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

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 $z\sim 3$ galaxies in the literature, the $z>5$ galaxies have similar [OIII]$\hat{I}»$5008/H${\hat{I}}^2$ ratios, similar [OIII]$\hat{I}»$4364/H${\hat{I}}^3$ ratios, and higher ($\sim$0.5 dex) [NeIII]$\hat{I}»$3870/[OII]$\hat{I}»$3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [NeIII]$\hat{I}»$3870/[OII]$\hat{I}»$3728, [OIII]$\hat{I}»$4364/H${\hat{I}}^3$, and [OIII]$\hat{I}»$5008/H${\hat{I}}^2$ 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]$\hat{I}»$4364/H${\hat{I}}^3$ and [OIII]$\hat{I}»$(4960+5008)/H${\hat{I}}^2$ line ratios indicate very high electron temperatures of $4.1<\log (T_e/\mathrm{K})<4.4$, 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.

Accepted for publication in AAS Journals. 14 pages, 6 figures, 3 tables