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Astronomers analyzed combined ultraviolet light from 104 galaxies observed when the universe was less than a billion years old (redshift z=6.5) using JWST spectroscopy. The galaxies show strong emission lines indicating active star formation and blueshifted absorption features revealing gas outflows moving at approximately 200 km/s. However, modeling suggests these stellar wind-driven outflows from detected galaxies are too slow and confined (within 3-9 kpc) to account for the widespread metal pollution observed in the early intergalactic medium.
Why it matters
This finding suggests that most of the chemical enrichment of the early universe came from numerous undetected low-mass dwarf galaxies rather than the brighter galaxies we can observe with current telescopes. Understanding this enrichment process is critical for reconstructing how the first generation of stars distributed heavy elements throughout cosmic space.
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⚠️ Preprint – Noch nicht peer-reviewed
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Abstract: We present a stacked JWST/NIRSpec spectrum of 104 galaxies at $lambda_{rm rest}=1200unicode{x2013}2000;mathrm{mathring{A}}$ 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$;alpha$ 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 $zsim6.5$ most star-forming galaxies are too young for winds at these velocities to propagate beyond $sim3unicode{x2013}9$ 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 ($M_starsim10^5unicode{x2013}10^6;mathrm{M_odot}$) clustered around the more massive systems detected in JWST/NIRCam WFSS surveys.