[论文解读] 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 Studies被引用 4

一句话总结

本研究识别出CEERS-1019，一个高红移（z=8.68）的极端N发射体，其N/O比值为超太阳系（log(N/O) = -0.18 ± 0.11），是原球状星团形成的强有力候选者。利用JWST/NIRSpec数据和光致电离建模，研究显示观测到的N/O丰度异常最可能由超大质量恒星（SMS）的富集所解释，为早期宇宙中SMS存在的最强间接证据，并将N发射体与球状星团形成及潜在的中等质量黑洞种子联系起来。

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.

研究动机与目标

• 识别并表征高红移处具有异常氮丰度比的极端N发射体。
• 确定这些N发射体是否为超大质量恒星（SMS）形成或原球状星团（GC）形成的标志。
• 区分大质量恒星风（WR情景）与SMS喷出物在塑造观测到的化学丰度中的作用。
• 评估支持原球状星团形成的物理条件，如紧凑性、高恒星质量面密度及星际介质（ISM）密度。

提出的方法

• 分析来自CEERS巡天的JWST/NIRSpec光谱数据，测量z=8.6782处CEERS-1019的静止紫外及光学发射线。
• 采用光致电离建模以推断电离源特性，并约束电离参数、电子密度和金属量。
• 将观测到的C、N、O和Ne丰度与大质量恒星风模型（WR情景）和超大质量恒星（SMS）喷出物模型的预测进行比较。
• 使用一致的光致电离与丰度分析技术，重新分析先前已知的N发射体（如Sunburst星团、SMACS2031、Lynx弧、Mrk 996）。
• 通过多波段SED拟合评估恒星质量面密度（ΣM⋆）与半光半径，以评估紧凑性及球状星团形成潜力。
• 应用SMS富集的“传送带模型”以解释CEERS-1019、SMACS2031和GN-z11中观测到的高N/O与低C/O变化。

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

实验结果

研究问题

• RQ1 CEERS-1019中极端的N/O丰度比是否可用标准大质量恒星（WR）风富集解释，还是必须依赖超大质量恒星（SMS）喷出物？
• RQ2 CEERS-1019的物理特性——如高恒星质量面密度（ΣM⋆ ≳ 10^3.5 M⊙ pc⁻²）和紧凑形态——是否支持其被归类为原球状星团？
• RQ3 透镜化N发射体（如SMACS2031、Sunburst星团）中观测到的丰度比与SMS与WR富集模型的预测相比如何？
• RQ4 是否存在超太阳系N/O的N发射体与中等质量黑洞形成之间的关联，特别是在GN-z11这类天体中？
• RQ5 ISM密度（ne ≈ 10⁴–10⁵ cm⁻³）和喷出物低稀释度在产生这些高红移星系中观测到的丰度异常中起何种作用？

主要发现

• CEERS-1019表现出超太阳系的N/O比值，log(N/O) = -0.18 ± 0.11，且12 + log(O/H) = 7.70 ± 0.18，表明金属量较低且氮元素极度富集。
• 该星系的恒星质量面密度达到log(ΣM⋆ / M⊙ pc⁻²) ≈ 3.55–4.14，其半光半径约为100–150 pc，与紧凑、致密的恒星形成系统一致。
• 高N/O比值最可能由SMS喷出物与未富集的ISM混合所解释，而C/O比值则与SMS富集的“传送带模型”一致。
• Lynx弧和Mrk 996表现出增强的C/O比值，表明其富集源于大质量WR恒星，因此被排除为原球状星团候选体。
• CEERS-1019、SMACS2031和Sunburst星团被确定为最可能的原球状星团宿主，因其具有紧凑性、高ISM密度及与SMS兼容的丰度模式。
• 本研究提出，尽管GN-z11中心存在活动星系核（AGN），其仍可能被SMS富集，暗示N发射体与中等质量黑洞形成之间可能存在关联。

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