JADES: Probing interstellar medium conditions at $z\sim 5.5-9.5$ with ultra-deep JWST/NIRSpec spectroscopy

Abstract

We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim 5.5-9.5$ with $-17.0<M_{1500}<-20.4$, measured from ultra-deep JWST/NIRSpec MSA spectroscopy from JADES. We use 28 hour deep PRISM/CLEAR and 7 hour deep G395M/F290LP observations to measure, or place strong constraints on, ratios of widely studied rest-frame optical emission lines including H$\hat{I}\pm$, H${\hat{I}}^2$, [OII] $\hat{I}»\hat{I}»$3726,3729, [NeIII] $\hat{I}»$3869, [OIII] $\hat{I}»$4959, [OIII] $\hat{I}»$5007, [OI] $\hat{I}»$6300, [NII] $\hat{I}»$6583, and [SII] $\hat{I}»\hat{I}»$6716,6731 in individual $z>5.5$ spectra. We find that the emission line ratios exhibited by these $z\sim 5.5-9.5$ galaxies occupy clearly distinct regions of line-ratio space compared to typical z~0-3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [OIII]/[OII] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [OIII]/[OII]>10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [OII]/H${\hat{I}}^2$ and [OIII]/[OII], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [NII] in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log $U\sim -1.5$), and a range of metallicities spanning from $\sim 0.1\times Z_{\odot }$ to higher than $\sim 0.3\times Z_{\odot }$, suggesting we are probing low-metallicity systems undergoing periods of rapid star-formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.

20 pages, 9 figures, submitted to Astronomy & Astrophysics, updated values in tables