High-z galaxies with JWST and local analogues -- it is not only star formation

TL;DR

This paper analyzes JWST NIRSpec data of high-redshift galaxies, introduces a modified abundance determination method, identifies potential AGN activity, and discusses implications for galaxy evolution and measurement biases.

Contribution

It proposes a new method for abundance determination less sensitive to calibration uncertainties and applies it to high-z galaxy data, revealing potential AGN activity and biases in metallicity measurements.

Findings

Three new galaxy redshifts identified from JWST data.

Modified abundance method reduces calibration sensitivity by a factor of ~3.

Potential AGN activity detected in some high-z galaxies.

Abstract

I present an analysis of the JWST NIRSpec data of SMACS 0723 released as Early Release Observations. As part of this three new redshifts are provided, bringing the total of reliable redshifts to 14. I propose a modification to the direct abundance determination method that reduces sensitivity to flux calibration uncertainties by a factor of ~3 and show that the resulting abundances are in good agreement with Bayesian photoionization models of the rest-frame optical spectrum. I also show that 6355 is most likely a narrow-line active galactic nucleus (AGN) with $M_* \sim 10^9$ Msun at z=7.66, and argue that 10612 might also have an AGN contribution to its flux through comparison to photoionization models and low-redshift analogues. Under the assumption that the lines come from star-formation I find that the galaxies have gas depletion times of ~$10^7$ years, comparable to similar galaxies locally. I also identify a population of possibly shock-dominated galaxies at z<3 whose near-IR emission lines plausibly come nearly all from shocks and discuss their implications. I close with a discussion of the potential for biases in the determination of the mass-metallicity relation using samples defined by detected [O III]4363 and show using low-z galaxies that this can lead to biases of up to 0.5 dex with a systematic trend with mass.