JWST Identification of Extremely Low C/N Galaxies with [N/O] ≳ 0.5 at z ∼ 6–10 Evidencing the Early CNO-cycle Enrichment and a Connection with Globular Cluster Formation

We present chemical abundance ratios of 70 star-forming galaxies at z ∼ 4–10 observed by the JWST/NIRSpec Early Release Observations, GLASS, and CEERS programs. Among the 70 galaxies, we have pinpointed two galaxies, CEERS_01019 at z = 8.68 and GLASS_150008 at z = 6.23, with extremely low C/N ([C/N] ≲ −1), evidenced with C iii]λλ1907,1909, N iii]λ1750, and N iv]λλ1483,1486, which show high N/O ratios ([N/O] ≳ 0.5) comparable with the one of GN-z11, regardless of whether stellar or active galactic nucleus radiation is assumed. Such low C/N and high N/O ratios found in CEERS_01019 and GLASS_150008 (additionally identified in GN-z11) are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. On the C/N versus O/H plane, these three galaxies do not coincide with Galactic H ii regions, normal star-forming galaxies, and nitrogen-loud quasars with asymptotic giant branch stars, but with globular-cluster (GC) stars, indicating a connection with GC formation. We compare the C/O and N/O of these three galaxies with those of theoretical models and find that these three galaxies are explained by scenarios with dominant CNO-cycle materials, i.e., Wolf–Rayet stars, supermassive (103–105 M ⊙) stars, and tidal disruption events, interestingly with a requirement of frequent direct collapses. For all the 70 galaxies, we present measurements of Ne/O, S/O, and Ar/O, together with C/O and N/O. We identify four galaxies with very low Ne/O, log(Ne/O) < −1.0, indicating abundant massive (≳30 M ⊙) stars.


INTRODUCTION
Chemical abundance ratios of the inter-stellar medium (ISM) in early galaxies are crucial to understanding stellar nucleosynthesis.Local dwarf star-forming galaxies isobe@icrr.u-tokyo.ac.jp have α-to-oxygen (α/O) ratios such as neon-to-oxygen (Ne/O) that are around solar abundances and remain mostly constant for their gas-phase metallicity (e.g., Izotov et al. 2006;Kojima et al. 2021;Isobe et al. 2022).These findings suggest that the majority of α elements are produced by massive stars evolving into core-collapse supernovae (CCSNe) and/or hypernovae (HNe; e.g., Nomoto et al. 2013).On the other hand, nitrogen-to-oxygen (N/O) ratios of star-forming galaxies increase with metallicity (e.g., Izotov et al. 2006;Pilyugin et al. 2012;Kojima et al. 2021;Isobe et al. 2022), thought to originate from primary oxygen production by massive stars and secondary nitrogen production by low and intermediate-mass metal-rich stars evolving to asymptotic giant branch (AGB) stars (e.g., Vincenzo et al. 2016).Similarly, carbon-to-oxygen (C/O) ratios of star-forming galaxies increase with metallicity due to massive stars and AGB stars (e.g., Berg et al. 2019).
Before the arrival of the James Webb Space Telescope (JWST), these kinds of chemical abundance studies were restricted to at most intermediate redshifts, even with stacking analysis (Steidel et al. 2016) or lensed objects (e.g., Kojima et al. 2017).However, Near Infrared Spectrograph (NIRSpec) on JWST can spectroscopically observe a near-infrared (NIR) wavelength range of 1-5 µm 10-1000 times more deeply than other spectrographs.This great advancement has led to several emission line detections of high-redshift (z 4) galaxies in restframe ultraviolet (UV) to optical ranges, which are vital for chemical abundance measurements.Shortly after Early Release Observations (ERO; Pontoppidan et al. 2022; PID: 2736) became public, Arellano-Córdova et al. (2022) have reported Ne/O ratios of 3 ERO galaxies and the C/O ratio of one ERO galaxy at z > 7. The 3 and 1 galaxies have Ne/O and C/O ratios comparable to those of local galaxies at a given metallicity, respectively.
However, Jones et al. (2023) have found that a z = 6.23 galaxy observed by the GLASS-JWST Early Release Science (ERS) program (hereafter GLASS ;Treu et al. 2022; PID: 1324) has a very low log(C/O) value of −1.01 ± 0.22.As the low C/O value is consistent with those expected from yields of pure core-collapse supernovae (Nomoto et al. 2013), Jones et al. (2023) have interpreted the galaxy as having a young chemical composition where carbon production by AGB stars is negligible.
Moreover, GN-z11 at z = 10.6, photometricly identified by Oesch et al. (2016) and spectroscopically confirmed by the JWST Advanced Deep Extragalactic Survey (JADES; Bunker et al. 2023), has a super-solar value of log(N/O) −0.5 (Cameron et al. 2023;Senchyna et al. 2023), which is significantly higher than those of local galaxies at a given metallicity.Given the ultrahigh redshift of z = 10.6 corresponding to 440 Myr after the Big Bang, nitrogen production by AGB stars is difficult to explain the N/O enhancement of GN-z11 (e.g., Cameron et al. 2023).Since the report, several studies have been trying to explain the high N/O ratio of GN-z11 (see Section 4.2 for more details).However, it remains unknown whether there is another galaxy with super-solar N/O ratios at high redshift.The carbonnitrogen (C/N) ratio has also not been studied, although it is usually assumed that both carbon and nitrogen originate from AGB stars.
Furthermore, given that chemical evolution models (Watanabe et al. 2023) predict significant changes of Ne/O ratios from the solar abundances at the early formation phase (more details in Section 5.2), Ne/O ratios potentially evolve toward higher redshifts.
The aim of this paper is to study the neon, carbon, nitrogen, and oxygen abundances of high-z galaxies using data from the NIRSpec public surveys to characterize the nature of star formation at high redshifts.This paper is organized as follows.Section 2 explains the data and sample we use.Our analysis is described in Section 3. We report and discuss our results of nitrogen, carbon, and oxygen abundances in Sections 4. Results and discussions of Ne/O ratios are described in 5. Our findings are summarized in Section 6.We assume a standard ΛCDM cosmology with parameters of (Ω m , Ω Λ , H 0 ) = (0.3, 0.7, 70 km s −1 Mpc −1 ).Throughout this paper, we use the solar abundance ratios of Asplund et al. (2021).The notation [A/B] is defined as log(A/B) subtracted by the solar abundance log(A/B) .

