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

TL;DR

This study probes the interstellar medium (ISM) of galaxies during the first billion years by measuring rest-frame optical emission-line ratios in a sample of z>5.5 galaxies with ultra-deep JWST/NIRSpec spectroscopy. Using PRISM/CLEAR and G395M/F290LP data, the authors derive lines such as Hα, Hβ, [O II], [Ne III], and [O III], revealing a population with high ionisation parameters and sub-solar metallicities, yet substantial diversity in ISM conditions. Photoionisation models with young-star spectra reproduce the observed line-ratio space, suggesting metallicities of roughly $0.07-0.30 Z_\\odot$ and $-2.0 \lesssim \log U \lesssim -1.0$ for the most extreme systems, and highlighting that high $O32$ values could relate to LyC leakage. The results indicate rapid, low-metallicity star formation in the early universe and demonstrate the value of deep JWST spectroscopy for characterising diversity within early galaxy populations and their role in reionisation.

Abstract

We present emission line ratios from a sample of 26 Lyman break galaxies from $z\sim5.5-9.5$ 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$α$, H$β$, [OII] $λλ$3726,3729, [NeIII] $λ$3869, [OIII] $λ$4959, [OIII] $λ$5007, [OI] $λ$6300, [NII] $λ$6583, and [SII] $λλ$6716,6731 in individual $z>5.5$ 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 $\sim0.1\times Z_\odot$ 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.

Figures (9)

• Figure 1: Example spectra from five galaxies included in our sample. Left: Low-resolution PRISM/CLEAR spectra shifted to the rest-frame according to the observed redshift. Flux is shown in $f_{\lambda}$ but has been renormalised relative to the peak flux from the [O iii] $\lambda$ 5007 line. Solid horizontal lines show the zero flux of each spectrum. The shaded region around the 'zero' line indicates the 1-$\sigma$ noise spectrum in the same renormalised units. We show only the subset of spectral coverage that includes the key emission lines used in this study. Vertical dotted lines show the expected centroids of the [O ii] $\lambda$ 3727, [Ne iii] $\lambda$ 3869, [Ne iii] $\lambda$ 3967, H$\delta$, H$\gamma$, [O iii] $\lambda$ 4363, H$\beta$, [O iii] $\lambda$ 4959, [O iii] $\lambda$ 5007, [O i] $\lambda$ 6300, H$\alpha$, [N ii] $\lambda$ 6583, [S ii] $\lambda$ 6716, and [S ii] $\lambda$ 6731 lines respectively (moving left to right). Right: Zoom in on the H$\alpha$+[N ii] complex as observed in the G395M/F290LP grating spectra of these same galaxies. Observed spectra are shown in the same way as for the left panel (except renormalised to H$\alpha$, rather than [O iii]). Vertical dotted lines show the expected centroids of H$\alpha$ and [N ii] $\lambda$ 6583.
• Figure 2: Top: Redshift histogram of galaxies observed in this study. We divide the sample into two sub-samples '$z\sim$6' and '$z\sim8$', based simply on cutting at $z=7$ since this is the redshift at which H$\alpha$ is redshifted beyond the spectral coverage of NIRSpec. Thus, for JADES '$z\sim8$' we only have coverage of lines from [O ii] $\lambda\lambda$ 3726, 3729 to [O iii] $\lambda$ 5007, while for JADES '$z\sim6$' we can in principle observe all the rest-frame optical lines out to [S ii] $\lambda\lambda$ 6716, 6731. The orange hatched histogram indicates galaxies for which we can constrain the [S ii] $\lambda\lambda$ 6716, 6731 flux. Green hatching is the same but for [N ii] $\lambda$ 6583. Bottom: Redshift vs. $M_{1500}$ for the full $z\sim5.5-9.5$ sample. We derived $M_{1500}$ directly from the PRISM spectra as described in Section \ref{['sub:muv_calc']}.
• Figure 3: BPT and VO87 diagrams showing the comparison between JADES $z\sim6$ galaxies and samples across various redshifts. Individual measurements are shown as solid purple points, while values dervied from stacked JADES spectra are shown as the open magenta diamonds. The background grey 2D PDF shows $z\sim0$ galaxies from SDSS. Extremely metal-poor $z\sim0$ galaxies from Izotov2018 and Izotov2019 are shown as yellow pentagons. MOSDEF $z\sim2$ galaxies are shown as green circles Kriek2015. The $z\sim5.6$ composite CEERS spectra presented by Sanders2023 are shown as the blue open circles. The solid navy lines in each panel show the theoretical maximum starbust lines from Kewley2001, while the the dotted line in the left panel shows the empirical demarcation derived by Kauffmann2003. Left: Classical BPT or '$N2$-BPT' -- Purple triangles provide 3-$\sigma$ upper limits on the locations of our JADES galaxies since we are unable to detect [N ii] $\lambda$ 6583 in any of the 18 galaxies for which we have G395M/F290LP spectral coverage of the H$\alpha$ complex, although the $R3$ ratio is very well constrained in all cases from PRISM data. Even after stacking the grating spectra of all galaxies, we do not recover a robust detection of [N ii] $\lambda$ 6583. Centre: In the $S2$-VO87 diagram (often referred to as '$S2$-BPT'), 3/20 of our JADES galaxies show detections of [S ii] in individual spectra. The tight 3-$\sigma$ upper limits we place on our non-detections highlight that there must be more than an order of magnitude in scatter in the $S2$ ratio within the sample. Right: We find one tentative detection of [O i] $\lambda$ 6300 in our $O1$-VO87 diagram.
• Figure 4: The $R2-R3$ diagram showing our two JADES sub-samples at $z\sim6$ (purple points) and $z\sim8$ (maroon points). Comparison samples from SDSS and MOSDEF are shown as in Figure \ref{['fig:n2_bpt']}. We additionally show comparison with two populations of $z\sim0$ extreme starbursts: green peas (Yang2017_GreenPea; orange plusses) and blueberries (Yang2017_Blueberry; blue crosses). The three well-studied $z>7.5$ galaxies from the SMACS 0723 ERO observations are shown as lime green pentagons, adopting the ratio values presented by Nakajima2023. Unlike the BPT and VO87 diagrams, the majority of our sample have $>3\sigma$ detections of all lines in individual spectra, revealing a large diversity within the sample. Ratios measured from stacked JADES spectra are shown as the open magenta and red diamonds (see Section \ref{['sub:stacking']}).
• Figure 5: The $R23$-$O32$ diagram, probing the ionisation and excitation of the ISM. The JADES sub-samples at $z\sim6$ (purple points) and $z\sim8$ (maroon points) lie at generally high excitation an high ionisation. Comparison samples, including SDSS, MOSDEF, green peas, blueberries, and SMACS 0723, are shown as in Figure \ref{['fig:r2_r3']}. $z\sim4.5-8$ galaxies from GLASS are shown as grey diamond Mascia2023. Blue and orange open circles show the $z\sim5.6$ and $z\sim7.5$ composite CEERS spectra from Sanders2023, while the $z\sim7.7$ composite CEERS spectrum from Tang2023 is shown as the green open hexagon. Stacked JADES spectra are shown as the open magenta and red diamonds.