Extreme N-emitters at high redshift: Possible signatures of supermassive stars and globular cluster or black hole formation in action

Abstract

Context. Recent James Webb Space Telescope (JWST) spectroscopic observations of the z = 10.6 galaxy GN-z11 have revealed a very peculiar UV spectrum exhibiting intense emission lines of nitrogen, which are not typically detected in galaxy spectra. This observation indicates a super-solar N/O abundance ratio at low metallicity, which only resembles the abundances seen in globular cluster (GC) stars. This discovery suggests that we might be seeing proto-GCs in formation or possibly even signatures of supermassive stars. Aims. To examine whether other objects with strong N IV and/or N III emission lines (N-emitters, hereafter) exist and to better understand their origin and nature, we have examined the available JWST spectra and data from the literature. Methods. Using the NIRSpec/JWST observations from CEERS, we found an extreme N-emitter, CEERS-1019 at z = 8.6782, showing intense N IV] λ1486 and N III] λ1750 emission. From the observed rest-UV and optical lines, we conclude that it is compatible with photoionization from stars and we have determined accurate abundances for C, N, O, and Ne, relative to H. We also (re-)analyzed other N-emitters from the literature, including three lensed objects at z = 2.3 − 3.5 (Sunburst cluster, SMACS2031, and Lynx arc) and a low-redshift compact galaxy, Mrk 996. We carried out a comparison among the observed abundance ratios to observations from normal star-forming galaxies, predicted wind yields from massive stars, and predictions from supermassive stars (SMS with ∼104 − 105M⊙). Results. For CEERS-1019, we find a highly supersolar ratio log(N/O)= − 0.18 ± 0.11, and abundances of log(C/O)= − 0.75 ± 0.11 and log(Ne/O)= − 0.63 ± 0.07, which are normal compared to other galaxies at the low metallicity (12 + log(O/H) = 7.70 ± 0.18) of this galaxy. The three lensed N-emitters also show strongly enhanced N/O ratios and two of them normal C/O. The high N/O abundances can be reproduced by massive star winds assuming a special timing and essentially no dilution with the ambient interstellar medium (ISM). Alternatively, these N/O ratios can be explained by mixing the ejecta of SMS with comparable amounts of unenriched ISM. Massive star ejecta (from WR stars) are needed to explain the galaxies with enhanced C/O (Lynx arc, Mrk 996). On the other hand, a SMS in the “conveyer-belt model” (put forward to explain globular clusters) would 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, and other properties (e.g., their compactness and high ISM density), we discuss which objects could contain proto-GCs. We suggest that this is the case for CEERS-1019, SMACS2031, and the Sunburst cluster. Enrichment in the Lynx arc and Mrk 996 is likely due to normal massive stars (WR), which implies that the star-forming regions in these objects cannot become 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. Conclusions. 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 objects such as GN-z11, which may possibly host an active galactic nucleus (AGN).

GN-z11 in Context: Possible Signatures of Globular Cluster Precursors at Redshift 10

“…While their nature likely varies, some other high-redshift detections of prominent N III] and N IV] emission lines beyond GN-z11 and the Sunburst Arc (Section 4.3) may hint at similar conditions. If globular clusters are a major source of UV light at z > 6 (e.g., Boylan-Kolchin 2018), it is likely that other precursor systems will be caught at similar phases to GN-z11 especially among UV-selected samples (and initial searches for such objects have already revealed other reionization era candidates; Isobe et al 2023b;Marques-Chaves et al 2024), enabling more detailed investigation of the timescales and processes involved in building up their stellar mass and peculiar abundances. The possible presence of an accreting black hole in GN-z11 (Maiolino et al 2024, see Section 3.3) also hints at the potential importance of such dense highly clustered star-forming environments for seeding and growing massive black holes (e.g., Portegies Zwart & McMillan 2002), and the significance of constraining these processes directly at z > 6.…”

Which Came First: Supermassive Black Holes or Galaxies? Insights from JWST

“…They suggest an intimate connection between black hole feedback and rapid (massive) star formation, indicative of what is often referred to as positive feedback, because the chemical timescales must be relatively short. This connection, resulting in the speed-up of chemical evolution, would be further enhanced by including the effects of stellar tidal disruptions as suggested to account for the relative-to-solar nitrogen-to-carbon abundance ratio enhancement and abundance pattern consistent with a single stellar tidal disruption event in a nearby AGN (Miller et al 2023), or by the possible role of very massive or even supermassive stars (Marques-Chaves et al 2024;Vink 2023).…”

CLASSY. IX. The Chemical Evolution of the Ne, S, Cl, and Ar Elements

“…In addition to oxygen, strong emission lines associated with other elements are now being clearly detected at z > 4 (e.g., Arellano-Córdova et al 2022a;Isobe et al 2023b;Jones et al 2023;Nakajima et al 2023;Marques-Chaves et al 2024). An example includes Ne, S, and Ar, which are the most typical α-elements observed in nebular studies in the local Universe (Izotov et al 2006;Croxall et al 2016).…”

“…Chemical abundances of SFGs have been studied in surveys like CEERS, GLASS, and JADES (Treu et al 2022;Finkelstein et al 2023;Eisenstein et al 2024). For example, the abundance ratio of C/O has been reported in a few galaxies at z > 6 (e.g., Jones et al 2012;Arellano-Córdova et al 2022a;Cameron et al 2023b;Marques-Chaves et al 2024), while the chemical abundances of Ne, S, and Ar have been determined for multiple SFGs at z = 4-8 (e.g., Isobe et al 2023a). Arellano-Córdova et al (2022a) reported Ne/O abundance ratios for three galaxies at z > 7 using spectra from the JWST Early Release Observations (ERO; Pontoppidan et al 2022).…”

“…early Universe (e.g.,Arellano-Córdova et al 2022a;Isobe et al 2023b;Marques-Chaves et al 2024;Rogers et al 2024). Using these studies, we can investigate the evolution of relative α-abundance patterns of O, Ne, S, and Ar for the first time.…”