DATA AND SAMPLE
We utilize JWST/NIRSpec data from the Early Release Observations (ERO; Pontoppidan et al. 2022), taken in the SMACS 0723 lensing cluster field (hereafter referred to as ERO data), the GLASS survey (Treu et al. 2022; hereafter referred to as GLASS data), and the CEERS survey (Finkelstein et al. 2022; hereafter referred to as CEERS data).The ERO data were acquired using medium-resolution (R ∼ 1000) filter-grating pairs of F170LP-G235M and F290LP-G395M, covering the wavelength ranges of 1.7-3.1 and 2.9-5.1 µm, respectively.The total exposure time for the ERO data is 4.86 hours for each filter-grating pair.The GLASS data were collected using high-resolution (R ∼ 2700) filtergrating pairs of F100LP-G140H, F170LP-G235H, and F290LP-G395H, covering the wavelength ranges of 1.0-1.6,1.7-3.1, and 2.9-5.1 µm, respectively.The total exposure time for the GLASS data is 4.9 hours for each filter-grating pair.The CEERS data were obtained using medium-resolution filter-grating pairs of F100LP-G140M, F170LP-G235M, and F290LP-G395M, covering the wavelength ranges of 1.0-1.6,1.7-3.1, and 2.9-5.1 µm, respectively.The total exposure time for the CEERS data is 0.86 hours for each filter-grating pair.
We use spectroscopic data that has been reduced by Nakajima et al. (2023).Nakajima et al. (2023) have extracted the raw data from the MAST archive and performed level-2 and 3 calibrations using the JWST Science Calibration Pipeline with the reference file jwst 1028.pmap,whose flux calibration is based on in-flight flat data.Checking the data, we identify 5, 15, and 50 galaxies at z > 4 in the ERO, GLASS, and CEERS data, respectively.We refer to these 70 (= 5 + 15 + 50) galaxies as our sample galaxies, hereafter.O iii]λλ1661,1666, respectively, for simplicity.Thirtyfive and 7 of our sample galaxies have S/N > 3 detections of [Ne iii] and C iii], respectively.We find that one of our sample galaxies, GLASS 150008, has a detection of N iii] with S/N = 4.2.We also identify the detection of N iv] (S/N = 4.3) from one of our sample galaxies, CEERS 01019, which has also been reported by Larson et al. (2023).

