
⁹ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁰ Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, USA
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Institute of Physics, Laboratory of Galaxy Evolution, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹³ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹⁴ Department of Physics and Astronomy, Colby College, Waterville, ME 04901, USA
¹⁵ Department of Physics and Astronomy, University of California Riverside, 900 University Avenue, Riverside, CA 92521, USA
¹⁶ Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro, 2, 00185 Roma, Italy
¹⁷ ESA/AURA Space Telescope Science Institute, Baltimore, USA
¹⁸ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
¹⁹ Institute of Space Sciences and Astronomy, University of Malta, Msida, MSD 2080, Malta

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

Received: 8 March 2024
Accepted: 9 August 2024

Abstract

Context. Extreme emission line galaxies (EELGs) are typically characterized by high equivalent widths (EWs) which are driven by elevated specific star formation rates (sSFRs) in low-mass galaxies with subsolar metallicities and little dust. Such extreme systems are exceedingly rare in the local universe, but the number density of EELGs increases with increasing redshift. Such starburst galaxies are currently strongly presumed to be the main drivers of hydrogen reionization over 5.5 < z < 15, which serves to motivate many of the searches for high-z EELGs.

Aims. We aim to characterize the physical properties of a sample of ∼730 EELGs at 4 ≲ z < 9 photometrically selected from the CEERS survey using JWST/NIRCam. We validate our method and demonstrate the main physical properties of a subset of EELGs using NIRSpec spectra.

Methods. We create synthetic NIRCam observations of EELGs using empirical templates based on ∼2000 local metal-poor starbursts to select EELGs based on color-color criteria. We study their properties based on SED fitting and flux excess from emission lines in the photometric filters.

Results. Our sample of EELGs has a mean stellar mass of 10^7.84 M_⊙ with high sSFRs from SED fitting with a mean value of 10^−7.03 yr⁻¹. We consider a delayed-τ model for the star formation history and find our sample of EELGs are young with a mean value of the time after the onset of star formation of 45 Myr. We find that they have similar line ratios to local metal-poor starburst galaxies with high log([OIII]/Hβ) ≳ 0.4−1 which indicates that star formation may be the dominant source of ionization in these galaxies. Based on the photometric fluxes and morphologies, we find an increase of EW([OIII]+Hβ) with sSFR and Σ_SFR, and a decrease with age and stellar mass. The sample of EELGs can reach Σ_SFR > 10 M_⊙ yr⁻¹ kpc⁻² which indicate they are strong candidates of LyC leakers. Another indirect indicator is the high values of O32 > 5 that can be reached for some galaxies in the sample. This indicates that they may have the conditions to facilitate the escape of ionizing photons.