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

We present emission-line ratios from a sample of 27 Lyman-break galaxies from z similar to 5:5 9:5 with -17:0 < M-1500 < -20:4, measured from ultra-deep JWST /NIRSpec multi-object spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES). We used a combination of 28 h deep PRISM /CLEAR and 7 h deep G395M/F290LP observations to measure, or place strong constraints on, ratios of widely studied rest-frame optical emission lines including H alpha, H beta, [O II] lambda lambda 3726, 3729, [Ne III] lambda 3869, [O III] lambda 4959, [O III] lambda 5007, [O I] lambda 6300, [N II] lambda 6583, and [S ii] lambda lambda 6716, 6731 in individual z > 5:5 spectra. We find that the emission-line ratios exhibited by these z similar to 5.5-9.5 galaxies occupy clearly distinct regions of line-ratio space compared to typical z similar to 0 3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [O III] /[O II] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [O III] /[O II] >10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [O II] /H beta and [O III] /[O II], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [N ii] lambda 6583 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 (logU similar to -1:5), and a range of metallicities spanning from similar to 0:1 x Z(circle dot) to higher than similar to 0:3 x Z(circle dot), 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.

Automated summary

We present emission-line ratios from a sample of 27 Lyman-break galaxies at z similar to 5.5-9.5, showing that they occupy distinct regions of line-ratio space compared to typical low-redshift galaxies. The scatter in line ratios suggests significant diversity in ISM conditions within the sample. The observed emission-line ratios are generally consistent with galaxies with high ionisation parameters and a range of metallicities, indicating low-metallicity systems undergoing rapid star formation.