Emission-line Diagnostics
The left panels of Figure 1 compare N iii]/O iii] and N iv]/O iii] ratios of GLASS 150008 (double red circle) and CEERS 01019 (red square) with those of GN-z11 (magenta square) as a function of N iv]/N iii], while we use N iii] and N iv] fluxes measured by Maiolino et al. (2023) and the upper limit of O iii] (Bunker et al. 2023) 2 .The N iii] and N iv] fluxes from Maiolino et al. (2023) and the O iii] upper limit from Bunker et al. (2023) are scaled by C iii] lines.We find that GLASS 150008 and CEERS 01019 have high N iii]/O iii] and N iv]/O iii] ratios, respectively, both of which are comparable to those of GN-z11, which is reported to have a super-solar N/O ratio (Section 1).The right panels of Figure 1 also show that GLASS 150008 and CEERS 01019 have low C iii]/N iii] and C iii]/N iv] ratios, respectively, both of which are comparable to those of GN-z11.
To compare the observed emission-line ratios, we construct photoionization models using Cloudy (Ferland et al. 2013) for both stellar and AGN radiations.To model young star-forming galaxies, we use BPASS (Stanway & Eldridge 2018) binary stellar radiations under the assumptions of the instantaneous star-formation history with the stellar age of 10 Myr, upper star mass cut of 100 M with the Salpeter (1955) initial mass function (IMF), and the hydrogen density n H of 300 cm −3 .The n H value is inferred from a typical value of the electron density (n e ) of 300 cm −3 in the ISM of z > 4 galaxies (Isobe et al. 2023).We also fix He/H and metalto-oxygen ratios to be the solar abundances.We vary O/H and ionization parameter (U ) values within the ranges of −2 ≤ [O/H] ≤ 1 and −3.5 ≤ log(U ) ≤ −0.5 in 0.25 and 0.25 increments, respectively.We set the stellar metallicity equal to the nebular metallicity defined by the O/H ratio.We refer to this model as the young stellar model, hereafter.We also construct the photoionization models for extremely young star-forming galaxies with very massive stars (extremely young massive stellar model, hereafter), assuming the stellar age of 1 Myr and upper star mass cut of 300 M .The other parameters are the same as those of the young stellar model.We also compute the stellar photoionization models with [N/O] = 1, whose other parameters are the same as those of the young stellar model and extremely young massive stellar model.
We also construct AGN photoionization models whose incident radiation is parameterized by the following 4 parameters: the big-bump temperature T BB , the X-ray to UV ratio α OX , the low-energy slope of the big-bump component α UV , and the X-ray component slope α X .Typical AGNs have α OX ∼ −1.4 (Zamorani et al. 1981) and α UV ∼ −0.5 (Francis 1993;Elvis et al. 1994).We refer to the AGN model with the parameter set of T BB = 1.5×10 5 K, α OX = −1.4,α UV = −0.5, and α X = −1.0 as the default AGN model.The flux density per frequency f ν is expressed by:

GN-z11 CEERS 01019
where a is a coefficient corresponding to α OX and T IR is an infrared cutoff temperature at kT IR = 0.01 Ryd.The continuum above 100 keV is also assumed to cut off by ν −2 .Using the AGN radiation, we calculate emission line ratios under the assumption of n H = 300 cm −3 .We vary O/H, U , and N/O within the ranges of and −1 ≤ [C/O] ≤ 0 in 0.25, 0.25, 1, and 1 increments, respectively.We also fix He/H and metal-tooxygen ratios other than N/O to be the solar abun-dances.Note that different assumptions of n H from 10 to 10 6 cm −3 can change key emission-line ratios of In Figure 1, we plot the young stellar models (solid) and the default AGN models (dashed) with [N/O] = 0 and 1 (corresponding to [C/N] = 0 and −1).We find that GLASS 150008, and CEERS 01019, and GN-z11 have high N iii]/O iii] and N iv]/O iii] ratios close to those predicted by both the young stellar model and the default AGN model with [N/O] = 1.Similarly, the low C iii]/N iii] and C iii]/N iv] ratios in the 3 galaxies are comparable to those of the models with [C/N] = −1.These results indicate that the 3 galaxies have very high N/O and low C/N ratios significantly above and below the solar abundance, respectively.These results are not likely affected by the metallicity changes from log(Z/Z ) = −1 (blue) to 0.25 (red) as shown in Figure 1, while for accuracy, we derive metallicities first in Sections 3.3 and 3.5.

Nebular Property
We derive color excesses E(B − V ) from Balmer line ratios of Hβ/Hα, Hγ/Hα, and Hγ/Hβ as many as possible by assuming the dust attenuation curve of Calzetti et al. (2000) and the case B recombination.Intrinsic values of the Balmer line ratios are calculated by PyNeb (Luridiana et al. 2015;v1.1.15).If none of the above Balmer line ratios are available, we use a median value of E(B − V ) = 0.07 for the other galaxies in our sample.
Using the E(B − V ) values and Calzetti et al. (2000)'s curve, we correct emission-line ratios for dust attenuation.
For the 9 galaxies with the measurements of electron temperatures of O iii (T e (O iii)) by Nakajima et al. (2023), we use the T e values and their errors.In addition, we derive T e (O iii) values of 5 galaxies with [O iii]λ4363 and/or O iii]λ1666.The other galaxies in our sample are assumed to have T e (O iii) = 15000 K with 1σ uncertainty of 5000 K.We calculate T e (O ii) from T e (O iii) and the empirical relation of Garnett (1992).We assume n e values to be 300 cm −3 .
We We then obtain gas-phase metallicity 12+log(O/H) from the equation O/H = O + /H + + O 2+ /H + by ignoring neutral oxygen and O 3+ and higher-order oxygen ions as in e.g., Izotov et al. (2006).
We include errors of the used fluxes as well as uncertainties of the assumptions of T e and [O ii] for some of our sample galaxies into 12 + log(O/H) errors by Monte Carlo simulations.We derive 1000 values of 12 + log(O/H) with T e and flux values such as [O ii] randomly fluctuated by their errors under the assump-tion of the normal distribution.Then, we derive the 16th and 84th percentiles of the distribution of the 1000 12 + log(O/H) values as the ±1σ confidence interval of 12 + log(O/H).We note that we cannot measure 12+log(O/H) of 7 galaxies in our sample due to the lack of Hβ.The derived T e , E(B − V ), and 12 + log(O/H) values are summarized in Table A1.

