Extreme emission line galaxies detected in JADES JWST/NIRSpec I: inferred galaxy properties

Abstract

Extreme emission line galaxies (EELGs) exhibit large equivalent widths (EW) in their rest-optical emission lines ([OIII]$\lambda 5007$ or H$\hat{I}\pm$ rest-frame EW$>750\tilde{A}$) 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 ($<10$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 old) 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 ($61\pm 4\%$ 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, ionisation parameter and star formation histories with redshift. We report that the EELGs within our sample exhibit a higher average ionisation efficiency (${\log }_{10}(\hat{I}{¾}_{ion}^{HII}/$erg${}^{-1}$Hz)$=25.5\pm 0.2$) than the non-EELGs. High-redshift EELGs therefore comprise a population of efficient ionising photon producers. Additionally, we report that $53\%$ (9/17) of EELGs at $z>5.7$ have observed Lyman-$\hat{I}\pm$ emission, potentially lying within large ionised regions. The high detection rate of Lyman-$\hat{I}\pm$ emitters in our EELG selection suggests that the physical conditions associated with entering an EELG phase also promote the escape of Lyman-$\hat{I}\pm$ photons.

34 pages, 25 figures