11institutetext: Observatoire de Genève, Université de Genève, Chemin Pegasi
51, 1290 Versoix, Switzerland 22institutetext: CNRS, IRAP, 14 Avenue E. Belin,
31400 Toulouse, France 33institutetext: Institut d’Astrophysique de Paris, UMR
7095 CNRS, Sorbonne Université, 98bis, Bd Arago, 75014 Paris, France
44institutetext: Bogolyubov Institute for Theoretical Physics, National
Academy of Sciences of Ukraine, 14-b Metrolohichna str., Kyiv, 03143, Ukraine
55institutetext: The Oskar Klein Centre, Department of Astronomy, Stockholm
University, AlbaNova, SE-10691 Stockholm, Sweden 66institutetext: Cosmic Dawn
Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128,
København N, DK-2200, Denmark

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

R. Marques-Chaves 11 D. Schaerer 1122 A. Kuruvanthodi 11 D. Korber 11 N.
Prantzos 33 C. Charbonnel 1122 A. Weibel 11
Y. I. Izotov 44 M. Messa 1155 G. Brammer 66 M. Dessauges-Zavadsky 11 P. Oesch
1166

(Received date; accepted date)

###### Abstract

Context. Recent JWST spectroscopic observations of the $z=10.6$ galaxy GN-z11
have revealed a very peculiar UV spectrum showing intense emission lines of
nitrogen, which are generally not detected in galaxy spectra. This observation
indicates a super-solar N/O abundance ratio at low metallicity, resembling
only 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 if other objects with strong N iv and/or Niii emission lines
(N-emitters, hereafter) exist and to better understand their origin and
nature, we have examined 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] $\lambda$1486 and N
iii] $\lambda$1750 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. We also
(re-)analyze other N-emitters from the literature, including three lensed
objects at $z=2.3-3.5$ (the Sunburst cluster, SMACS2031, and Lynx arc) and a
low-redshift compact galaxy, Mrk 996. We compare the observed abundance ratios
to observations from normal star-forming galaxies, predicted wind yields from
massive stars and predictions from supermassive stars (SMS with $\sim
10^{4}-10^{5}$ M⊙).

Results. For CEERS-1019 we find a highly supersolar ratio $\log({\rm
N/O})=-0.18\pm 0.11$, and abundances of $\log({\rm C/O})=-0.75\pm 0.11$ and
$\log({\rm Ne/O})=-0.63\pm 0.07$, which are normal compared to other galaxies
at the low metallicity ($12+\log({\rm O/H})$$=7.70\pm 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
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, SMS in the “conveyer-belt model” put forward to
explain globular clusters, 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, such as 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 $z\sim 2-3$. 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.

###### Key Words.:

Galaxies: high-redshift – Galaxies: ISM – Galaxies: clusters: general –
(Galaxies:) quasars: supermassive black holes Cosmology: dark ages,
reionization, first stars

##  1 Introduction

Long known as the most distant spectroscopically-confirmed galaxy (Oesch et
al. 2016), GN-z11 has recently lead to new exciting and intriguing results,
after the first spectra of this galaxy were obtained with the JWST. Indeed,
the JWST/NIRSpec observations of Bunker et al. (2023) allowed to confirm a
very high redshift of this source ($z=10.60$) and showed the presence of
hydrogen, carbon, oxygen, magnesium, and neon emission lines in the rest-UV
and rest-optical spectrum, often seen in star-forming galaxies at low-redshift
and detected at $z\sim 4-8$ in other JWST spectra (see e.g., Schaerer et al.
2022; Cameron et al. 2023b; Nakajima et al. 2023; Tang et al. 2023). Most
surprisingly, however, the spectrum of GN-z11 revealed the presence of strong
N iii] $\lambda$1750 and N iv] $\lambda$1486 lines (Bunker et al. 2023), which
are very rarely detected in galaxies (see e.g., Barchiesi et al. 2023).
Furthermore, the object is found to be very compact (Tacchella et al. 2023),
which could indicate the presence of massive compact star clusters or point to
an active galactic nucleus (AGN) (Bunker et al. 2023; Tacchella et al. 2023;
Charbonnel et al. 2023; Maiolino et al. 2023).

The discovery of the peculiar emission line spectrum has triggered a series of
papers discussing in particular their origin and the nature of GN-z11. Bunker
et al. (2023) first suggested that the strong N emission lines may imply an
unusually high N/O abundance. They also discussed whether the emission would
be powered by star formation or photoionization from an AGN, without reaching
clear conclusions on this issue. The quantitative analysis of the emission
line spectrum of GN-z11 by Cameron et al. (2023a) confirmed the high N/O
abundance, with a lower limit of four times solar, finding also possibly a
less extreme C/O ratio, and a metallicity (O/H), which is sub-solar, although
not well constrained. Using a suite of photoionization models, Senchyna et al.
(2023) inferred the N/O abundance with a lower uncertainty and constrained the
metallicity to $12+\log({\rm O/H})=7.84^{+0.06}_{-0.05}$, confirming in
particular a large overabundance of N/O $\approx 3\times$ solar.

The finding of an exceptionally high N/O abundance at low metallicity
(typically ten times the normal N/O value at this O/H) has triggered different
speculations about the sources and processes explaining this enrichment. The
scenarii discussed include enrichment from massive stars winds (WR stars) or
AGB stars, i.e. relatively “classical scenarii”, or more “exotic” options such
as pollution from PopIII star-formation, tidal disruption of stars from
encounters with black holes, ejecta from very massive stars formed through
collisions in dense clusters, and supermassive stars (see: Cameron et al.
2023a; Watanabe et al. 2023; Senchyna et al. 2023; Charbonnel et al. 2023;
Nagele & Umeda 2023). Supermassive stars, for example, have been invoked by
Charbonnel et al. (2023) and Nagele & Umeda (2023) since very strong
enrichment of N and low metallicity is difficult to explain and requires
fairly fined-tuned conditions with classical scenarios (see also Cameron et
al. 2023a; Watanabe et al. 2023). Furthermore, such stars (with masses
$M_{\star}\ga 1000$ M⊙) have been proposed to form by runaway collisions in
very dense stellar clusters, and they could explain the long-standing problem
of multiple stellar populations and peculiar abundance patterns observed in
globular clusters (GC), as discussed by Gieles et al. (2018) and Denissenkov &
Hartwick (2014). If correct, this would probably represent the first
observational evidence of supermassive stars, which are also of great
interest, for example for understanding the seeds of supermassive black holes
(e.g., Portegies Zwart & McMillan 2002; Woods et al. 2019; Trinca et al. 2023,
and references therein) .

