JADES: Probing interstellar medium conditions at <i>z</i> ∼ 5.5–9.5 with ultra-deep JWST/NIRSpec spectroscopy
Alex J. Cameron, Aayush Saxena, Andrew J. Bunker, Francesco D’Eugenio, Stefano Carniani, R. Maiolino, Emma Curtis-Lake, Pierre Ferruit, P. Jakobsen, Santiago Arribas, Nina Bonaventura, S. Charlot, Jacopo Chevallard, Mirko Curti, Tobias J. Looser, Michael V. Maseda, Tim Rawle, Bruno Rodríguez Del Pino, Renske Smit, Hannah Übler, Chris J. Willott, Joris Witstok, Eiichi Egami, Daniel J. Eisenstein, Benjamin D. Johnson, Kevin Hainline, Marcia Rieke, Brant Robertson, Daniel P. Stark, Sandro Tacchella, Christina C. Williams, Christopher N. A. Willmer, Rachana Bhatawdekar, R. A. A. Bowler, Kristan Boyett, Chiara Circosta, Jakob M. Helton, Gareth C. Jones, Nimisha Kumari, Zhiyuan Ji, Erica J. Nelson, Eleonora Parlanti, Lester Sandles, Jan Scholtz, Fengwu Sun
Abstract
We present emission-line ratios from a sample of 27 Lyman-break galaxies from z ∼ 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 G 395 M / F 290 LP observations to measure, or place strong constraints on, ratios of widely studied rest-frame optical emission lines including H α , H β , [O II ] λλ 3726, 3729, [Ne III ] λ 3869, [O III ] λ 4959, [O III ] λ 5007, [O I ] λ 6300, [N II ] λ 6583, and [S II ] λλ 6716, 6731 in individual z > 5.5 spectra. We find that the emission-line ratios exhibited by these z ∼ 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 [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 β and [O III ]/[O II ], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [N II ] λ 6583 in our sample, either in individual galaxies, or a stack of all G 395 M / F 290 LP spectra. The emission-line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log U ∼ −1.5), and a range of metallicities spanning from ∼0.1 × Z ⊙ to higher than ∼0.3 × Z ⊙ , 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.