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 λrest=1200unicodex20132000 mathringA 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 MUV=−20.7, and stellar mass M⋆=108.93 M⊙. The stacked continuum closely resembles lower-redshift Lyman Break Galaxy composites, with a few notable differences. Lyα 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 ∼200 km s−1, again indicating outflows driven by stellar winds or entrained metal-enriched ISM. A simple model suggests that at z∼6.5 most star-forming galaxies are too young for winds at these velocities to propagate beyond ∼3unicodex20139 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⋆∼105unicodex2013106 M⊙) clustered around the more massive systems detected in JWST/NIRCam WFSS surveys.
Related
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BibTeX
@misc{depalma2026composite,
title = {{A Composite Rest-Frame UV Spectrum of Galaxies at $5.7 < z < 6.9$: Ionization, Outflows, and Implications for Intergalactic Metal Enrichment}},
author = {David DePalma and Robert A. Simcoe and Anna-Christina Eilers and Rongmon Bordoloi and Daichi Kashino and Gauri Kotiwale and Jorryt Matthee},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.33154},
eprint = {2610.33154},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.33154},
}
RIS
TY - UNPB
TI - A Composite Rest-Frame UV Spectrum of Galaxies at 5.7 < z < 6.9: Ionization, Outflows, and Implications for Intergalactic Metal Enrichment
AU - David DePalma
AU - Robert A. Simcoe
AU - Anna-Christina Eilers
AU - Rongmon Bordoloi
AU - Daichi Kashino
AU - Gauri Kotiwale
AU - Jorryt Matthee
PY - 2026
DA - 2026/10/23
PB - arXiv
DO - 10.48550/arxiv.2610.33154
UR - https://arxiv.org/abs/2610.33154
AB - We present a stacked JWST/NIRSpec spectrum of 104 galaxies at λ_rest=1200unicodex20132000 mathringA 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_UV=−20.7, and stellar mass M_⋆=10^8.93 M_⊙. The stacked continuum closely resembles lower-redshift Lyman Break Galaxy composites, with a few notable differences. Lyα 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 ∼200 km s^−1, again indicating outflows driven by stellar winds or entrained metal-enriched ISM. A simple model suggests that at z∼6.5 most star-forming galaxies are too young for winds at these velocities to propagate beyond ∼3unicodex20139 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_⋆∼10^5unicodex201310^6 M_⊙) clustered around the more massive systems detected in JWST/NIRCam WFSS surveys.
ER -