Not only the abundance ratios observed in GN-z11 resemble those of GC stars.
Its compactness and high ISM density also indicate conditions expected in
young very massive clusters, which could be proto-GCs (Senchyna et al. 2023;
Charbonnel et al. 2023). GN-z11 might thus also be the first high-redshift
object where the long sought-for peculiar abundance patterns characterizing
GCs are observed (e.g., Renzini 2017; Gratton et al. 2019, and references
therein). These exciting and surprising findings obviously pose the question
of the uniqueness of GN-z11, beg for more examples, and call for a better
understanding of similar objects, if they exist.

Indeed, although very rare, other galaxies showing emission lines of N iii]
$\lambda$1750 or N iv] $\lambda$1486 in the UV (referred to as N-emitters
subsequently) are known, as pointed out by Senchyna et al. (2023) and found in
the compilation of Barchiesi et al. (2023). Apart from objects clearly
identified as AGN, the Lynx arc, a lensed $z=3.36$ galaxy identified for N iv]
$\lambda$1486 and He ii $\lambda$1640 emission is probably the first N-emitter
studied in detail (Fosbury et al. 2003; Villar-Martín et al. 2004). From
photoionization modeling Villar-Martín et al. (2004) derive a high N/O ratio
and sub-solar metallicity. Another strongly lensed object at $z=2.37$, the
multiply-imaged compact star cluster in the Sunburst arc which has extensively
been studied in recent years (e.g. Rivera-Thorsen et al. 2019; Vanzella et al.
2022), shows N iii] $\lambda$1750 emission, as shown in the high S/N spectrum
of Meštrić et al. (2022). Pascale et al. (2023) have shown that N/O is also
elevated ($\sim 4\times$ solar) at a metallicity $\sim 1/5$ solar. Finally, in
the low-redshift Universe, Mrk 996 uniquely stands out as the only galaxy
showing strong N iii] $\lambda$1750 emission in the UV (see Mingozzi et al.
2022), and this blue compact dwarf galaxy has long been known as very
peculiar, showing e.g. a high electron density, the presence of strong
emission lines from WR stars in the optical, and a high N/O abundance, at
least in its core (e.g. Thuan et al. 1996; James et al. 2009; Telles et al.
2014).

Here we present a detailed analysis of the $z=8.68$ galaxy CEERS-1019 observed
with NIRSpec/JWST by the public CEERS survey (Finkelstein et al. 2017). This
object has previously been studied by several authors (Tang et al. 2023;
Nakajima et al. 2023; Larson et al. 2023), but none of these have analysed the
carbon and nitrogen abundance and its rest-UV spectrum. Only very recently,
Isobe et al. (2023) have analyzed the UV spectrum in detail. Similarly to
GN-z11, this galaxy exhibits a very peculiar rest-UV spectrum, making it
clearly an N-emitter. Showing numerous emission lines of H, C, N, O, Ne, and
the auroral [O iii]$\lambda 4363$ line, it allows us to accurately determine
the chemical abundances of these elements and offers thus a unique opportunity
to study the second N-emitter in the early Universe and to enlarge the sample
of these rare objects. We also analyze the other known N-emitters and compare
their properties to those of CEERS-1019 and GN-z11. Finally, we confront the
observed abundance patterns with predictions from normal massive stars and
with predicted enrichment patterns from supermassive stars.

The paper is structured as follows. In Sect. 2 we describe the observational
data, reduction, and measurements used in this work. We then discuss the
nature of the ionizing source of CEERS-1019 (Sect. 3). The chemical abundances
and other physical properties of CEERS-1019 are derived in Sect. 4. In Sect. 5
we compare the abundance ratios of CEERS-1019 to other N-emitters and normal
star-forming galaxies, and we present different chemical enrichment scenarios
to explain them. We also discuss the possible link between CEERS-1019 and
proto-GCs. The main results of our work are summarized in Sect. 6. Throughout
this work, we assume concordance cosmology with $\Omega_{\rm m}=0.274$,
$\Omega_{\rm\Lambda}=0.726$, and $H_{0}=70$ km s-1 Mpc-1.

##  2 CEERS-1019: a new strong N emitter at high redshift

CEERS-1019 ($\alpha$, $\delta$ [J2000] = 215.0354∘, 52.8907∘) was initially
identified as a $z_{\rm phot}\simeq 8.6$ dropout galaxy by Roberts-Borsani et
al. (2016) and spectroscopically confirmed at $z_{\rm spec}=8.683$ by Zitrin
et al. (2015) through strong Ly$\alpha$ emission (see also Mainali et al. 2018
and Witten et al. 2023). It is one of the most distant Ly$\alpha$ emitter
known and is thought to reside in an over-dense region and ionized bubble
boosting substantially its Ly$\alpha$ transmission (Larson et al. 2022;
Leonova et al. 2022; Whitler et al. 2023). Mainali et al. (2018) also report a
tentative detection of N v $\lambda 1240$ ($4.6\sigma$), suggesting a hard
ionizing spectrum of this source.

Recently, much deeper spectroscopy of CEERS-1019 was reported and analyzed by
Tang et al. (2023), Nakajima et al. (2023), and Larson et al. (2023) using
NIRSpec, along with NIRCam and MIRI imaging. Although with some discrepancies,
these works derived important physical properties of CEERS-1019 such as its
stellar mass (log($M_{\star}/M_{\odot}\simeq 8.7-10.1$), gas-phase
metallicities ($12+\log({\rm O/H})$$\simeq 7.6-8.0$), ionizing indicators
(e.g., O32 $\simeq 13-18$), among others. Interestingly, Larson et al. (2023)
reported a tentative ($2.5\sigma$) detection of a broad component in H$\beta$
that could be related to AGN activity (the presence of an AGN will be further
discussed in Section 3). Here, we re-analyze the available JWST data of
CEERS-1019.

###  2.1 JWST NIRSpec and NIRCam observations

JWST/NIRSpec spectra are available for CEERS-1019 as part of the Cosmic
Evolution Early Release Science (CEERS111https://ceers.github.io/; Finkelstein
et al. 2023) program. These observations include both low-resolution PRISM and
medium-resolution grating (G140M/F100LP, G235M/F170LP, and G395M/F290LP),
providing spectral resolution of $R\simeq 100$ and $R\simeq 1000$,
respectively, and a spectral coverage $\simeq 1-5\mu$m. Standard 3-shutter
slits and a 3-point nodding pattern were used. The total exposure time for
each medium-resolution grating was 3107 seconds, split into three individual
exposures of 14 groups each. Deeper observations were obtained with the low-
resolution PRISM, with a total exposure time of 6214 seconds. Both PRISM and
medium-resolution observations were obtained with an aperture position angle
$\rm PA\simeq 89.32$ deg (see Figure 1).

