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

abstract · 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.

S2.SS4: II.4 Emission and Absorption Line Fits / block 1 · html

Figure 4: ( Left ) Summary plot showing individual emission line centroids and the velocity uncertainty on the centroid measurement. We use O iii] and C iii] , which should closely track the ISM systemic rest frame, to cross-validate the wavelength solutions for our NIRSpec (MASQUERADE) and NIRCam (EIGER) data reductions. These lines are consistent with zero velocity offset relative to the rest frame, which we define by convention using the [O iii ] doublet wavelength. The vertical solid shaded regions show the inverse-variance weighted average of these line centroids. Ly- α \alpha and He ii are redshifted relative to the rest frame. In the case of Ly- α \alpha , this is interpreted as back-scattered emission from a receding shell behind the galaxy. He ii does not share this behavior but may be affected by foreground resonant absorption. We therefore exclude Ly α \alpha and He II from the average velocity and indicate so by shading them gray. ( Right ) This plot shows a summary of fit centroids for absorption lines as labeled. The vertical colored lines and regions show respectively the mean and standard error on the mean for all absorption lines, which is blueshifted by 180 ​ – ​ 200 180\text{--}200 km s -1 relative to the rest frame. N v exhibits a larger blueshift but we view it as unreliable because of continuum absorption in the same spectral region. It is therefore shaded gray and not included in the average outflow velocity calculation.

S3.SS3: III.3 Emission Lines’ Summary / block 1 · html

The Ly α \alpha profile has similar strength to z ∼ 3 z\sim 3 stacks and is redshifted by ∼ 250 \sim 250 km s -1 relative to the systemic frame. Likewise there is tentative evidence of broadening in the [O iii ] profile. We interpret these as velocity signatures of outflows.

S3.SS4: III.4 Absorption Line Properties / block 1 · html

Figure 4 provides a visual summary of the centroid measurements in Table 1 , showing broad agreement around a Δ ​ v ∼ − 200 \Delta v\sim-200 km s -1 velocity offset for Si ii , C ii , O i , Si iv , and C iv . The N v doublet exhibits an even larger blueshift, but we discard it from subsequent analysis because the continuum level in that region is compromised by proximity to Ly α \alpha in emission and absorption. Fe ii is the weakest line detection and is slightly offset from the mean value for the MSAExp reduction, but not at a statistically significant level. Including all lines except N v , we find a weighted-average velocity offset of − 178 ± ± 36 km s − 1 -178\pm\pm 36\text{\,}\mathrm{km}\text{\,}{\mathrm{s}}^{-1} at 4.9 ​ σ 4.9\sigma significance [ − 204 ± ± 35 km s − 1 -204\pm\pm 35\text{\,}\mathrm{km}\text{\,}{\mathrm{s}}^{-1} at 5.8 ​ σ 5.8\sigma significance].

S4.SS1: IV.1 Outflow Trajectories / block 1 · html

These results are summarized in the histogram of D max D_{\rm max} in Figure 6 . The largest wind bubble, not shown in Figure 6 for compactness, implied by this exercise is ∼ 37 \sim 37 kpc, comparable to the virial radius R 200 R_{200} for our average sample galaxy. Most objects yield D max ≈ 3 D_{\rm max}\approx 3 – 9 9 kpc, meaning they can lie well outside the stellar component but may not yet have escaped their dark-matter halos into the IGM.

S4.SS2: IV.2 Implications for Production of Metal Absorption in Quasar Sightlines / block 1 · html

The principal caveat to this conclusion is that MASQUERADE survey galaxies are brighter (by design) than randomly selected counterparts from blind surveys like EIGER. The average object in Higginson et al. [40] or Bordoloi et al. [7] has an order-of-magnitude lower stellar mass which corresponds to a ∼ 3 × \sim 3\times reduction in halo mass decelerating the wind. Integrating Equation 7 with a 3 × 3\times lower halo mass reveals that the final radius in an NFW halo depends on M h M_{h} to a greater degree than a point mass but still with a fairly shallow slope. The maximum radius increases by 50%, which is significant but not sufficient to bridge the gap between inferred and observed impact parameters. While a reduction in mass alone is not sufficient to launch winds to large distance for any single halo, a combination of slightly larger wind bubbles and increased number density of fainter galaxies can increase the probability of coincident identifications.

S5: V Conclusions / block 1 · html

We detect clear blueshifted absorption lines from low- and high-ionization tracers of stellar and/or galactic winds. MCMC fits yield mean velocity offsets of ∼ 170 \sim 170 – 200 200 km s -1 relative to systemic.

