Extreme emission line galaxies detected in JADES JWST/NIRSpec – I. Inferred galaxy properties

Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([O III]λ5007 or H α rest-frame EW> 750 Å) which can be tied to a recent upturn in star formation rate (SFR), due to the sensitivity of the nebular line emission and the rest-optical continuum to young (< 10 Myr) and evolved stellar populations, respectively. By studying a sample of 85 star-forming galaxies (SFGs), spanning the redshift and magnitude interval 3 < z < 9.5 and −16 >M_UV > −21, in the JWST Advanced Deep Extragalactic Survey (JADES) with NIRSpec/prism spectroscopy, we determine that SFGs initiate an EELG phase when entering a significant burst of star formation, with the highest EWs observed in EELGs with the youngest luminosity-weighted ages (< 5 Myr) and the highest burst intensity (those with the greatest excess between their current and long-term average SFR). We spectroscopically confirm that a greater proportion of SFGs are in an EELG phase at high redshift in our UV-selected sample (61 ± 4 per cent in our z > 5.7 high-redshift bin, compared to 23⁺₋₁⁴% in our lowest redshift bin 3 < z < 4.1) due to the combined evolution of metallicity, ionization parameter, and star formation histories with redshift. We report that the EELGs within our sample exhibit a higher average ionization efficiency (log₁₀(ξ_ion^HII/erg⁻¹Hz) = 25.5 ± 0.2) than the non-EELGs. High-redshift EELGs therefore comprise a population of efficient ionizing photon producers. Additionally, we report that 53 per cent (9/17) of EELGs at z > 5.7 have observed Ly α emission, potentially lying within large ionized regions. The high detection rate of Ly α emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Ly α photons.