2024MNRAS.535.1796B

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$\gt 750$ 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 ($\lt 10$ Myr) and evolved stellar populations, respectively. By studying a sample of 85 star-forming galaxies (SFGs), spanning the redshift and magnitude interval $3 \lt z\lt 9.5$ and $-16\gt $M$\rm _{UV}\gt -21$, in the JWST Advanced Deep Extragalactic Survey (JADES) with NIRSpec/prism spectroscopy, we determine that SFGs initiate an EELG phase when entering a significant burst of star formation, with the highest EWs observed in EELGs with the youngest luminosity-weighted ages ($\lt 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 ($61\pm 4~{{\ \rm per\ cent}}$ in our $z\gt 5.7$ high-redshift bin, compared to $23^{+4}_{-1}\%$ in our lowest redshift bin $3\lt z\lt 4.1$) 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 ($\log _{10}(\xi _\mathrm{ion}^\mathrm{HII}/{\rm erg^{-1}Hz}) =25.5\pm 0.2$) 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 $z\gt 5.7$ 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.