S5: V Conclusions / block 1 · html

A simple kinematic model indicates that a 200 km s -1 wind propagating for a typical integration time t 20 ∼ 38 Myr t_{20}\sim$38\text{\,}\mathrm{M}\mathrm{y}\mathrm{r}$ will reach R max ∼ 3 R_{\rm max}\sim 3 – 9 9 pkpc. Most galaxies have R max R_{\rm max} limited by travel time; a few reach a terminal radius and remain bound, falling back toward the center.

S5: V Conclusions / block 1 · html

We estimate the contribution of [O iii ] emitters to the absorber incidence d ​ N / d ​ X dN/dX and find that R max ∼ 5 R_{\rm max}\sim 5 kpc wind bubbles from these galaxies can account for less than a percent of observed absorption systems in quasar spectra.

S2.SS2: II.2 Stacking / block 1 · html

Rest wavelengths of several strong interstellar absorption and emission lines are indicated in green. Although many individual spectra have low SNR, the PypeIt and MSAExp composites display consistent continuum shapes and similar emission and absorption features, indicating that our results are relatively insensitive to systematic effects introduced by pipeline extraction, wavelength calibration, or flux calibration.

S3: III Steep dust extinction curve / block 1 · html

We apply the best-fit dust extinction model to de-redden the observed spectra (Fig. 2 , left panel), revealing good agreement with the template continuum outside of wavelength ranges of major emission lines (illustrated for α λ = 1.5 \alpha_{\lambda}=1.5 ). The resulting extinction curve ( A λ / A V A_{\lambda}/A_{\mathrm{V}} v.s. 1 / λ 1/\lambda ) is shown in the right panel of Fig. 2 , including the 1 ​ σ 1\sigma uncertainty band derived from the MCMC posterior. The extinction toward UDS-27023 (catalog ) is distinctive when compared to standard benchmarks, including the Milky Way [ 7 ] , SMC [ 25 ] extinction curves, the Calzetti et al. [6] starburst attenuation laws, and the attenuation curves inferred for z > 4 z>4 galaxies observed by JWST/NIRSpec [ 54 ] . We observe moderate optical attenuation of A V = 0.72 ± 0.15 A_{\mathrm{V}}=0.72\pm 0.15 with a canonical total-to-selective extinction ratio R V ≡ A V A B − A V = 3.13 − 0.10 + 0.07 R_{\mathrm{V}}\equiv\frac{A_{V}}{A_{B}-A_{V}}=3.13_{-0.10}^{+0.07} , where A B A_{B} is the B-band (0.44 μ \mu m) dust extinction. However, the extinction curve exhibits a significantly steeper rise into the UV, with a measured effective UV-optical slope (steepness) of S ≡ A 1500 ​ Å / A V = 8.0 − 0.9 + 1.4 S\equiv A_{1500\text{\AA}}/A_{\mathrm{V}}=8.0_{-0.9}^{+1.4} . This steep slope, combined with moderate V-band attenuation, positions the dust properties of UDS-27023 (catalog ) at the extreme end of the currently known distribution (Fig. 3 , left panel).

S4.SS2: IV.2 Origin, distribution, and fate of small dust grains / block 1 · html

In this way, UDS-27023 (catalog ) exemplifies how AGN at high redshift can play a dual role in the dust lifecycle: they may both process pre-existing grains through shocks and turbulence, and produce new dust directly within dense QSO-driven winds. The resulting small grains can subsequently grow in the ISM or be redistributed into the circumgalactic medium. Steep-extinction QSOs therefore bridge supernova dust injection, interstellar dust processing, QSO-wind dust formation, and halo enrichment, thereby closing the observational loop between the massive dust reservoirs seen in early galaxies and the small-grain-rich halos that regulate future star formation.

S6: VI Conclusions / block 1 · html

We have presented the first discovery of extreme nitrogen enrichment from Wolf Rayet nitrogen stars (WN) in the metal-poor ( ∼ 10 % ​ Z ⊙ \sim 10\%\ Z_{\odot} ), high surface-density ( 1.1 × 10 4 ​ M ⊙ 1.1\times 10^{4}\ M_{\odot} pc -2 ), high-redshift ( z = 6.1025 z=6.1025 ), lensed galaxy RXCJ2248-ID3. These measurements were made possible by exceptionally deep JWST/NIRSpec medium-resolution spectroscopy of RXCJ2248-ID3, obtained as part of the GLIMPSE-D survey. The unprecedented depth and S/N of the GLIMPSE-D spectrum allow spectral measurements typically limited to the nearby universe, including consistent broad components in the Balmer series and [O III] λ \lambda 4364 and λ \lambda λ \lambda 4960,5008 lines, faint [Ar IV] λ \lambda λ \lambda 4713,4741 emission, and signatures of WR stars. Specifically, we detected emission characteristic of WN-type stars, including strong N III] λ \lambda 4642 and broadened He II λ \lambda 1640 and λ \lambda 4687 emission, marking RXCJ2248-ID3 as the most distant galaxy with direct WR features to date.