Abundance Ratio
We derive Ne/O, C/O, and N/O ratios from ionic abundance ratios with similar ionization energies of Ne 2+ /O 2+ , C 2+ /O 2+ , and N 2+ /O 2+ to minimize systematics of ionization correction factors (ICFs).We calculate Ne and N 2+ /O 2+ ratios with less systematics.For the other galaxies in our sample, we calculate C 2+ /O 2+ and N 2+ /O 2+ ratios from [O iii] lines.We calculate these ionic abundance ratios using PyNeb with the same transition probabilities and collision strengths listed in Table 1.
To derive chemical abundance ratios (Ne/O, C/O, and N/O) from the ionic abundance ratios (Ne  A1. We have confirmed that our derived values of C/O and 12 + log(O/H) for GLASS 150008 are consistent with those measured by Jones et al. (2023) within 1σ level.We find that GLASS 150008 has a super-solar N/O value of log(N/O) = −0.40+0.05  −0.07 as predicted in Figure 1.The derived properties of GLASS 150008 are listed in Table 2.

Abundance Ratio Re-estimation Based on AGN Radiation
It should be noted that GLASS 150008 does not have very high ionization lines that require the presence of AGN.We have also checked that GLASS 150008 does not have a clear broad component in the Hβ line profile.We thus conclude that GLASS 150008 does not have AGN signatures at least in the current data set.Contrary to GLASS 150008, Larson et al. (2023) have reported that CEERS 01019 with the N iv] detection has an Hβ broad component with the full-width half maximum of ∼ 1200 km s −1 , which can be an evidence of AGN.Maiolino et al. (2023) have also reported that GN-z11 has a detection of [Ne iv]λλ2422,2424 lines that require high-energy photons beyond 63.5 eV, which can also suggest that GN-z11 is an AGN.
Here, we check if emission line ratios of CEERS 01019 and GN-z11 cannot be reproduced by stellar radiations.The solid curves in the top panels of Figure 2  Then, we revisit metallicity and N/O measurements of CEERS 01019 and GN-z11 with AGN radiations.We find that CEERS 01019 has the lower limit of N iv]/N iii] and the upper limit of He ii/Hδ simultaneously reproduced by the default AGN model.We also find that both [Ne iv]/[Ne iii] and He ii/Hδ ratios of GN-z11 measured by Maiolino et al. (2023)

Obs.
Obs.  −0.05 (Senchyna et al. 2023).This is because the AGN radiation can enhance [O iii]λ4363, which results in overestimation of T e .Our derived metallicity is consistent with that inferred from the deep blueshifted absorption of C iv λλ1548,1550 (Maiolino et al. 2023).

AGN model AGN model
In addition, Figure 3 compares the metallicity of GN-z11 (magenta square) based on the AGN radiation with those of First0 (black) and First1 (blue) galaxies of the cosmological hydrodynamics zoom-in simulation FOR-EVER22 (Yajima et al. 2022a,b).Although the metallicities of the simulated galaxies calculated for the total gas (solid) are lower than that of GN-z11, we find that the simulated galaxies can instantaneously have supersolar metallicities within their half-mass radii (∼ 100 pc at z = 10.6,where GN-z11 is located).This suggests that it is not very strange even for high-z galaxies such as GN-z11 (and CEERS 01019 at z = 8.679; red square) to have high metallicities around the solar value.
We We have confirmed that the C/N value based on C iii]/N iii] is consistent with that based on C iii]/N iv] within the error level.We summarize the metallicities and the abundance ratios of GLASS 150008, CEERS 01019, and GN-z11 in Table 2.

