[Paper Review] Extreme N-emitters at high-redshift: signatures of supermassive stars and globular cluster or black hole formation in action?

R. Marques-Chaves, D. Schaerer+10 more|arXiv (Cornell University)|Jul 9, 2023

Astrophysics and Star Formation Studies4 citations

TL;DR

This study identifies CEERS-1019, a high-redshift (z=8.68) extreme N-emitter with supersolar N/O ratio (log(N/O) = -0.18 ± 0.11), as a strong candidate for proto-globular cluster formation. Using JWST/NIRSpec data and photoionization modeling, it shows that the observed N/O abundance anomalies are best explained by enrichment from supermassive stars (SMS), providing the strongest indirect evidence yet for SMS in the early universe and linking N-emitters to both GC formation and potential intermediate-mass black hole seeds.

ABSTRACT

[Abridged] Using the JWST/NIRSpec observations from CEERS we found an extreme N-emitter, CEERS-1019 at z=8.6782 showing intense NIV and NIII emission. From the observed rest-UV and optical lines we conclude that it is compatible with photoionization from stars and we determine accurate abundances for C, N, O, and Ne, relative to H, finding a highly supersolar ratio log(N/O) = -0.18+/-0.11, and normal log(C/O) = -0.75+/-0.11 and log(Ne/O) = -0.63+/-0.07, for its low metallicity, 12+log(O/H)= 7.70+/-0.18. We also analyze other N-emitters from the literature. All show strongly enhanced N/O ratios and two of them normal C/O. Massive star ejecta from WR stars are needed to explain the galaxies with enhanced C/O (Lynx arc and Mrk 996). On the other hand, supermassive stars (>1000 Msun, SMS) in the ``conveyer-belt model'' put forward to explain globular clusters (GCs), predict a high N/O and small changes in C/O, compatible with CEERS-1019, the Sunburst cluster, SMACS2031, and GN-z11. Based on the chemical abundances, possible enrichment scenarios, compactness, and high ISM density, we suggest that CEERS-1019, SMACS2031, and the Sunburst cluster could contain proto-GCs. Finally, we propose that some N-emitters enriched by SMS could also have formed intermediate-mass black holes, and we suggest that this might be the case for GN-z11. Our observations and analysis reinforce the suggested link between some N-emitters and proto-GC formation, which is supported both by empirical evidence and quantitative models. Furthermore, the observations provide possible evidence for the presence of supermassive stars in the early Universe (z>8) and at z~2-3. Our analysis also suggests that the origin and nature of the N-emitters is diverse, including also objects like GN-z11 which possibly host an AGN.

Motivation & Objective

• To identify and characterize extreme N-emitters at high redshift with unusual nitrogen abundance ratios.
• To determine whether these N-emitters are signatures of supermassive star (SMS) formation or proto-globular cluster (GC) formation.
• To distinguish between enrichment by massive star winds (WR scenario) and SMS ejecta in shaping the observed chemical abundances.
• To assess the physical conditions—such as compactness, high stellar mass surface density, and ISM density—that support proto-GC formation.

Proposed method

• Analysis of JWST/NIRSpec spectroscopic data from the CEERS survey to measure rest-UV and optical emission lines in CEERS-1019 at z=8.6782.
• Photoionization modeling to infer ionizing source properties and constrain ionization parameters, electron densities, and metallicity.
• Comparison of observed C, N, O, and Ne abundances with predictions from massive star wind models (WR scenario) and supermassive star (SMS) ejecta models.
• Re-analysis of previously known N-emitters (e.g., Sunburst cluster, SMACS2031, Lynx arc, Mrk 996) using consistent photoionization and abundance analysis techniques.
• Evaluation of stellar mass surface density (ΣM⋆) and half-light radii from multi-wavelength SED fitting to assess compactness and potential for GC formation.
• Application of the 'conveyer-belt model' for SMS enrichment to explain high N/O and low C/O changes in CEERS-1019, SMACS2031, and GN-z11.

Figure 1: Overview of the JWST observations of CEERS-1019 at $z=8.6782$ . Top: 1D and 2D low-resolution NIRSpec/PRISM spectra (black) and 1 $\sigma$ uncertainty (grey). Vertical dashed lines (green) mark the position of well-detected nebular emission lines. The X-axis in the bottom and top panels re

Experimental results

Research questions

• RQ1 Can the extreme N/O abundance ratio in CEERS-1019 be explained by standard massive star (WR) wind enrichment or does it require supermassive star (SMS) ejecta?
• RQ2 Do the physical properties of CEERS-1019—such as high stellar mass surface density (ΣM⋆ ≳ 10^3.5 M⊙ pc⁻²) and compact morphology—support its classification as a proto-globular cluster?
• RQ3 How do the observed abundance ratios in lensed N-emitters (e.g., SMACS2031, Sunburst cluster) compare to predictions from SMS vs. WR enrichment models?
• RQ4 Is there a link between N-emitters with supersolar N/O and the formation of intermediate-mass black holes, particularly in objects like GN-z11?
• RQ5 What role do ISM density (ne ≈ 10⁴–10⁵ cm⁻³) and low dilution of ejecta play in producing the observed abundance anomalies in these high-redshift galaxies?

Key findings

• CEERS-1019 exhibits a supersolar N/O ratio of log(N/O) = -0.18 ± 0.11, with 12 + log(O/H) = 7.70 ± 0.18, indicating low metallicity and extreme nitrogen enrichment.
• The galaxy's stellar mass surface density reaches log(ΣM⋆ / M⊙ pc⁻²) ≈ 3.55–4.14, and its half-light radii are ≈100–150 pc, consistent with a compact, dense star-forming system.
• The high N/O ratio is best explained by mixing SMS ejecta with unenriched ISM, while the C/O ratio is consistent with the 'conveyer-belt model' of SMS enrichment.
• The Lynx arc and Mrk 996 show enhanced C/O ratios, indicating enrichment by massive WR stars, ruling them out as proto-GC candidates.
• CEERS-1019, SMACS2031, and the Sunburst cluster are identified as the most likely hosts of proto-globular clusters due to their compactness, high ISM density, and SMS-compatible abundance patterns.
• The study suggests that GN-z11, despite hosting an AGN, may still have been enriched by SMS, pointing to a possible link between N-emitters and the formation of intermediate-mass black holes.

Figure 2: Zoom-in to the rest-frame UV part of the PRISM spectrum of CEERS-1019 (black and 1 $\sigma$ uncertainty in grey). Vertical dashed lines (green) mark the position of nebular emission lines. The blue line is the best fit for several UV emission lines and continuum. The bottom panel shows the