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 blue line is the best fit for several UV
emission lines and continuum. X-axis in the bottom and top panels refer to the
observed ($\mu$m) and rest-frame wavelengths (Å), respectively. Bottom right:
NIRSpec/G395M medium-resolution spectrum of CEERS-1019. Bottom left: JWST
NIRCam cutout of CEERS-1019 in the F200W filter. CEERS-1019 is composed of
three resolved clumps. The inferred positions of the NIRSpec MSA shutters are
overlaid in blue.

Data reduction was performed using the official JWST pipeline 222https://jwst-
pipeline.readthedocs.io/ for Level 1 data products and
msaexp333https://github.com/gbrammer/msaexp for Levels 2 and 3. Bias and dark
current are subtracted followed by the correction of the $1/f$ noise and the
“snowball” events. We use the calibration reference data system (CRDS) context
jwst_1063.pmap to correct spectra for flat-field and implement the wavelength
and photometric calibrations. 2D spectra of each slitlet are then drizzle-
combined and the background is subtracted following the three-shutter dither
pattern. Finally, 1D spectra are extracted using the inverse-variance weighted
kernel following Horne (1986). Figure 1 shows the NIRSpec spectra of
CEERS-1019.

CEERS-1019 was also observed with JWST/NIRCam with the F115W, F150W, F200W,
F277W, F356W, F410M, and F444W filters with exposure times of $\sim 3000$
seconds (Finkelstein et al. 2023). NIRCam images were reduced using the grizli
reduction pipeline (Brammer 2023), which includes procedures for masking the
“snowball” artifacts and minimizing the impact of $1/f$ noise. Photometry of
CEERS-1019 is performed using SExtractor (Bertin & Arnouts 1996) in dual mode.
For each NIRCam filter, we use the point-spread functions (PSFs) provided by
G. Brammer within the grizli PSF library,444
https://github.com/gbrammer/grizli-psf-library which are based on models from
webbpsf (Perrin et al. 2014). Images are then PSF-matched to F444W, which has
the largest PSF within the NIRCam filters. We measure the flux of CEERS-1019
in each filter using a circular aperture of $0.16^{\prime\prime}$ radius (4
pix) and apply an aperture correction derived in F444W using the ”FLUX_AUTO”
measured in a Kron-like aperture with default Kron parameters. Then, we scale
all fluxes to total fluxes based on the encircled energy of the circularized
Kron aperture on the F444W PSF from webbpsf (see Weibel et al. in prep. for
more details) As shown in the bottom left panel of Figure 1, CEERS-1019 shows
a complex morphology with three compact clumps.

###  2.2 Emission line measurements

As shown in Figure 1, CEERS-1019 presents intense nebular emission in the
rest-frame UV and optical. As a first step, we determine the systemic redshift
of CEERS-1019 using well-detected ($\geq 10\sigma)$ and uncontaminated (i.e.,
not blended) emission lines detected in the G395M spectrum. Using the
centroids of [Ne iii] $\lambda$3869, H$\gamma$, H$\beta$, and [O iii]
$\lambda\lambda$4959,5007 we derive the mean value and scatter of $z_{\rm
sys}=8.6782\pm 0.0006$.

Several rest-frame UV lines are detected with high significance ($\geq
5\sigma$) in the deep PRISM spectrum, such as N iv] $\lambda$1486555Unless
stated otherwise, N iv] $\lambda$1486 refers to the sum of the forbidden [N
iv] $\lambda 1483$ and the semi-forbidden N iv] $\lambda 1486$ lines, which
are not resolved in the Prism spectrum., C iv $\lambda$1550, O iii]
$\lambda$1666, and C iii] $\lambda$1909\. This contrasts with the shallower
medium-resolution G140M spectrum that shows only Ly$\alpha$ and N iv] at $\geq
3\sigma$. Thus we use the much higher signal-to-noise ratio (S/N) PRISM
spectrum to measure the fluxes of the rest-frame UV lines. We fit
simultaneously several Gaussian profiles to account for the emission of N iv],
C iv, O iii], N iii], and C iii], and a power-law in the form of
$f_{\lambda}\propto\lambda^{\beta}$ to fit the continuum level between
$1.3-2.1\mu$m ($\lambda_{0}\simeq 1300-2200$Å). Since these lines are not
resolved in the PRISM spectrum,666N iv] $\lambda 1486$ presents an observed
line width $\rm FWHM=394\pm 95$ km s-1 in the medium-resolution G140M
spectrum. we fixed the line widths of each line to the expected instrumental
resolution at their corresponding wavelengths ($R\simeq
30-45$)777https://jwst-docs.stsci.edu/jwst-near-infrared-spectrograph/nirspec-
instrumentation/nirspec-dispersers-and-filters. We repeat the fit 500 times
while bootstrapping the spectrum according to its $1\sigma$ error, and
consider the standard deviation of each parameter as its $1\sigma$
uncertainty. Table 1 summarizes our flux measurements. Along with Ly$\alpha$,
N iv] is found to be the strongest emission line in the rest-UV, stronger than
C iv and C iii] by a factor $\simeq 1.8$ and $\simeq 1.5$, respectively. We
also infer a steep UV slope of $\beta_{\rm UV}^{\rm spec}=-2.11\pm 0.09$ from
the spectrum, which is consistent with the photometric one ($\beta_{\rm
UV}^{\rm phot}=-2.11\pm 0.15$) using the apparent magnitudes in the F150W and
F200W filters ($\rm F150W=25.25\pm 0.08$ and $\rm F200W=25.29\pm 0.07$).