Result
The top left panel of Figure 4 shows N/O ratios as a function of metallicity.We find that CEERS 01019 (red square) and GLASS 150008 (double red circle) show [N/O] 0.5.These 2 galaxies are the second and third examples of super-solar N/O galaxies at z > 6 after the report of GN-z11.Hereafter we refer to these 3 galaxies  (Senchyna et al. 2023;Berg et al. 2016).Local galaxies (Berg et al. 2016;Berg et al. 2019;Izotov et al. 2006) and Galactic H ii regions (García-Rojas & Esteban 2007) are shown by the gray dots, while the gray curves are the empirical relation of the Galactic stars (Nicholls et al. 2017).The purple circles are dwarf stars in a globular cluster (GC) NGC6752 (Carretta et al. 2005), while those O/H values are taken from Senchyna et al. (2023).The pink crosses are carbon-enhanced metal-poor (CEMP) stars (Norris et al. 2013) and nitrogen-enhanced metal-poor (NEMP) stars (Beveridge & Sneden 1994).The cyan and brown shaded regions indicate yields of CCSN (Watanabe et al. 2023) and the equilibrium value of the CNO cycle (Maeder et al. 2015), respectively.The orange dashed line shows a chemical evolution model that reproduces emission-line ratios of quasars (Hamann & Ferland 1993), while nitrogen enrichment in the model is mainly caused by asymptotic giant branch (AGB) stars.The blue dashed, green solid, and yellow dashed-dotted lines illustrate chemical evolution models of Wolf-Rayet (WR) stars, supermassive stars (SMS), and tidal disruption events (TDE), respectively, where most of massive stars are assumed to undergo the direct collapse (Watanabe et al. 2023;Watanabe et al. in prep.).The potential change of N/O and C/O by dust depletion (Ferland et al. 2013) is indicated by the length of the small black arrow at the bottom left corner of the bottom right panel.(CEERS 01019, GLASS 150008, and GN-z11) as JWST N-rich galaxies.The super-solar N/O values are significantly higher than those of typical local dwarf galaxies, Galactic H ii regions (gray dots), and typical Galactic stars (gray curve) at a given 12 + log(O/H), but comparable to that of a Wolf-Rayet galaxy, Mrk996.
Contrary to the high N/O ratios, the top right panel of Figure 4 illustrates that CEERS 01019 has a C/O ratio comparable to those of the typical local galaxies and stars (gray).GLASS 150008 (larger double red circle) shows a C/O ratio even lower than those of the typical local galaxies and stars but comparable to some of our sample galaxies (white circle).
This discrepancy between the high N/O and low C/O ratios is illustrated by the bottom left panel of Figure 4, which shows the C/N ratios as a function of metallicity.We find that all the JWST N-rich galaxies have [C/N] values less than ∼ −1, which are significantly lower than those of the typical local galaxies and stars.
Such low C/N ratios are expected to be observed in nitrogen-loud quasars (e.g., Batra & Baldwin 2014), while those O/H ratios have not fully been investigated.Alternatively, we plot a chemical evolution model of Hamann & Ferland (1993) reproducing observed emission-line ratios of quasars.Nitrogen in the model is mainly enriched by AGB stars.Although the quasar model with AGB stars can produce low C/N ratios down to [C/N] ∼ −1, it does not reproduce the (relatively-)low O/H ratios of the JWST N-rich galaxies simultaneously.This is because AGB stars require much delay time ( 1 Gyr) to decrease the C/N ratio to [C/N] ∼ −1, which increases O/H ratios too much.This result also suggests that the JWST N-rich galaxies are not likely to be nitrogen-loud quasars whose nitrogen is enriched by AGB stars.
The bottom right panel of Figure 4 shows the relations between N/O and C/O.We confirm that the JWST Nrich galaxies have N/O ratios higher than those of the typical local galaxies and stars at a given C/O ratios, which indicate that only nitrogen is selectively enriched in the JWST N-rich galaxies with respect to the typical local galaxies and stars.

Origin of Low C/N
In the bottom panels of Figure 4, we plot the abundance ratios processed by the CNO cycle at the equilibrium state (Maeder et al. 2015) by the brown shaded regions, while core-collapse supernova yields (Watanabe et al. 2023) are shown by the cyan shaded regions.In contrast to the typical local galaxies and stars, we find that the N-rich galaxies have the abundance ratios similar to those of the equilibrium state of the CNO cycle rather than those of the CCSN yields.Thus, we need scenarios that can enrich gaseous materials originating from the CNO cycle while suppressing oxygen supply by CCSNe in the relatively low-metallicity environments.
In fact, these requirements are met by the scenarios presented to explain nitrogen enrichment of GN-z11 such as Wolf-Rayet (WR) stars (Senchyna et al. 2023;Watanabe et al. 2023), supermassive stars (SMS; Charbonnel et al. 2023;Nagele & Umeda 2023), and tidal disruption events (TDE; Cameron et al. 2023).In all the 3 scenarios, CNO-cycle material can be ejected from the outermost hydrogen-burning layer of stars via stellar winds (for the WR star and SMS scenarios) or gravitational interactions with black holes (for the TDE scenario).Lowmetallicity massive stars can also undergo direct collapse (i.e., no ejecta) due to the low opacity of the hydrogen envelope, resulting in a heavier helium core during collapse (Heger et al. 2003).The blue dashed, green solid, and yellow dashed-dotted lines in the bottom panel of Figure 4 represent chemical evolution models of WR stars, SMSs, and TDEs, respectively, all of which are constructed by Watanabe et al. (2023) and Watanabe et al. (in prep.).The WR star and SMS models include the yields of WR stars with 25-120 M (Limongi & Chieffi 2018) and SMSs with 10 5 M (Nagele & Umeda 2023), respectively.The TDE model includes TDE yields calculated by Watanabe et al. (in prep.) with the nucleosynthesis code of Tominaga et al. (2007), assuming the destruction of stars with 9-40 M .Note that 100%, 99.99%, and 90% of massive stars are assumed to undergo direct collapse (i.e., no ejecta) in the WR star, SMS, and TDE models, respectively, and that the remaining massive stars explode as CCSNe whose yields are taken from Nomoto et al. (2013).We confirm that all the 3 models can reproduce the observed N/O and C/O ratios of the JWST N-rich galaxies.We have also checked that the N/O and C/O ratios of the wind yields from the 10 3 M SMS (Nagele & Umeda 2023) are also comparable to those of the JWST N-rich galaxies.These findings suggest the presence of high-z galaxies affected by WR stars, SMSs, and/or TDEs with frequent direct collapses.

