The JWST Spectroscopic Properties of Galaxies at $z=9-14$

TL;DR

This study analyzes JWST spectra of galaxies at redshifts 9-14, revealing significant evolution in emission line properties and star formation activity compared to lower redshift galaxies, indicating more extreme nebular conditions and rapid star formation episodes at the universe's earliest epochs.

Contribution

First comprehensive spectroscopic analysis of galaxies at z=9-14 using JWST, identifying key changes in emission lines and star formation behaviors at these high redshifts.

Findings

Increased prevalence of large equivalent width emission lines at z>9.

Evidence of rapid star formation upturns in early galaxies.

Detection of galaxies with moderate dust or dense nebular conditions.

Abstract

We characterize the JWST spectra of $61$ galaxies at $z=9-14$, including $30$ newly-confirmed galaxies. We directly compare the $z>9$ spectroscopic properties against $401$ galaxies at $6<z<9$, with the goal of identifying evolution in the star formation histories and ISM. We measure rest-UV emission line properties and UV continuum slopes, while also investigating the rest-optical emission lines for the subset of galaxies at $9.0<z<9.6$. With these spectra, we constrain the stellar masses, specific star formation rates, dust attenuation, and the average metallicity and abundance pattern of $z>9$ galaxies. Our dataset indicates that the emission lines undergo a marked change at $z>9$, with extremely large CIII], H$\beta$, and H$\gamma$ EWs becoming $2-3\times$ more common at $z>9$ relative to $6<z<9$. Using the spectra, we infer the distribution of SFRs on short (SFR$_{\rm 3Myr}$) and medium (SFR$_{\rm 3-50Myr}$) timescales, finding that rapid SFR upturns (large SFR$_{\rm 3Myr}$/SFR$_{\rm 3-50Myr}$ ratios) are significantly more likely among $z>9$ galaxies. These results may reflect a larger dispersion in UV luminosity at fixed halo mass and larger baryon accretion rates at $z>9$, although other physical effects may also contribute. We suggest that the shift in star formation conditions explains the prevalence of extreme nebular spectra that have been detected at $z>9$, with hard ionizing sources and nitrogen-enhancements becoming more typical at the highest redshifts. Finally, we identify five $z>9$ spectroscopically confirmed galaxies with red UV colors ($\beta\gtrsim-1.5$), either revealing a small population with moderate dust attenuation ($\tau_V=0.23-0.35$) or very high density nebular-dominated galaxies with hot stellar populations.