Table 1: Flux measurements of CEERS-1019.  Line | Flux | Grism/Prism
---|---|---
| [$\times 10^{-18}$ erg s-1 cm-2] |
Ly$\alpha$ $\lambda 1215$ | $2.38\pm 0.49$ | G140M
N v $\lambda 1240$ |  $<1.44$ (3$\sigma$) | G140M
N iv] $\lambda\lambda 1483,1486$ | $3.75\pm 0.40$ | Prism
C iv $\lambda\lambda 1548,1550$ | $2.10\pm 0.42$ | Prism
He ii $\lambda 1640$ |  $<1.20$ (3$\sigma$) | Prism
O iii] $\lambda\lambda 1661,1666$ | $1.64\pm 0.32$ | Prism
N iii] $\lambda 1750$ | $0.73\pm 0.30$ | Prism
C iii] $\lambda\lambda 1907,1909$ | $2.43\pm 0.36$ | Prism
[O ii] $\lambda\lambda 3727,3729$ | $1.29\pm 0.14$ | G395M
[Ne iii] $\lambda 3869$ | $1.08\pm 0.16$ | G395M
H8+He i $\lambda 3889$ | $0.24\pm 0.08$ | G395M
[Ne iii] $+$ H7 $\lambda 3968$ | $0.60\pm 0.11$ | G395M
H$\delta$ $\lambda 4101$ | $0.34\pm 0.11$ | G395M
H$\gamma$ $\lambda 4340$ | $1.10\pm 0.20$ | G395M
[O iii] $\lambda 4363$ | $0.42\pm 0.12$ | G395M
H$\beta$ $\lambda 4861$ | $2.14\pm 0.22$ | G395M
[O iii] $\lambda 4959$ | $4.50\pm 0.24$ | G395M
[O iii] $\lambda 5007$ | $14.05\pm 0.28$ | G395M

Flux measurements of rest-optical lines are obtained using the G395M spectrum,
which presents a similar depth as the PRISM spectrum but with a much higher
resolution. Optical lines are fitted separately over relatively narrow
spectral windows (100 Å, rest-frame) and a constant is assumed for the
continuum level. The width of the lines is set as a free parameter. In total,
we detect up to ten optical emission lines with high significance (Table 1),
including Balmer lines that are useful for the determination of dust
attenuation.

To account for wavelength-dependent slit losses and absolute flux calibration,
we derive the synthetic photometry of NIRSpec spectra (PRISM and gratings)
through each NIRCam filter bandpass and matched it to that obtained from
observed photometry. In this process, we use a wavelength-dependent polynomial
function yielding scaling factors for the slit-loss correction ranging from
approximately 2.0 (F150W) to 3.6 (F444W).

Using fluxes and equivalent widths of the detected Balmer lines H$\beta$,
H$\gamma$, and H$\delta$, we iteratively derive the dust attenuation
$E(B-V)=0.12\pm 0.11$ using the Reddy et al. (2016) attenuation curve and
following the methodology of Izotov et al. (1994), which accounts for the
internal extinction and underlying hydrogen stellar absorption. Other
important lines, such as those that are sensitive to the electron temperature
($T_{\rm e}$, [O iii]$\lambda 4363$) and density ($n_{e}$, N iv]
$\lambda\lambda 1483,1486$ and [O ii]$\lambda\lambda 3727,3729$) are also
detected and are analyzed in more detail in Section 4. For the N iv] and [O
ii] doublets we fit two Gaussian profiles with similar widths and use the
expected separation between the two transitions. We find line ratios of
$F_{1483}/F_{1486}=0.50\pm 0.22$ and $F_{3727}/F_{3729}=0.98\pm 0.27$ for the
N iv] and [O ii] doublets, respectively.

We also check for the presence of spectral features that are usually
associated with Wolf-Rayet (WR) stars. The so-called blue bump around
4600–4700Å, encompassing the emission from N iii $\lambda 4640$, C iii
$\lambda 4650$, and He ii $\lambda$4686, is detected neither in the G395M nor
the PRISM spectra. We derive a $3\sigma$ upper limit relative to H$\beta$ of
He ii/H$\beta\leq 0.26$. Similarly, the rest-UV He ii $\lambda$1640 line is
not detected. Despite its low resolution, the PRISM spectrum clearly suggests
no emission at the expected position of He ii, while the close O iii] emission
is well detected (see Figure 1).

##  3 The nature of the ionizing source: star formation versus AGN

We now discuss the nature of the ionizing source of CEERS-1019, building upon
the recent findings by Mainali et al. (2018) and Larson et al. (2023), who
suggest a possible AGN activity. In their study, Mainali et al. (2018)
reported the detection of N v $\lambda 1242$ emission with an integrated flux
of $(2.8\pm 0.6)\times 10^{-18}$ erg s-1 cm-2 with a narrow profile $\rm
FWHM<90$ km s-1 (unresolved in the MOSFIRE spectrum). However, the G140M
spectrum does not exhibit any significant emission around the expected
position of N v $\lambda\lambda 1238,1242$ (Figure 2, top left). By
considering the flux uncertainty around $1.2\mu$m from the G140M error
spectrum and assuming an unresolved line width of $\rm FWHM=352$ km s-1, we
infer a $3\sigma$ limit of $1.44\times 10^{-18}$ erg s-1 cm-2. This limit
stands well below the reported value of Mainali et al. (2018). Furthermore,
according to Morton (1991), N v $\lambda 1238$ is expected to be twice as
strong as N v $\lambda 1242$ under standard conditions. Hence, considering the
reported flux of Mainali et al. (2018) for N v $\lambda 1242$, we would expect
$11.6\sigma$ and $5.8\sigma$ detections for N v $\lambda 1238$ and $\lambda
1242$, respectively. These limits, however, are incompatible with our
observations.

Figure 2: Star formation and AGN diagnostics. The top left panel shows the
G140M spectrum of CEERS-1019 (in black and 1$\sigma$ uncertainty in grey)
around the expected positions of N v $\lambda\lambda 1238,1242$ (marked with
vertical lines), which are not detected ($1.44\times 10^{-18}$ erg s-1 cm-2 at
$3\sigma$). Ly$\alpha$ emission is also marked. The top middle and right
panels show the best fits (green) to the H$\beta$ emission line using dual-
component (middle, narrow and broad components in blue and red, respectively)
and single-component (right) Gaussian profiles. The bottom panels show star
formation (blue) and AGN (red) photoionization models using several rest-frame
UV lines. The position of CEERS-1019 (dark blue circle) aligns with the
predictions of star-forming models in all diagnostic diagrams. The location of
other known star-forming galaxies with strong Nitrogen emission, GN-z11
(Bunker et al. 2023), the Lynx arc (Villar-Martín et al. 2004), SMACS2031
(Patrício et al. 2016), and Mrk 996 (Mingozzi et al. 2022) are also marked
with different symbols as indicated in the legend.