Local Object with Similar C, N, O Abundances
We find that Mrk996, a local metal-poor dwarf starforming galaxy with a high value of log(N/O) ∼ 0 (Senchyna et al. 2023), has a low C/N value similar to the JWST N-rich galaxies (bottom left panel of Figure 4).The bottom right panel of Figure 4 4, while our sample galaxies with moderately high Ne/O ratios are shown in semi-transparent for presentation purposes.The gray shaded region in the right panel corresponds to the 16th and 84th percentiles of the Ne/O distribution of the local dwarf galaxies (Izotov et al. 2006;Kojima et al. 2021;Isobe et al. 2022).The blue curves represent chemical evolution models (Watanabe et al. 2023) with different metallicities.The cyan lines denote isochrones of the chemical evolution models at the model ages of 10 6.7 , 10 6.8 , 10 6.9 , and 10 7.0 year that correspond to the lifetime of progenitor stars with 40, 30, 25, and 20 M , respectively (Portinari et al. 1998).Telles et al. 2014).Similar to Mrk996, the JWST N-rich galaxies may also host WR stars.
We also identify some of carbon-enhanced metalpoor (CEMP) stars (Norris et al. 2013) and nitrogenenhanced metal-poor (NEMP) stars (Beveridge & Sneden 1994) in the Galactic halo with the C/N vs. O/H and N/O vs. C/O relations comparable to those of the JWST N-rich galaxies as shown by the pink crosses in Figure 4.It should be noted that some of such stars are giants undergoing the conversion of C to N in their hydrogen layers.
Moreover, the purple circles in the bottom panels of Figure 4 indicate dwarf stars in a globular cluster (GC) NGC6752, which have been reported to have high N/O ratios similar to GN-z11 (Senchyna et al. 2023).We find that the GC dwarf stars also have low C/N ratios and N/O vs. C/O relations similar to those of the JWST N-rich galaxies.As surface abundance of dwarf stars are likely unevovled, the gas composition at the time of GC formation may also have similar abundance ratios to the JWST N-rich galaxies.This implies that the JWST N-rich galaxies may be progenitors of GCs.

Result
The left panel of Figure 5 shows Ne/O ratios of our sample galaxies (red) as a function of 12 + log(O/H).Although the majority of our sample galaxies have Ne/O ratios comparable to those of local dwarf galaxies (gray dot; Izotov et al. 2006;Kojima et al. 2021;Isobe et al. 2022), 4 of our sample galaxies (GLASS 100003, CEERS 00698, CEERS 01143, and CEERS 01149) have log(Ne/O) values of < −1.0, which are significantly lower than those of the local dwarf galaxies and the other our sample galaxies.The right panel of Figure 5 illustrates the Ne/O ratios as a function of redshift.Interestingly, all the 4 galaxies with the low Ne/O ratios are located at z > 6.

Origin of Low Ne/O
To explore the origins of the low Ne/O, we compare these data points with (Watanabe et al. 2023)'s chemical evolution models accumulating the CCSN yields calculated by the nucleosynthesis code of Tominaga et al. (2007) under the assumption of Kroupa (2001) IMF.We assume the metallicity evolution of the Milky Way (Suzuki & Maeda 2018), and convert the maximum stellar ages of the models (Age mod ) to 12 + log(O/H).The blue curves in Figure 5 correspond to the chemical evolution models that accumulate CCSN ejecta with fixed metallicities of Z = 0, 0.001, and 0.004.We also illustrate isochrones of the models at log(Age mod /yr) = 6.7, 6.8, 6.9, and 7.0, which correspond to lifetimes of stars with 40, 30, 25, and 20 M , respectively (Portinari et al. 1998).As shown in the top left panel of Figure 5, the models predict an increase in Ne/O with Age mod .This increase can originate from the prediction that CCSNe with more massive progenitors have higher temperatures in the carbon-burning layer (e.g., Woosley & Janka 2005), which reduces neon abundance.
As shown in the top left panel of Figure 5, we find that the models with log(Age mod /yr) 6.8 (i.e., Age mod 6 Myr) reproduce the low Ne/O ratios of the 4 galaxies.This suggests that the 4 galaxies have an abundance population of massive stars beyond ∼ 30 M .This implication agrees with the fact that the 4 galaxies are located at higher redshifts beyond 6, because galaxies at higher redshifts are expected to be younger and/or have top-heavy IMFs due to the metal-poorer environment.

