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 ([OIII]λ5007 or Hα rest-frame EW> 750Å) which can be tied to a recent upturn in star formation rate, due to the sensitivity of the nebular line emission and the rest-optical continuum to young (<10Myr) and evolved stellar populations, respectively. By studying a sample of 85 star forming galaxies (SFGs), spanning the redshift and magnitude interval 3 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 mass-weighted ages (<5 Myr old) and the highest burst intensity (those with the highest proportion of their total stellar mass formed in the last 10 Myr). We spectroscopically confirm that a greater proportion of SFGs are in an EELG phase at high redshift in our UV-selected sample (61±4% in our z>5.7 high-redshift bin, compared to 23^+4_-1% in our lowest-redshift bin 35.7 have observed Lyman-α emission, potentially lying within large ionised regions. The high detection rate of Lyman-α emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Lyman-α photons.