Larson et al. (2023) reported a $2.5\sigma$ detection of a broad ($\simeq
1200$ km s-1) component in H$\beta$ using the medium-resolution NIRSpec G395M
spectrum. This broad component is not seen in stronger, forbidden lines like
[O iii] $\lambda\lambda 4960,5008$, from which they suggest conditions similar
to the broad line region (BLR) of an AGN. Using our own reduction of the G395M
spectrum and a dual-component Gaussian profile to H$\beta$, we find a
$2.2\sigma$ detection for the broad component (Figure 2, top middle). Clearly,
deeper observations of H$\beta$ (or H$\alpha$ with MIRI) are needed to
unambiguously confirm the presence and nature of the broad component in
H$\beta$, as already discussed and suggested by Larson et al. (2023). Indeed,
if a single Gaussian profile is used to fit the H$\beta$ profile, a good fit
is also found without penalizing significantly the residuals (Figure 2, top
right). In this case, we find $\rm FWHM(H\beta)=452\pm 68$ km s-1 which
differs only by $1.2\sigma$ from the nominal $\rm FWHM=369\pm 16$ km s-1
obtained for the much brighter [O iii] $\lambda 5008$ line.

If the existence of this broad component can be confirmed and attributed to
the BRL, it would be expected that high-ionization semi-forbidden lines such
as N iv], C iv, or C iii], which probe high-density regimes ($n_{\rm
crit}\gtrsim 10^{9}$ cm-3), would display similar broad Doppler widths as
observed in type-1 AGNs (e.g., Pâris et al. 2011). However, these lines appear
narrow in CEERS-1019, especially N iv] which exhibits a high-significance
detection and an intrinsic $\rm FWHM\simeq 160$ km s-1 after correcting for
instrumental broadening. Thus, our results suggest that the aforementioned
semi-forbidden lines unlikely originate from the broad line region. Instead,
the properties of these lines, such as the narrow widths and the N iv] line
ratio $F_{1483}/F_{1486}=0.50\pm 0.22$ (implying densities $n_{e}\approx
10^{4-5}$ cm-3, see Section 4.3), are consistent with narrow line regions of
AGN or H ii regions. In the following, we discuss these two scenarios.

The lower panels of Figure 2 present several diagnostic diagrams using
different UV nebular lines: C iii]/He ii versus C iv/He ii, [O iii]/He ii, and
N v/N iv]. Photoionization models of star-forming galaxies from Gutkin et al.
(2016) and narrow-line regions of AGN from Feltre et al. (2016) are shown in
blue and red, respectively. In the right panel of Figure 2 we show models of
star-forming galaxies from the updated BOND grid using Cloudy (Ferland et al.
2017), which also includes N iv] and is available from the
3MdB888https://sites.google.com/site/mexicanmillionmodels/ (Morisset et al.
2015). These models encompass a wide range of parameters, including the
ionizing parameter ($-4.0\leq\log U\leq-1.0$), hydrogen number density
($10^{2}\leq n_{\rm H}\rm/cm^{3}\leq 10^{4}$), and the power law index of the
ionizing spectrum ($-2.0\leq\alpha\leq-1.2$). We have selected models with
metallicities within the range $0.05\leq Z/Z_{\odot}\leq 0.20$, which
corresponds to the inferred metallicity for CEERS-1019 (12+log(O/H) $=7.70\pm
0.18$, as indicated in Table 2). As illustrated in this figure, the position
of CEERS-1019 (indicated by the blue circle) aligns with the predictions of
star-forming models in all diagnostic diagrams. Clearly, the absence of He ii
and N v, which probe energies $>54$ eV and $>77$ eV, respectively, places
CEERS-1019 far away from the region occupied by AGN models. It is worth noting
that Isobe et al. (2023) suggested recently that the high N iv]/N iii] ratio
observed in CEERS-1019 is hardly reproduced by star formation models, pointing
to an AGN contribution. However, the 3MdB photoionization models used here do
predict very high ratios even well above the observed N iv]/N iii] $=5.1\pm
2.2$, although requiring fairly high ionization parameters
(log($U)\lesssim-2$).

Other spectral features observed in CEERS-1019, such as the intense N iv]
emission compared to other UV lines (N iv]/C iv $\simeq 1.8$, N iv]/C iii]
$\simeq 1.5$, N iv]/N v $\geq 2.6$), and narrow profiles ($\rm FWHM\simeq 160$
km s-1 for N iv]) differ from those observed in AGNs, even those showing
unusually strong Nitrogen lines (e.g., Bentz et al. 2004; Jiang et al. 2008).
The so-called Nitrogen-loud QSOs exhibit much weaker N iv] compared to other
lines (e.g., N iv]/C iv$\simeq 0.02-0.38$, Batra & Baldwin 2014, Dhanda et al.
2007) and, as expected, they present very broad Doppler widths ($\rm
FWHM\simeq 1500-6000$ km s-1, Jiang et al. 2008). Similarly, some type-2 AGNs
also present N iv] emission (e.g., Hainline et al. 2011; Alexandroff et al.
2013), but notably weaker compared to other high-ionization lines (N iv]/C iv
$\simeq 0.15$, N iv]/C iii] $\simeq 0.34$, or N iv]/N v $\simeq 0.30$;
Hainline et al. 2011). An exception may be GS-14, a type 1.8 AGN at $z\simeq
5.55$ recently analyzed by Übler et al. (2023). GS-14 exhibits broad
components in Hydrogen and Helium lines ($\rm FWHM\simeq 3400$ km s-1, Übler
et al. 2023) as well as narrow N iv] emission ($\rm FWHM\simeq 430$ km s-1,
Vanzella et al. 2010, Barchiesi et al. 2023), but it also shows clear nebular
emission in N v $\lambda 1240$ and O vi $\lambda 1033$ (Grazian et al. 2020;
Barchiesi et al. 2023) which are not detected in CEERS-1019.

In contrast, the spectrum of CEERS-1019 resembles those of other, yet also
rare star-forming galaxies with intense emission in Nitrogen lines. Examples
such as the Lynx arc (Fosbury et al. 2003; Villar-Martín et al. 2004),
SMACS-2031 (Christensen et al. 2012; Patrício et al. 2016), Mrk 996 (James et
al. 2009; Mingozzi et al. 2022), and the Sunburst cluster show narrow and
prominent N iv] and/or [N iii] lines suggestive of high electron temperatures
and densities like CEERS-1019 (see Section 4) and without any hint of AGN
activity. The bottom panels of Figure 2 also show the location of these strong
N-emitters, all consistent with star-forming models like CEERS-1019. The case
of GN-z11, another strong N-emitter reported by Bunker et al. (2023), appears
to be ambiguous, consistent with both models of AGN and star formation, as
already discussed in Bunker et al. (2023) and Maiolino et al. (2023). In
conclusion, our results suggest that, regardless of the presence of an AGN
whose confirmation awaits deeper data, the high-ionization lines observed in
CEERS-1019 are consistent with stellar photoionization.

