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]lambda 5007 or H alpha rest-frame EW>750 angstrom) 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(-1)(+4)% 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)(xi(HII)(ion)/erg(-1)Hz)=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 alpha emission, potentially lying within large ionized regions. The high detection rate of Ly alpha emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Ly alpha photons.