[Paper Review] High-z galaxies with JWST and local analogues -- it is not only star formation

J. Brinchmann|arXiv (Cornell University)|Aug 15, 2022

Astronomy and Astrophysical Research5 citations

TL;DR

This paper presents a revised method for direct oxygen abundance determination in high-redshift galaxies using JWST NIRSpec data, reducing sensitivity to flux calibration by a factor of ~3. It identifies AGN activity in two high-z galaxies (including a low-mass AGN at z=7.66) and finds shock-dominated emission in some z<3 galaxies, while cautioning that [O III]4363-based sampling biases the mass-metallicity relation by up to 0.5 dex.

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&lt;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.

Motivation & Objective

• To improve the accuracy of direct oxygen abundance measurements in high-redshift galaxies by reducing sensitivity to flux calibration and dust attenuation uncertainties.
• To assess the presence of active galactic nuclei (AGN) in high-redshift galaxies using emission line ratios and photoionization modeling.
• To investigate the role of shocks in shaping near-IR emission line spectra in low-redshift analogues and high-z galaxies.
• To evaluate the impact of selecting galaxies based on detectable [O III]4363 on the derived mass-metallicity relation.
• To validate Bayesian photoionization modeling as a robust alternative to temperature-based abundance determinations in high-redshift samples.

Proposed method

• A modified direct abundance method uses the double line ratio [O III]4363/Hγ and [O III]5007/Hβ to reduce sensitivity to flux calibration and dust extinction by ~3×.
• Ionic abundances are referenced to nearby hydrogen recombination lines to further minimize flux calibration and reddening effects.
• Photoionization models are applied to rest-frame optical spectra to infer physical conditions and validate abundance results.
• Line ratio comparisons with low-redshift AGN analogues and ionization parameter modeling are used to assess AGN contributions.
• Shock-dominated emission is evaluated using strong-line diagnostics like [Fe II]/Paγ and comparison with shock models.
• Bayesian photoionization modeling grids are used to estimate abundances in galaxies where [O III]4363 is undetected, reducing selection bias.

Experimental results

Research questions

• RQ1 Can the direct abundance method be improved to reduce sensitivity to flux calibration and dust attenuation in high-redshift galaxies?
• RQ2 What evidence supports the presence of AGN activity in high-redshift galaxies like 6355 and 10612?
• RQ3 To what extent do shock processes contribute to the observed emission line spectra in low-redshift and high-redshift galaxies?
• RQ4 How does selecting galaxies based on the detection of [O III]4363 affect the derived mass-metallicity relation?
• RQ5 Can Bayesian photoionization modeling provide reliable abundance estimates when temperature-sensitive lines are undetected?

Key findings

• The modified direct abundance method reduces sensitivity to flux calibration and dust attenuation by a factor of approximately 3, improving accuracy in high-redshift abundance determinations.
• Galaxy 6355 at z=7.66 is identified as a narrow-line AGN with a stellar mass below 10^9 M☉, based on the detection of [Ne IV]2422,2424 and line ratios consistent with AGN excitation.
• Galaxy 10612 shows line ratios and ionization parameters consistent with AGN excitation, suggesting a possible AGN contribution despite no clear [Ne IV] detection.
• Three low-redshift galaxies (z<3) exhibit strong [Fe II]1.257/ Paγ emission, indicating shock-dominated excitation, possibly due to a high supernova remnant rate.
• Photoionization modeling of the rest-frame optical spectra yields oxygen abundances in good agreement with those from the direct method, validating its use in high-redshift studies.
• Selecting galaxies based on detectable [O III]4363 can bias the mass-metallicity relation by up to 0.5 dex, with a systematic trend toward underestimating metallicity at higher masses.