A Fleeting GLIMPSE of N/O Enrichment at Cosmic Dawn: Evidence for Wolf Rayet N Stars in a z = 6.1 Galaxy

TL;DR

This paper reports the first direct evidence that Wolf Rayet nitrogen stars drive chemical enrichment in a galaxy formed near cosmic dawn, RXCJ2248-ID3 at $z=6.1025$. Using ultra-deep JWST/NIRSpec spectroscopy, the authors measure a direct oxygen abundance of $12+\log(\rm O/H)=7.749\pm0.023$ and an elevated $\log(\rm N/O)=-0.390\pm0.035$, with C/O markedly depleted to $\log(\rm C/O)=-0.796\pm0.052$, consistent with nitrogen production dominated by WN winds and little WC contribution. They detect broad WR features, notably a strong N$^{+}$-driven N3$\lambda$4642 blue bump and broad He II lines, indicating a metal-poor WN-dominated population at high redshift. A modified dual-burst chemical evolution model suggests a short, WR-dominated enrichment window of a few Myr after a second burst, producing the observed N/O enhancement and C/O deficiency; this yields a mass budget compatible with the measured ionized N mass. The results imply that N/O outliers are a natural, transient phase tied to very young, intense bursts in the early universe and provide a timing argument for bursty star formation cycles shaping galaxies at cosmic dawn.

Abstract

We present the discovery of extreme nitrogen enrichment by Wolf Rayet nitrogen stars (WN) in the metal-poor ($\sim10\%Z_\odot$), lensed, compact ($R_{\rm eff}\sim20$ pc) galaxy RXCJ2248 at $z=6.1$, revealed by unprecedentedly deep JWST/NIRSpec medium-resolution spectroscopy from the GLIMPSE-D Survey. The exquisite S/N reveals multiple high-ionization nebular lines and broad Balmer and [OIII] components (FWHM$\sim700-3000$ km s$^{-1}$). We detect broadened HeII $λ$1640 and $λ$4687 (FWHM$\sim530$ km s$^{-1}$) and strong NIII] $λ$4642 emission consistent with a population of WN stars, making RXCJ2248 the most distant galaxy with confirmed WR features to date. We measure the multi-phase nebular density across five ions, the direct-method metallicity ($12+\log(\rm O/H)= 7.749\pm0.023$), and a non-uniform elemental enrichment pattern of extreme N/O enhancement ($\log(\rm N/O)=-0.390\pm0.035$ from N$^+$, N$^{+2}$, and N$^{+3}$) and suppressed C/O relative to empirical C/N trends. We show that this abundance pattern can be explained by enrichment from a dual-burst with a low WC/WN ratio, as expected at low metallicities. Crucially, these signatures can only arise during a brief, rare evolutionary window shortly after a burst ($\sim3-6$ Myr), when WN stars dominate chemical feedback but before dilution by later yields (e.g., supernovae). The observed frequency of strong N emitters at high$-z$ implies a $\sim50$ Myr burst duty cycle, suggesting that N/O outliers may represent a brief but ubiquitous phase in the evolution of highly star-forming early galaxies. The detection in RXCJ2248, therefore, provides the first direct evidence of WN-driven chemical enrichment in the early Universe and a novel timing argument for the bursty star formation cycles that shaped galaxies at cosmic dawn.

Figures (7)