SUMMARY
We present N/O, C/O, C/N, and Ne/O ratios of 70 star-forming galaxies at z ∼ 4-10, observed by the JWST/NIRSpec ERO, GLASS, and CEERS programs.We derive these abundance ratios from emissionline ratios of similar ionization energies, accounting for both stellar and AGN radiation, particularly for CEERS 01019 and GN-z11, which exhibit AGN signatures of high-ionization lines.Our findings are summarized below: • Among the 70 galaxies, we have pinpointed 2 galaxies with unique characteristics: CEERS 01019 at z = 8.68 and GLASS 150008 at z = 6.23.We find the low C/N and high N/O ratios these galaxies, which are also found in GN-z11, closely align with the equilibrium of the CNO cycle.This suggests that these 3 galaxies have the gas component dominated by metals processed by the CNO cycle.
• The C/O and N/O ratios of these 3 galaxies are reproduced by chemical evolution models dominated by CNO-cycle materials involving Wolf-Rayet stars, supermassive stars, and tidal disruption events.Interestingly, these scenarios would require frequent direct collapses.
• On the C/N vs. O/H plane, these 3 galaxies do not align with Galactic H ii regions, typical Galactic stars, typical star-forming galaxies, or nitrogenloud quasars whose nitrogen is assumed to originate from AGB stars.They do, however, coincide with GC dwarf stars.We may see the site of GC formation in these 3 galaxies.

Figure 1 .
Figure 1.N iii]/O iii] (top left), N iv]/O iii] (bottom left), C iii]/N iii] (top right), and C iii]/N iv] (bottom right) ratios as a function of N iv]/N iii].Observed emission-line ratios of GLASS 150008, CEERS 01019, and GN-z11 are represented by the double red circle, the red square, and the magenta square, respectively.The emission-line ratios of GN-z11 are measured by Maiolino et al. (2023) and Bunker et al. (2023).Cloudy (Ferland et al. 2013) photoionization models with [N/O] = 0 and log(Z/Z ) = −0.5 based on the stellar and AGN radiations are shown by the black solid and dashed curves, respectively.These models with [N/O] = 0 and different metallicities of log(Z/Z ) = −1 and 0.25 are shown by the semi-transparent blue and red lines, respectively.Detailed measurements of the N/O and C/N ratios are made in Sections 3.4 and 3.5.
derive O + /H + from [O ii]/Hβ and T e (O ii) and O 2+ /H + from [O iii]λλ4959,5007/Hβ and T e (O iii).In our sample galaxies where only [O ii] upper limits are present, we presume the smallest values of [O ii] fluxes to be the ones obtained from [O iii] under the assumption of log(U ) = −1.For these galaxies, we also suppose the true and maximum values of [O ii] fluxes to be their 1σ and 3σ upper limits, respectively.For our sample galaxies where [O ii] falls outside the wavelength coverage of NIRSpec, we assume the true [O ii] fluxes of the galaxies to be [O iii] divided by a median [O iii]/[O ii] ratio of the other galaxies in our sample.The minimum and maximum values are assumed to be [O iii] divided by [O iii]/[O ii] at log(U ) = −1 and −3, respectively.
and N 2+ /O 2+ ), we extract ICFs from the young stellar model (Section 3.2).We have checked that our ICFs provide Ne/O and C/O ratios consistent with those based on Izotov et al. (2006) and Berg et al. (2019)'s ICFs, respectively.We have also confirmed that ICF(Ne 2+ /O 2+ ), ICF(C 2+ /O 2+ ), and ICF(N 2+ /O 2+ ) of all our sample galaxies are ∼ 1.In addition, the extremely young massive stellar model provides Ne/O, C/O, and N/O values similar to those of the young stellar model.We derive the errors of the Ne/O, C/O, and N/O ratios by Monte Carlo simulations in the same way as for 12+log(O/H) (Section 3.3).For our sample galaxies without the detections of [Ne iii], [C iii], or [N iii], we use 3σ upper limits of these lines to obtain upper limits of Ne/O, C/O, or N/O ratios.We summarize the abundance ratios in Table represent predicted emission line ratios of the extremely young massive stellar model.The model contains very massive stars up to 300 M , which are expected to produce harder radiation than normal stellar populations.However, we confirm that even the extremely young massive stellar model cannot reproduce the observed N iv]/N iii] and [O iii]/[O ii] relation of CEERS 01019 (red circle) or the observed [Ne iv]λλ2422,2424/[Ne iii] and N iv]/N iii] relation ofGN-z11 (magenta square;Maiolino et al. 2023).These result suggest that CEERS 01019 and GN-z11 require hard emissions such as AGN as discussed byLarson et al. (2023) andMaiolino et al. (2023), respectively.
are reproduced by the default AGN model except for α OX = −1.5 (lowα OX AGN model, hereafter).These AGN models are shown by the color-coded dashed lines in the top panels of Figure 2, reproducing the observed emission-line ratios of both CEERS 01019 and GN-z11.We then re-estimate 12 + log(O/H) and N/O of CEERS 01019 and GN-z11 by using these AGN models.The middle left panel of Figure 2 shows the observed [O iii]/Hβ ratio of CEERS 01019 (red solid line; 1σ uncertainty is shown by the red shaded region) and those predicted by the default AGN models with the U value derived from the observed [O iii]/[O ii] ratio (gray dashed line; 1σ uncertainty based on the observed [O iii]/[O ii] uncertainty is shown by the gray shaded region) as a function of 12 + log(O/H).Note that [O iii]/Hβ values slightly decrease with increasing N/O.With the default AGN models with [N/O] = 1, we find that the observed and model-predicted [O iii]/Hβ ratios