S6: VI Conclusions / block 1 · html

The CNO abundance pattern is best reproduced by a modified version of the dual-burst chemical evolution model from Kobayashi and Ferrara [69] that reduces the contribution from WC stars relative to WN stars, as expected in metal-poor environments. The resulting short-lived WN phase and ejects N-rich, C-poor material. Further, we use this chemical evolution model to assess whether the observed N mass can plausibly arise from the recent star formation in RXCJ2248-ID3 and estimate an ionized N mass of 1250 ​ M ⊙ 1250\ M_{\odot} . This value is consistent with the N mass estimated from the observed emission lines of ∼ 920 ​ M ⊙ \sim 920\ M_{\odot} if ∼ 74 % \sim 74\% of the N gas is ionized.

S6: VI Summary / block 1 · html

The C II/C IV column density ratio at z = 7.03 z=7.03 falls within 0.3 dex of the empirical redshift-dependent trend established by the XQR-30 legacy survey of z ≳ 5 z\gtrsim 5 quasar absorbers [ 42 ] , consistent with the global trend toward increasingly neutral, low-ionization CGM environments in the Epoch of Reionization (see Section IV.2 ). This concordance arises because the extraordinary metal column densities are enhanced uniformly across all ionization phases, preserving the standard relative ionic balance despite the absolute enrichment. At the current single-object precision the observation does not require any significant departure from the low- z z ionization balance.

S6: VI Summary / block 1 · html

The unique physical regime revealed by this system, an extreme absorber intercepted at only 16 kpc from a heavily dust-obscured, vigorously star-forming host, points to two natural next steps. First, higher-spectral-resolution (e.g., R ∼ 10000 R\sim 10000 ) follow-up will be essential to constrain individual cloud b b -parameters. Only with the, break the saturation degeneracy in the column-density estimates, and more robustly characterize the underlying nucleosynthetic pattern. Second, if the enhanced [C/O] observed here is indeed a Pop III imprint, quasar-absorption spectroscopy toward confirmed high- z z galaxy–absorber pairs may prove a complementary route to probing Pop III enrichment, sidestepping the challenge of directly detecting individual Pop III stellar systems. Realizing both goals requires a systematic effort to identify high- z z absorber hosts even when they are NIR-dark, motivating targeted millimeter and sub-millimeter follow-up of high- z z metal-absorber fields with ALMA and NOEMA.

S8: 8 Summary / block 1 · html

Galaxies in overdense environments exhibit a steeper MZR slope ( γ = 0.42 ± 0.04 \gamma=0.42\pm 0.04 ) compared to field galaxies ( γ = 0.21 ± 0.06 \gamma=0.21\pm 0.06 ), and they are more metal-rich especially at higher masses log ⁡ ( M ∗ / M ⊙ ) > 8 \log(M_{*}/M_{\odot})>8 . We build a mass-matched sample of field galaxies and still find that the galaxies in overdense environments are more metal-rich by ∼ 0.2 \sim 0.2 dex than their field counterparts with the same stellar masses.

S8: 8 Summary / block 1 · html

We compare the FMR of our sample galaxies with the local FMR using α = 0.66 \alpha=0.66 ( Andrews & Martini, 2013 ) . We find that our sample galaxies are metal-deficient by 0.2 0.2 dex compared to the local FMR. However, the galaxies in overdense environments align closer to the local FMR predictions.

S2.SS1: II.1 Chandra X-ray Observations / block 1 · html

Table 1: Summary of Chandra ACIS-S Observations

S4: IV Results and Discussion / block 1 · html

In this section, we study the X-ray photometric and spectroscopic properties of quasars at z > 6.5 z>6.5 , and investigate their relations with quasar and host galaxy properties measured from NIR spectroscopy and ALMA observations.

S5.SS3: V.3 Strong High-ionization Emission and the Continuum State of the Host / block 1 · html

A second unexpected result of this work is the connection between the relative strength of [Ne V] and the continuum properties of the host. The strong [Ne V]/[Ne III] sources do not differ significantly in M UV M_{\rm UV} from the sensitivity-qualified lower-ratio comparison sample. They do, however, have redder UV slopes and stronger measured Balmer breaks. In the redshift-matched comparison, the differences are Δ ​ β = 0.63 \Delta\beta=0.63 and Δ ​ log 10 ⁡ ( B Balmer ) = 0.183 \Delta\log_{10}(B_{\rm Balmer})=0.183 dex, corresponding to a Balmer break approximately 1.5 times stronger on average. The association is therefore with continuum shape rather than simply with UV luminosity.