##  4 Observational and derived physical properties of CEERS-1019

###  4.1 ISM properties and element abundances

The rich set of emission lines detected from the rest-frame UV-to-optical
spectrum allows us to determine the electron temperature and density in the
gas and the detailed abundances of numerous elements including H, C, N, O, and
Ne. The derived quantities are summarized in Table 2.

###  4.2 Electron temperature

To derive physical conditions and element abundances we follow the
prescriptions of Izotov et al. (2006). Briefly, these authors adopt the
classical three zone model of the H ii region with electron temperatures
$T_{\rm e}$(O iii) for the high-ionization zone, and $T_{\rm e}$(O ii) for the
low-ionization zone. The intermediate-ionization zone is not used here, since
no such lines are detected.

The electron temperature $T_{\rm e}$(O iii) is derived both from the ratio of
[O iii] line fluxes $\lambda$4363/$\lambda$(4959+5007) and from the UV-to-
optical line ratio of $\lambda$1660/$\lambda$5007\. The former ratio (rest-
optical) is determined from the medium-resolution spectrum, the latter from
the PRISM spectrum. In both cases we obtain $T_{e}\approx 18000$ K, consistent
within 1 $\sigma$, and with uncertainties between 1151 and 3252 K.
Subsequently, we adopt the electron temperature from the optical line ratios
($T_{e}=18849\pm 3252$ K) with the larger uncertainty, which is primarily due
to the low S/N detection of [O iii]$\lambda 4363$. The electron temperature in
the low-ionization region is derived from relations obtained from the
photoionization models of Izotov et al. (2006).

###  4.3 Electron density

Several density indicators exist in the observed spectral range, but few can
be used here in practice. In the UV, the C iii] $\lambda$1909, Si iii]
$\lambda$1883,1892, and N iv] $\lambda$1486 doublets are density estimators.
However, the PRISM spectrum is of insufficient resolution to resolve any of
these doublet lines. Si iii] $\lambda$1883,1892 is not detected, and C iii]
$\lambda$1909 has too low S/N in the medium-resolution spectrum. Although of
fairly low S/N, the N iv] $\lambda$1486 doublet is detected with a ratio
$\lambda$1483/$\lambda$1487 $=0.50\pm 0.22$ which indicates a fairly high
electron density of $n_{e}\approx 10^{4-5}$ cm-3 (Kewley et al. 2019). In the
optical, the [O ii] $\lambda$3727 doublet is clearly detected, but not
resolved from the medium-resolution spectra. Our measured line ratio
$\lambda$3727/$\lambda$3729 $=0.98\pm 0.23$ is consistent within the
uncertainties with that obtained by Larson et al. (2023) ($0.639\pm 0.255$),
and compatible with $n_{e}>10^{3}$ cm-3 (Kewley et al. 2019).

Figure 3: Best fit (blue) of density-sensitive emission lines, N iv]
$\lambda\lambda 1483,1486$ (left) and [O ii] $\lambda\lambda 3727,3729$
(right), using G140M and G395M medium-resolution spectra (black and $1\sigma$
uncertainty in grey), respectively. The fit uses two Gaussian profiles with
similar widths and the expected position and separation between the two
transitions (vertical lines).

The two density estimates could indicate a density gradient between the low
and high ionization regions, but are also compatible with a single, relatively
high density of $n_{e}\approx 10^{4-5}$ cm-3, whose origin we discuss below.
In any case, the most important point to take away from this is that the
electron density, although high, is lower than the critical densities of all
the relevant emission lines used for the subsequent abundance determinations.
This holds for the (semi-)forbidden lines of [O iii] at 1666, 4363, 4959, 5007
(with critical densities $n_{\rm crit}\geq 6.9\times 10^{5}$ cm-3), the two
components of the C iii] $\lambda$1909 doublet ($n_{\rm crit}=8.7\times
10^{4}$ cm-3 for 1907 and $10^{9}$ cm-3 for 1909), C iv $\lambda$1550 ($n_{\rm
crit}=2\times 10^{15}$ cm-3), N iii] $\lambda$1750 (a multiplet whose
components have $n_{\rm crit}\geq 10^{9}$ cm-3), N iv] $\lambda$1486 ($n_{\rm
crit}=3\times 10^{9}$ cm-3), and [Ne iii] $\lambda$3869 ($n_{\rm crit}=1\times
10^{8}$ cm-3) (see e.g. Hamann et al. 2002; Dere et al. 2019). Only the [O ii]
$\lambda$3727 doublet, whose components have relatively low critical densities
of $n_{\rm crit}=1(4)\times 10^{3}$ cm-3 for 3728 (3726), is therefore
affected by the high density inferred for CEERS-1019, whereas all other lines
can safely be used to determine abundances, to which we now proceed.

Table 2: ISM properties, ionic and total heavy element abundances  Property | Quantity
---|---
$n_{e}$ [cm-3] | $10^{4}-10^{5}$
$T_{\rm e}$(O iii) – UV [K] | $17839\pm 1151$
$T_{\rm e}$(O iii) – opt [K] | $18849\pm 3252$
$T_{\rm e}$(O ii) [K] | $14937\pm 944$
12+log(O+/H+) | $6.73\pm 0.14$
12+log(O2+/H+) | $7.68\pm 0.18$
12+log(O/H) | $7.70\pm 0.18$
log((N2+\+ N3+)/O2+) – UV only (V+04) | $-0.13\pm 0.11$
log((N2+\+ N3+)/O2+) – UV only (H+02) | $-0.16\pm 0.17$
log((N2++N3+)/O2+) – UV+opt | $-0.18\pm 0.28$
log((C2+\+ C3+)/O2+) – UV only (V+04) | $-0.75\pm 0.11$
log((C2+\+ C3+)/O) – UV+opt (V+04) | $-0.79\pm 0.22$
log((C2+\+ C3+)/O2+) – UV only (PM17) | $-0.76\pm 0.11$
log(C2+/O2+) – UV only (I+23) | $-0.92\pm 0.12$
log(C2+/O2+) – UV+opt only (I+23) | $-0.84\pm 0.22$
ICF(C2+/O2+) = 1.1 |
ICF(Ne2+/O2+) | 1.04
log(Ne/O) | $-0.63\pm 0.07$

###  4.4 Ionic and total metal abundances

The electron temperature $T_{\rm e}$(O iii) is used to obtain abundances of
ions O2+, N3+, N2+, C3+, C2+, and Ne2+; the temperature in the low-ionization
region, $T_{\rm e}$(O ii), to derive the ionic abundance of O+. Ionic
abundances are derived following Izotov et al. (2006) for the optical lines,
and comparing different methods for the UV lines. For C, N, and O, the
observations provide two ionization stages, hence the ionic abundances will be
close to the total abundances, and we neglect further ionization corrections.
For Ne2+ we use the ionization correction factor (ICF) following Izotov et al.
(2006). The results are listed in Table 2.

