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

This is the authors' abstract. We don't add key points for this paper.

Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([O iii]λ 5007 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>MUV> -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⁺⁴₋₁% 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 (log ₁₀(ξ ᵢonHII/ 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.