A Composite Rest-Frame UV Spectrum of Galaxies at $5.7 < z < 6.9$: Ionization, Outflows, and Implications for Intergalactic Metal Enrichment

Abstract

We present a stacked JWST/NIRSpec spectrum of 104 galaxies at $λ_{\rm rest}=1200\unicode{x2013}2000\;\mathrm{\mathring{A}}$ to study nebular emission and absorption from the interstellar medium (ISM) and stellar winds. The sample is drawn from known [O III] emitters with mean redshift $z=6.50$, absolute UV magnitude $M_\mathrm{UV}=-20.7$, and stellar mass $M_{\star}=10^{8.93}\;\mathrm{M_\odot}$. The stacked continuum closely resembles lower-redshift Lyman Break Galaxy composites, with a few notable differences. Ly$\;α$ has comparable strength and is redshifted relative to the [O III] systemic frame, consistent with production in gaseous outflows. High-ionization emission lines including O III], C III], and He II are slightly stronger than in lower-redshift stacks but exhibit narrow widths, disfavoring ionization from a broad-line AGN. Emission-line ratios slightly favor ionization by a hard stellar continuum, though a contribution from a narrow-line nuclear source cannot be strictly excluded. Numerous strong absorption lines at both high and low ionization are blueshifted by $\sim200\;\mathrm{km\,s^{-1}}$, again indicating outflows driven by stellar winds or entrained metal-enriched ISM. A simple model suggests that at $z\sim6.5$ most star-forming galaxies are too young for winds at these velocities to propagate beyond $\sim3\unicode{x2013}9$ proper kpc, implying that the observed [O III] emitter population alone cannot explain the frequency of heavy-element absorbers seen in quasar spectra. In this picture, early enrichment of the intergalactic medium likely arises from undetected low-mass galaxies ($M_\star\sim10^5\unicode{x2013}10^6\;\mathrm{M_\odot}$) clustered around the more massive systems detected in JWST/NIRCam WFSS surveys.