We derive a total oxygen abundance of $12+\log({\rm O/H})=7.70\pm 0.18$, which
is dominated by the ionic abundance of O2+/H+ (see Table 2). Given the high
density, [O ii] $\lambda$3727 could be decreased and hence the O+/H+ abundance
underestimated. However, in view of the high excitation observed from lines
with high critical densities, it is likely that O2+ is the dominant ionization
stage over the majority of the Hii region and hence the determination of O/H
close to the correct value.

With available line detections the N/O abundance can be determined in
different ways. First we use only the UV lines to compute the ionic abundance
ratio (N2++N3+)/O2+ using the expressions from Villar-Martín et al. (2004)
(V+04) and Hamann et al. (2002) (H+02), assuming the low-density regime. Then
we determine N/H from the UV and optical line ratio (N and H$\beta$) and use
O/H determined from the optical lines. Both methods, marked as ”UV only” and
”UV+opt” respectively, yield values compatible within the errors, and
consistent with a high N/O abundance $\log{(\rm N/O)}\approx-0.15\pm 0.17$.

Similarly, for C/O we use the expressions from Villar-Martín et al. (2004),
Pérez-Montero & Amorín (2017) (PM17, and Izotov et al. (2023) (I+23) using
either only the rest-UV or a combination of the UV and optical lines. As seen
from Table 2 the ionic abundance ratios derived in this manner are compatible
within uncertainties. For the total C/O abundance we adopt $\log({\rm
C/O})=-0.75\pm 0.11$ as our default value. The C/O ratio is therefore clearly
subsolar, and in fact very similar to the average of normal star-forming
galaxies at the same O/H (see below).

Finally we also derive the Neon abundance from the [Ne iii] $\lambda$3869 and
H$\beta$ lines and applying an ICF from the oxygen lines, following Izotov et
al. (2006). We find an abundance ratio of $\log({\rm Ne/O})=-0.63\pm 0.07$,
somewhat higher than the average value of $\log({\rm Ne/O})=-0.78\pm 0.01$
determined for normal star-forming galaxies by Guseva et al. (2011) at the
same metallicity.

Although the abundances derived here assume low densities they are not altered
by density effects at the density derived for CEERS-1019, as already discussed
above. Most importantly, the critical densities for the N iii] $\lambda$1750,
N iv] $\lambda$1486, and O iii] $\lambda$1666 lines involved in the
(N2++N3+)/O2+ ratio derived from the UV are all very high ($n_{\rm
crit}>10^{9}$ cm-3), which further shows that this important ionic abundance
ratio can be determined accurately.

Taken together, the derived abundances of CEERS-1019 show that this object has
a “metallicity” (O/H) of approximately 1/10 solar (asssuming a solar value of
$12+\log({\rm O/H})$=8.69 Asplund et al. 2009, 2021), an exceptionally high
N/O abundance, and a normal C/O abundance, when compared to galaxies of
similar metallicity (see Fig. 5). The interpretation of these abundances and
implications will be discussed below (Sect. 5).

###  4.5 Comparison with other studies and caveats

ISM properties and abundances of CEERS-1019 have been determined by several
other studies, with whom we now briefly compared our results.

Larson et al. (2023) argue that the [O ii] $\lambda$3727 doublet can be
deblended, from which they infer an electron density of $n_{e}=(1.9\pm
0.2)\times 10^{3}$ cm-3. From inspection of the C iii] $\lambda$1909 doublet
they suggest that the density could be higher than $n_{e}>10^{4}$ cm-3. The
density inferred here from the N iv] $\lambda$1486 doublet ($n_{e}\approx
10^{4}-10^{5}$ cm-3) is compatible with their finding. Most importantly for
the abundance determinations, all available density estimates indicate that
the main emission lines should not be affected by density effects.

From their 3-$\sigma$ detection of [O iii]$\lambda 4363$ Larson et al. (2023)
inferred $T_{e}=18630\pm 3682$ K, in excellent agreement with our
determination. Based on the $T_{e}$ determination they infer $12+\log({\rm
O/H})=7.664\pm 0.508$ from an average relation between $T_{e}$ and O/H
determined empirically by Pérez-Montero & Amorín (2017). Tang et al. (2023)
determined $12+\log({\rm O/H})=7.72^{+0.17}_{-0.14}$ using the direct method.
Within the quoted uncertainties, our results agree with both of these
determinations. A slightly higher O/H abundance ($12+\log({\rm O/H})=7.97\pm
0.16$), but still compatible with the uncertainties, has been derived by
Nakajima et al. (2023) using a less accurate R23 strong-line calibration.
Finally, assuming AGN models, Isobe et al. (2023) have obtained a higher
metallicity for CEERS-1019, but similar N/O, C/O, and Ne/O ratios as derived
here.

Note also that the abundance ratios determined here assume a homogeneous
medium both in abundance and density. If pockets of high density and enriched
gas coexist with lower density gas with say normal abundance ratios, only a
relatively small fraction of enriched gas – i.e. relatively low amounts of
Nitrogen – might suffice to explain the observed emission line ratios, since
the emissivity of the forbidden line depends on the density (see e.g Izotov et
al. 2006). However, in this case the inferred N/O abundance would also be
lower limit of the true N/O ratio in the enriched pocket.

Figure 4: Left panel shows cutout images around CEERS-1019 in the NIRCam
filters. In the F150W sub-panel, we show the positions of the three compact
clumps resolved only at short wavelength, labeled as A, B, and C (blue, green,
and red, respectively) The right panel shows the SED best-fit models using
CIGALE (Boquien et al. 2019) of the integrated light of CEERS-1019 (”Total” in
black), as well as the individual clumps (A, B, and C in blue, green, and red,
respectively). Observed fluxes are marked with circles, while the predicted
fluxes from the best fit are marked with crosses.

