[Paper Review] JADES: Probing interstellar medium conditions at $z\sim5.5-9.5$ with ultra-deep JWST/NIRSpec spectroscopy

Alex J. Cameron, Aayush Saxena+43 more|arXiv (Cornell University)|Feb 8, 2023

Galaxies: Formation, Evolution, Phenomena22 citations

TL;DR

This paper presents ultra-deep JWST/NIRSpec spectroscopy of 27 Lyman break galaxies at z~5.5–9.5 from the JADES survey, measuring rest-frame optical emission lines to diagnose ISM conditions and chemical enrichment.

ABSTRACT

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$β$ 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 $\sim0.1 imes Z_\odot$ to higher than $\sim0.3 imes Z_\odot$, 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.

Motivation & Objective

• Motivate the use of ultra-deep JWST/NIRSpec spectroscopy to probe ISM conditions in the early Universe (z>5.5).
• Measure rest-frame optical emission lines in individual z>5.5 galaxies to derive line ratios diagnostic of metallicity, ionisation, and radiation field.
• Characterize diversity and typical ISM properties (ionisation parameter, metallicity range) in an extremely high-redshift sample.
• Compare high-z ISM conditions to lower-redshift analogues to understand galaxy evolution and star-formation processes at early times.

Proposed method

• Utilize ultra-deep JWST/NIRSpec observations from the JADES survey, including PRISM/CLEAR (R~30-300) and G395M/F290LP (R~700-1300) configurations.
• Measure fluxes of key rest-frame optical lines: [O II] 3726,3729, [Ne III] 3869, Hβ, [O III] 4959,5007, [O I] 6300, Hα, [N II] 6583, [S II] 6716,6731.
• Model the continuum with pPXF using high-resolution SSP libraries to fit emission lines as Gaussians grouped into four kinematic classes.
• Correct line fluxes for dust using the SMC law and Balmer decrements when available; constrain [N II]/Hα upper limits from high-resolution spectra.

Experimental results

Research questions

• RQ1 What are the rest-frame optical emission-line ratios in z~5.5–9.5 galaxies observed with ultra-deep NIRSpec spectroscopy?
• RQ2 How do these high-redshift line ratios compare to typical z~0–3 galaxies and to lower-redshift extreme populations in diagnostic diagrams?
• RQ3 What do the line ratios imply about ISM properties such as ionisation parameter, metallicity, and radiation field in early galaxies?
• RQ4 Is there evidence for nitrogen enrichment (N/O) or other abundance patterns influencing the observed spectra?
• RQ5 What is the level of diversity in ISM conditions among individual galaxies at z>5.5?

Key findings

• More than half of the sample have [O III]/[O II] > 10, indicating high ISM ionisation.
• Emission line ratios show greater than an order of magnitude scatter in ratios like [O II]/Hβ and [O III]/[O II], implying significant diversity in ISM conditions.
• No convincing detections of [N II] λ6583 are found in either individual galaxies or a stack of G395M/F290LP spectra.
• Observed ratios are generally consistent with very high ionisation parameters (log U ~ -1.5) and metallicities ranging from ~0.1 Z⊙ to >0.3 Z⊙, suggesting low-metallicity systems with vigorous star formation and strong radiation fields.
• The deep observations demonstrate the value of high-S/N, deep spectroscopy for constraining ISM properties of individual z>5.5 galaxies and revealing population diversity.