• Figure 1: JWST/NIRSpec MSA slits targeting RXCJ2248-ID3 for each of the three exposures in the GLIMPSE-D program. The two pointings that are closely aligned (solid purple regions) have the same wavelength coverage, while the pointing offset to the lower left (dashed region) is has somewhat reduced blue coverage. All three pointings were used in the spectrum coaddition.
• Figure 2: JWST/NIRSpec rest-frame UV and optical spectra of RXCJ2248-ID3 highlighting the first object known with simultaneously detected emission from N$^+$, N$^{+2}$, and N$^{+3}$topping24a and WR features. The 2nd row shows the main emission UV emission line detections from the archival G140M/F100LP spectrum, with significant detections of several high-ionization emission lines, including N4] $$$$1483,1486, C4 $$$$1548,1550, He2 $$1640, O3] $$$$1661,1666, N3] $$1750, and C3] $$$$1907,1909. The 3rd row shows the blue end of the optical spectrum, where the left hand panel shows the archival G235M/F170LP spectrum, which includes the low-ionization [O2] $$$$3727,3729 doublet. The right hand panel of the 3rd row and the 4th row shows the extremely high S/N GLIMPSE-D optical spectrum, enabling detections of several weak features. Some of the important features to this work are highlighted in the zoom in panels in the top row. In particular, the last panel reveals the most distant WR detection to date, with the $$4650 WR bump showing emission from N3 $$4642, indicative of nitrogen enrichment from WN stars.
• Figure 3: Multi-component emission line fits to the GLIMPSE spectrum of RXCJ2248-ID for H$$$$4342 $+$ [O3] $$4364 (left panels), H$$$$4863 $+$ [O3] $$$$4960,5008 (middle panels), and H$$$$6565 $+$ [N2] $$$$6549,6585 (right panels). When fit with single, narrow Gaussian components (e.g., purple and yellow filled Gaussians), all three line complexes show strong, broad component residual flux. The resulting best fit to each line is comprised of a single narrow Gaussian plus two broad Gaussians, where the relevant component velocity widths are tied together: The H$$$$6565 $+$ [N2] $$$$6549,6585 complex fit provided the velocity width constraints for the H Balmer line narrow (purple Gaussians) and broad components (blue and green Gaussians) and, subsequently, the H$$$$4863 $+$ [O3] $$$$4960,5008 fit constrained the [O3] narrow (yellow Gaussian) and broad (orange and red Gaussians) velocity widths that were then used in the H$$$$4342 $+$ [O3] $$4364 fit. Note that faint lines (e.g., He1 $$5017) are not included in the fits shown here. Careful accounting for the residual broad flux has significant impact on the derived nebular reddening, temperature, metallicity, and N/O abundance.
• Figure 4: The blue WR region of the optical spectrum of RXCJ2248-ID3 (purple) is shown in comparison to the $z\sim3$ Sunburst Arc spectrum from rivera-thorsen24, which has been convolved to $R\sim1000$ to match RXCJ2248-ID3. Both galaxies show characteristic signs of hosting WN stars, but RXCJ2248-ID3 shows striking N3 $$4642 emission that is much stronger than the Sunburst Arc. On the other hand, the Sunburst Arc shows broader He2 emission, which is expected for a more metal-rich galaxy as stellar winds scale with metallicity.
• Figure 5: Relative C and N abundance trends versus metallicity. Nitrogen to oxygen ratio versus oxygen abundance for star-forming galaxies is plotted in the left panels, while carbon to oxygen ratio versus oxygen abundance is plotted in the middle panels, and carbon to nitrogen abundance versus oxygen abundance is plotted in the right panels. Top row: RXCJ2248-ID3 is shown relative to the observed $z\sim0$ trend and other high-$z$ galaxies. The abundances for RXCJ2248-ID3 are shown as purple diamonds, where multiple N/O points show the measurements for each ionic N/O calculation method. The typical bi-modal N/O trend is characterized by local dwarf vanzee06aberg12berg16berg19a and spiral galaxy esteban02esteban09esteban14pilyugin05garcia-rojas07lopez-sanchez07berg20H2 region measurements. The primary N/O plateau from berg19a is shown as a dashed purple line, while the solid green line is an empirical stellar curve from nicholls17. Additional C/O literature measurements for dwarf galaxies are from pena-guerrero17 and senchyna17. High-redshift ($z>2$) abundances from martinez25 are plotted as blue plus signs for galaxies with UV N$^{+2}$/O$^{+2}$ derived abundances and pentagons for optical N$^+$/O$^+$ derived abundances. Bottom row: The same observed samples are shown as the top row, but with the $z\sim0$ sample represented by the shaded gray regions. The observed abundances of RXCJ2248-ID3 are compared to updated dual-burst chemical evolution models of kobayashi24, color-coded by age since onset of the second burst. The models have been modified to reproduce both the enhanced N/O and relatively deficient C/O observed for RXCJ2248-ID3, which requires enrichment from WN but very little WC enrichment, as expected at low-metallicities.