The impact of star formation sampling effects on the spectra of lensed $z>6$ galaxies detectable with $\textit{JWST}$

TL;DR

This paper studies how star formation sampling effects influence the spectral analysis of faint, high-redshift galaxies detectable with JWST, highlighting significant impacts on spectral diagnostics due to rapid SFR variations.

Contribution

It demonstrates that star formation sampling effects, especially rapid SFR fluctuations, significantly affect spectral diagnostics of low-SFR galaxies observed by JWST, while IMF sampling is less impactful.

Findings

Star formation sampling affects Balmer emission line diagnostics.

Rapid SFR variations cause significant spectral effects.

IMF sampling has a minor impact compared to SFR sampling.

Abstract

The upcoming $\textit{James Webb Space Telescope}$ ($\textit{JWST}$) will allow observations of high-redshift galaxies at fainter detection levels than ever before, and $\textit{JWST}$ surveys targeting gravitationally lensed fields are expected to bring $z\gtrsim 6$ objects with very low star formation rate (SFR) within reach of spectroscopic studies. As galaxies at lower and lower star formation activity are brought into view, many of the standard methods used in the analysis of integrated galaxy spectra are at some point bound to break down, due to violation of the assumptions of a well-sampled stellar initial mass function (IMF) and a slowly varying SFR. We argue that galaxies with SFR$\sim 0.1\ M_\odot$ yr$^{-1}$ are likely to turn up at the spectroscopic detection limit of $\textit{JWST}$ in lensed fields, and investigate to what extent star formation sampling may affect the spectral analysis of such objects. We use the $\small{\text{SLUG}}$ spectral synthesis code to demonstrate that such effects are likely to have significant impacts on spectral diagnostics of, for example, the Balmer emission lines. These effects are found to stem primarily from SFRs varying rapidly on short ($\sim$ Myr) timescales due to star formation in finite units (star clusters), whereas the effects of an undersampled IMF is deemed insignificant in comparison. In contrast, the ratio between the HeII- and HI-ionizing flux is found to be sensitive to IMF-sampling as well as ICMF-sampling (sampling of the initial cluster mass function), which may affect interpretations of galaxies containing Population III stars or other sources of hard ionizing radiation.