S6: VI Conclusions / block 1 · html

[Ne V] selection overlaps only partly with other AGN diagnostics. Among SHINE-[NeV] galaxies securely classifiable in S2–VO87, 67% lie above the Kewley et al. [62] maximal-starburst relation, whereas only 14% lie above the more conservative high-redshift boundary of Scholtz et al. [110] . However, 32% of galaxies with constraining [Ne V] nondetections also lie above the Kewley relation, showing that this region is not specific to [Ne V] emitters. Five of the 25 SHINE-[NeV] galaxies have adopted broad-line classifications; for the remainder, broad emission is either not required in the assessable spectra or cannot be determined from the available data. Broad-line, narrow-line, and [Ne V] selection therefore provide complementary views of the high-redshift AGN population. Stellar masses are available for 20 sources and span approximately log ⁡ ( M ∗ / M ⊙ ) ∼ 7.4 \log(M_{\ast}/M_{\odot})\sim 7.4 –11, including one exceptionally low-mass source at log ⁡ ( M ∗ / M ⊙ ) ∼ 7.4 \log(M_{\ast}/M_{\odot})\sim 7.4 . Luminous very-high-ionization emission can therefore occur in low-mass hosts, although low stellar mass alone does not imply the presence of an intermediate-mass black hole.

S5.SS1.SSS1: 5.1.1 A high-z weak line quasar? / block 2 · html

5007 weakness observed in ∼ 70 % \sim 70\% of their sample is mainly driven by orientation effects. The NLR is photoionized within the polar ionization cone. When viewed more face-on, i.e., looking down the cone, the bright continuum outshines the projected NLR emission, resulting in a higher continuum-to-NLR ratio than in more edge-on orientations. In addition to high L / L Edd L/L_{\mathrm{Edd}} and orientation effects, obscuration by dust or dense gas has also been invoked to explain reduced [O III ] λ ​ λ \lambda\lambda 4959,5007 emission in quasars ( Temple et al., 2019 ; Wang et al., 2025 ) .

S5.SS2.SSS1: 5.2.1 Photoionization simulations / block 1 · html

Figure 9: Cloudy simulation results for the H β \beta shell. The upper and middle rows correspond to models using the AGN normal" and AGN soft" continua as ionizing sources, respectively, while the lower row corresponds to the stellar ionizing source. Each panel shows the predicted line ratio log 10 ⁡ ( [ O ​ III ] ​ 5007 / H ​ β ) \log_{10}([\mathrm{O\,III}]\,5007/\mathrm{H}\beta) as a function of gas density n H n_{\rm H} and metallicity Z Z , for inner shell radii r in = 0.1 r_{\rm in}=0.1 , 0.8 0.8 , and 1.7 1.7 kpc (left to right). The maps are spanned by constant-density shell models with outer radius r out = 1.8 r_{\rm out}=1.8 kpc. Regions enclosed by the black contour are consistent with the observed constraint log 10 ⁡ ( [ O , III ] / H ​ β ) < − 1.15 \log_{10}([\mathrm{O,III}]/\mathrm{H}\beta)<-1.15 from the extended emission in J0313-1806. The hatched areas indicate the radiation–pressure confinement (RPC) exclusion zone, where densities fall below the expected RPC floor and are therefore disfavored.

S1: I Introduction / block 1 · html

In Liu et al. [39] , we introduced a JWST cycle 2 NIRSpec/integral field unit (IFU) survey program #3428 (; PI W. Liu), which observed 27 luminous quasars at z ∼ \sim 5–6. In that paper, we presented novel results on the detection of more frequent extreme galaxy-scale outflows in the z ∼ \sim 5–6 quasars from the survey program than in the lower-z quasars. In this work, we report other key rest-frame optical spectral properties of these z ∼ \sim 5–6 quasars and their host galaxies/companion galaxies from the same program. This paper is organized as follows. In Section II , we describe the sample selection, observations and data reduction. In Section III , the spectral fitting and analysis are presented. The discussion on the quasar nuclear spectral properties and extended line emission is presented in IV and V , respectively. Finally, our main conclusions are summarized in Section VI . Throughout the paper, we assume a Λ \Lambda CDM cosmology with H 0 = H_{0}= 70 km s -1 Mpc -1 , Ω m \Omega_{\rm m} = 0.3, and Ω Λ = 0.7 \Omega_{\rm\Lambda}=0.7 . An arcsecond corresponds to 5.71–6.45 kpc at the redshifts of our objects.

S6: VI Conclusion / block 1 · html

We determine the systemic redshifts of our objects through the peak of the narrow [O iii ] component or the overall H β \beta component when no narrow [O iii ] emission line is detected. Our mean and median redshifts ∼ \sim 390 and ∼ \sim 40 km s -1 larger than the rest-frame UV emission line-based redshifts (including C IV , C III ] and/or Si IV ] ) in the literature, respectively, despite the large scatter.