Figure 2 .
Figure 2. Emission-line ratios of CEERS 01019 (red square and red shaded region) and GN-z11 (magenta square and magenta shaded region).(Top) Cloudy photoionization models of young massive stellar population and AGN are shown by the solid and dashed lines, respectively, color-coded by metallicity.The observed emission-line ratios are not reproduced by the stellar models but the AGN models.(Second top, second bottom, and bottom) Comparison with the observed line ratios and the AGN photoionization models with different 12 + log(O/H) (second top), N/O (second bottom), C/O (bottom left), and C/N (bottom right).Ionization parameters of the models are determined by [O iii]/[O ii] and N iv]/N iii] for CEERS 01019 and GN-z11, respectively.The gray shaded regions represent uncertainties of the observed ionization parameters and 12 + log(O/H) (for the bottom middle and bottom panels).

Figure 3 .
Figure3.Metallicity as a function of redshift.The metallicities of CEERS 01019 (red square) and GN-z11 (magenta square) are based on their O/H ratios.The black and blue symbols are FOREVER22(Yajima et al. 2022a,b) simulated galaxies of First0 and First1, respectively, while the dots and the solid lines denote metallicities measured within the half-mass radii (r half ) and the total gas of the galaxies, respectively.

Figure 4 .
Figure 4. N/O left), C/O (top right), C/N (bottom left) as a function of 12+log(O/H).N/O as a function of C/O (bottom right).The measurements of CEERS 01019 (red square) and GN-z11 (magenta square) are based on the AGN photoionization models (see Section 3.5 and Figure 2), while those of GLASS 150008 are based on the stellar photoionization model (Section 3.4).In the top right panel, C/O ratios of the other galaxies in our sample are shown by the smaller red circles (with C iii] detection) and the white circles (with strong upper limits of C iii]), respectively.The double circles correspond to the measurements with Te determinations.The magenta dashed lines and the magenta pentagon indicate the measurements byCameron et al. (2023) andSenchyna et al. (2023; gas only), respectively.Mrk996, a Wolf-Rayet galaxy, is represented by the black diamonds(Senchyna et al. 2023;Berg et al. 2016).Local galaxies(Berg et al. 2016;Berg et al. 2019;Izotov et al. 2006) and Galactic H ii regions (García-Rojas & Esteban 2007) are shown by the gray dots, while the gray curves are the empirical relation of the Galactic stars(Nicholls et al. 2017).The purple circles are dwarf stars in a globular cluster (GC) NGC6752(Carretta et al. 2005), while those O/H values are taken fromSenchyna et al. (2023).The pink crosses are carbon-enhanced metal-poor (CEMP) stars(Norris et al. 2013) and nitrogen-enhanced metal-poor (NEMP) stars(Beveridge & Sneden 1994).The cyan and brown shaded regions indicate yields of CCSN(Watanabe et al. 2023) and the equilibrium value of the CNO cycle(Maeder et al. 2015), respectively.The orange dashed line shows a chemical evolution model that reproduces emission-line ratios of quasars(Hamann & Ferland 1993), while nitrogen enrichment in the model is mainly caused by asymptotic giant branch (AGB) stars.The blue dashed, green solid, and yellow dashed-dotted lines illustrate chemical evolution models of Wolf-Rayet (WR) stars, supermassive stars (SMS), and tidal disruption events (TDE), respectively, where most of massive stars are assumed to undergo the direct collapse(Watanabe et al. 2023; Watanabe et al. in prep.).The potential change of N/O and C/O by dust depletion(Ferland et al. 2013) is indicated by the length of the small black arrow at the bottom left corner of the bottom right panel.

Figure 5 .
Figure 5. Ne/O ratio as a function of 12 + log(O/H) (left) and redshift (right).The symbols are the same as those in the top right panel of Figure4, while our sample galaxies with moderately high Ne/O ratios are shown in semi-transparent for presentation purposes.The gray shaded region in the right panel corresponds to the 16th and 84th percentiles of the Ne/O distribution of the local dwarf galaxies(Izotov et al. 2006;Kojima et al. 2021;Isobe et al. 2022).The blue curves represent chemical evolution models(Watanabe et al. 2023) with different metallicities.The cyan lines denote isochrones of the chemical evolution models at the model ages of 10 6.7 , 10 6.8 , 10 6.9 , and 10 7.0 year that correspond to the lifetime of progenitor stars with 40, 30, 25, and 20 M , respectively(Portinari et al. 1998).

Figure B1 .
Figure B1.Ar/O and S/O as a function of metallicity.The symbols are the same as those in Figure 4, while we add Watanabe et al. (2023)'s chemical evolution models of hypernovae with different metallicities (green) and isochrones (light green).

•
We identify 4 galaxies with very low Ne/O, with log(Ne/O) < −1.0.This low ratio indicates the abundance of massive ( 30 M ) stars.JST FOREST Program (JP-MJFR202Z).This research was supported by a grant from the Hayakawa Satio Fund awarded by the Astronomical Society of Japan.
TABLE OF METALLICITY AND ABUNDANCE RATIOS