The Physical Conditions of Emission-Line Galaxies at Cosmic Dawn from JWST/NIRSpec Spectroscopy in the SMACS 0723 Early Release Observations

TL;DR

This work targets the physical conditions of emission-line galaxies at cosmic dawn by measuring rest-frame optical line ratios in five $z>5$ galaxies with JWST/NIRSpec. Using post-processing to ensure reliable relative flux calibration and comparing to MAPPINGS V photoionization models, the authors find very high ionization ($\log(Q) \approx 8-9$), low metallicity ($Z/Z_\odot \lesssim 0.2$), and very high ISM pressure ($\log(P/k) \approx 8-9$), with electron temperatures $4.1 < \log(T_e/\mathrm{K}) < 4.4$ consistent with low-redshift metallicity calibrations. The observed line ratios, including $\mathrm{[Ne III] 3870/[O II] 3728}$ and $\mathrm{[O III] 4364/H\gamma}$, support a scenario of an intensely ionized, low-metallicity ISM in these early galaxies. These results provide a tantalizing glimpse into the ISM conditions during cosmic dawn and demonstrate JWST's capability to constrain fundamental gas properties in the early universe.

Abstract

We present rest-frame optical emission-line flux ratio measurements for five $z>5$ galaxies observed by the JWST Near-Infared Spectrograph (NIRSpec) in the SMACS 0723 Early Release Observations. We add several quality-control and post-processing steps to the NIRSpec pipeline reduction products in order to ensure reliable relative flux calibration of emission lines that are closely separated in wavelength, despite the uncertain \textit{absolute} spectrophotometry of the current version of the reductions. Compared to $z\sim3$ galaxies in the literature, the $z>5$ galaxies have similar [OIII]$λ$5008/H$β$ ratios, similar [OIII]$λ$4364/H$γ$ ratios, and higher ($\sim$0.5 dex) [NeIII]$λ$3870/[OII]$λ$3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [NeIII]$λ$3870/[OII]$λ$3728, [OIII]$λ$4364/H$γ$, and [OIII]$λ$5008/H$β$ emission-line ratios are consistent with an interstellar medium that has very high ionization ($\log(Q) \simeq 8-9$, units of cm~s$^{-1}$), low metallicity ($Z/Z_\odot \lesssim 0.2$), and very high pressure ($\log(P/k) \simeq 8-9$, units of cm$^{-3}$). The combination of [OIII]$λ$4364/H$γ$ and [OIII]$λ$(4960+5008)/H$β$ line ratios indicate very high electron temperatures of $4.1<\log(T_e/{\rm K})<4.4$, further implying metallicities of $Z/Z_\odot \lesssim 0.2$ with the application of low-redshift calibrations for ``$T_e$-based'' metallicities. These observations represent a tantalizing new view of the physical conditions of the interstellar medium in galaxies at cosmic dawn.

Figures (3)

• Figure 1: An illustration of the emission lines detectable by JWST/NIRSpec at different redshifts. The gray regions show the observed-frame wavelength range of three medium-resolution grating and filter combinations, with darker gray regions indicating the overlapping wavelength regions covered by two gratings. We do not use the G140M/F100LP grating/filter in this work and so show its wavelength coverage in a lighter shade for illustrative purposes only. In this work we use SMACS 0723 Early Release Observations with G235M/F170LP and G395M/F290LP to study rest-frame optical lines of five galaxies at $5<z<9$.
• Figure 2: The 2D profiles of the rest-frame optical emission lines of interest for the first visit of source 6355. The pixel scale is $0\hbox{$.\!\!^{\prime\prime}$}1$/pixel in the cross-dispersion (vertical) direction and 17 Å/pixel in the wavelength (horizontal) direction. The darker regions are individual nod positions and the brighter regions are from the coadd of the three nods. The emission lines are well-detected and have a consistent spectral trace over a broad range of wavelength in the reduced 2D spectra. The complete figure set (10 images) of all 10 sets of emission-line profiles (2 visits each for 5 sources) is available in the online journal.
• Figure 3: The spectrum and emission-line fits for the first visit of source 4590, in the rest-frame wavelength regions that include [O ii ] and [Ne iii ], H$$ and $\hbox{\rm [O}{\sc iii}{\rm ]} 4364$, H$$ and $\hbox{\rm [O}{\sc iii}{\rm ]} 4960,5008$. Flux is shown by the black histogram and flux uncertainty by the gray error bars. The blue line indicates the continuum and the red lines indicate the best-fit Gaussian functions to each emission line. The complete figure set (10 images) of all 10 spectra and emission-line fits (2 visits each for 5 sources) is available in the online journal.