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 James Webb Space Telescope 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 absolute spectrophotometry of the current version of the reductions. Compared to z ∼ 3 galaxies in the literature, the z > 5 galaxies have similar [O iii]λ5008/Hβ ratios, similar [O iii]λ4364/Hγ ratios, and higher (∼0.5 dex) [Ne III]λ3870/[O II]λ3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [Ne III]λ3870/[O II]λ3728, [O iii]λ4364/Hγ, and [O iii]λ5008/Hβ emission-line ratios are consistent with an interstellar medium (ISM) that has very high ionization ( log ( Q ) ≃ 8 − 9 , units of cm s−1), low metallicity (Z/Z ⊙ ≲ 0.2), and very high pressure ( log ( P / k ) ≃ 8 − 9 , units of cm−3). The combination of [O iii]λ4364/Hγ and [O iii]λ(4960 + 5008)/Hβ line ratios indicate very high electron temperatures of 4.1 < log ( T e / K ) < 4.4 , further implying metallicities of Z/Z ⊙ ≲ 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 ISM in galaxies at cosmic dawn.

CEERS Spectroscopic Confirmation of NIRCam-selected z ≳ 8 Galaxy Candidates with JWST/NIRSpec: Initial Characterization of Their Properties

“…The black lines are the best-fit relation at z = 0 (solid), and z = 3 (dashed;Sanders et al 2021), while the other color lines denote the predictions from simulations(Schaye et al 2015;Ma et al 2016;Pallottini et al 2022;Ucci et al 2023). The magenta circles are the measurements based on the [O III]4363 line for three lensed galaxies at z = 7.7-8.5 (e.g.,Curti et al 2023;Schaerer et al 2022;Trump et al 2023). The mass-metallicity relation at z = 8 − 9 is likely placed below the relationsat z = 0 − 3which is generally consistent with the predictions from simulations.…”

Spatially Resolved Properties of Galaxies at 5 < z < 9 in the SMACS 0723 JWST ERO Field

“…There have been multiple works on these targets, albeit always from an integrated perspective (e.g., Arellano-Córdova et al 2022;Brinchmann 2022;Carnall et al 2023;Curti et al 2022;Fujimoto et al 2022;Heintz et al 2023;Rhoads et al 2023;Schaerer et al 2022;Tacchella et al 2022;Trump et al 2023). These papers derive different stellar masses, some of them with extremely young ages of the stellar population.…”

A magnified compact galaxy at redshift 9.51 with strong nebular emission lines

“…Nebular emission lines are produced by clouds of interstellar gas within a galaxy; spectroscopic analysis of these lines can provide information about the density, temperature, and chemical composition of the gas. Spectroscopy has confirmed three high-redshift galaxies (7.66 < z < 8.50) with detections of strong nebular emission lines ( 14 ) and the temperature-sensitive [O iii ] 4363 Å emission line, which has been used to make direct electron temperature oxygen abundance measurements in galaxies at these redshifts ( 15 – 19 ). There has been further spectroscopic confirmation of seven galaxies from z = 7.762 to 8.998 ( 20 ).…”