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

Journal article

We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim5.5-9.5$ with $-17.05.5$ spectra. We find that the emission line ratios exhibited by these $z\sim5.5-9.5$ 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$\beta$ 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 $\sim0.1\times Z_\odot$ to higher than $\sim0.3\times Z_\odot$, 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.Comment: 20 pages, 9 figures, submitted to Astronomy & Astrophysics, updated values in table

Authors: Cameron, AJ; Saxena, A; Bunker, AJ; D’Eugenio, F; Carniani, S

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy & Astrophysics
• Volume: 677
• Pages: A115-A115
• Publication date: 2023-07-28
• Acceptance date: 2023-07-17
• DOI: 10.1051/0004-6361/202346107
• EISSN: 1432-0746
• ISSN: 0004-6361
• Language: English
• Keywords: Emission spectrum; Redshift; Galaxy; Astronomy; Line (geometry); Doubly ionized oxygen; Astrophysics; Spectroscopy; Physics; Spectral line; Interstellar medium