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

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$β$, and H$γ$ 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 ($β\gtrsim-1.5$), either revealing a small population with moderate dust attenuation ($τ_V=0.23-0.35$) or very high density nebular-dominated galaxies with hot stellar populations.

Figures (2)

• Figure 1: Spectroscopic redshifts and absolute UV magnitudes of the $61$ galaxies in our $z>9$ sample. We highlight sources with rest-frame UV emission line (N IV], C IV, He II, O III], N III], C III]) detections with solid blue circles. Those without UV line detections are shown by open dodger blue circles.
• Figure 2: Composite prism spectra of galaxies in our $z>9$ sample. The top panel shows the stack of all the $61$ galaxies at in our $z>9$ sample ($9.0<z<14.3$). The spectrum at rest-frame $\sim3500-4000$ Å is composed of $56$ galaxies at $9.0<z<12.6$. The bottom panel shows the composite at rest-frame optical wavelengths obtained by stacking $22$ galaxies at $9.0<z<9.6$. Emission line detections are marked by blue dashed lines. The grey shaded region presents the $1\sigma$ uncertainty of flux density.