###  4.6 Other physical properties

####  4.6.1 Morphology

As shown in the left panel of Figure 4 CEERS-1019 shows a complex morphology
in the NIRCam bands consistent with three different clumps/structures
separated by $\simeq 0.24^{\prime\prime}$, or $\simeq 1.12$ kpc at $z=8.678$
($4.68^{\prime\prime}$ kpc-1). These clumps, labeled as A, B, and C as
indicated in Figure 4, are very compact, only resolved in the NIRCam bands at
short wavelengths.

To investigate the morphology of CEERS-1019 in more detail, we model the three
galaxy substructures following accurately the methodology applied to the study
of stellar clumps in Messa et al. (2022) and Claeyssens et al. (2023).
Assuming that clumps have Gaussian profiles, we consider a 15$\times$15 pixel
region centered on the galaxy and we fit a model consisting of three 2D
Gaussian functions, convolved to the NIRCam instrumental PSF in this field
from the grizly library. The best fit to their observed profiles (given by
least-squares minimization) returns their fluxes and sizes. We assume that the
shape of each substructure is the same in all bands. For this reason, the fit
is initially performed in F200W, chosen as the reference filter, and then the
shape (size, axis ratio, and position angle) of each clump is kept fixed in
the other filters, where only the source flux is fitted. Uncertainties are
obtained from Monte Carlo sampling.

The results of the model analysis are presented in Table 3. Our findings
indicate that the morphologies of the three clumps in CEERS-1019 are compact,
with measured FWHMs of $48\pm 5$ mas, $62\pm 15$ mas, and $43\pm 4$ mas for
clumps A, B, and C, respectively. Following Peng et al. (2010) (see also:
Vanzella et al. 2017, Messa et al. 2022, and Claeyssens et al. 2023), the
inferred FWHM suggest that these clumps are resolved, albeit slightly, as
their sizes are larger than the pixel size of the NIRCam images (40 mas).
Translating these measurements into half-light radii, we find $r_{\rm
e}=112\pm 12$ pc, $145\pm 35$ pc, and $101\pm 9$ pc for clumps A, B, and C,
respectively.

Table 3: SED and morphological properties of the different substructures of
CEERS-1019.

ID | R.A. | Dec. | $r_{\rm eff}$ | Age | SFR10Myr | log($M_{\star}$) | log($\Sigma_{M}$) | log($\Sigma_{{\rm SFR}}$)
---|---|---|---|---|---|---|---|---
| [J2000] | [J2000] | [pc] | [Myr] | [$M_{\odot}$ yr-1] | [$M_{\odot}$] | [$M_{\odot}$ pc-2] | [M⊙ yr-1kpc-2]
(1) | (2) | (2) | (3) | (4) | (5) | (6) | (7) | (8)
A | 14:20:08.50 |  $+$52:53:26.37 | $112\pm 12$ | $4.0\pm 0.3$ | $148\pm 25$ | $8.76\pm 0.04$ | $3.86\pm 0.11$ | $3.27\pm 0.11$
B | 14:20:08.51 |  $+$52:53:26.51 | $145\pm 35$ | $5.7\pm 0.7$ | $83\pm 18$ | $8.66\pm 0.15$ | $3.55\pm 0.53$ | $2.81\pm 0.21$
C | 14:20:08.48 |  $+$52:53:26.36 | $101\pm 9$ | $15.0\pm 3.0$ | $2\pm 10$ | $8.94\pm 0.12$ | $4.14\pm 0.14$ | $<2.27$
Total | — | — | — | $14.4\pm 7.2$ | $161\pm 23$ | $9.31\pm 0.15$ | — | —

Notes. — (1) different components of CEERS-1019, (2) J2000 coordinates, (3)
the effective radius, (4) burst age for components A, B, and C, and age
assuming CSF for ”total”, (5) 10 Myr-weighted star-formation rate, (6) and (7)
stellar mass and mass surface density, defined as $\Sigma_{M}=M_{\star}/(2\pi
r_{\rm eff}^{2})$, and (8) SFR surface density.

####  4.6.2 Spectral Energy Distribution

We now analyze the spectral energy distributions (SEDs) of CEERS-1019 as a
whole (named Total) as well as its sub-components (A, B, and C). We use the
SED-fitting code CIGALE (Boquien et al. 2019, version 2022.1) using the
available NIRCam photometry from F115W to F444W, covering the rest-frame
wavelength $\sim 1200-4600$Å. Stellar population models from Bruzual & Charlot
(2003) are used along with the Chabrier (2003) Initial Mass Function (IMF) and
the Small Magellanic Cloud extinction curve ($R_{v}=2.93$, Pei 1992). The
metallicity is fixed to $Z=0.004$, the closest available value inferred for
CEERS-1019, and is assumed to be the same for nebular emission and starlight.
The dust attenuation ($E(B-V)$) and ionization paramater (log($U$)) are
treated as free parameters, ranging from $0.0-0.5$ mag and $-3.5$ to $-1.0$,
respectively. Finally, we explore two different star-formation histories: a
constant star-formation model applied to the integrated light of CEERS-1019
(Total) and instantaneous burst episodes for the three sub-components (A, B,
and C). For the former, we include the flux measurements of the H$\beta$ \+ [O
iii] $\lambda\lambda 4960,5008$ emission lines in the fitting process.

Starting with the integrated emission of CEERS-1019 (Total), the best-fit
model, shown in black in the right panel of Figure 4, finds a continuous star-
formation rate SFR$=161\pm 23$ M⊙ yr-1 over $14\pm 7$ Myr. The stellar mass is
$M_{\star}^{\rm total}/M_{\odot}=(2.0\pm 0.6)\times 10^{9}$ attenuated by
$E(B-V)=0.17\pm 0.02$, in agreement with the values reported in Larson et al.
(2023). For the three individual components A, B, and C we find burst masses
of $M_{\star}^{\rm A}/M_{\odot}=(5.7\pm 0.5)\times 10^{8}$, $M_{\star}^{\rm
B}/M_{\odot}=(4.6\pm 0.1)\times 10^{8}$, and $M_{\star}^{\rm
C}/M_{\odot}=(8.6\pm 0.2)\times 10^{8}$, respectively. Clumps A and B are
well-fitted with very young burst models, having ages of $4.0\pm 0.26$ Myr and
$5.6\pm 0.7$ Myr, respectively. On the other hand, clump C is older than the
other components, with a burst age of $15.0\pm 2.9$ Myr. Indeed, the color
obtained for clump C $\rm F356W-F444W=0.32\pm 0.29$ is significantly lower
than those measured in clumps A and B, $\rm F356W-F444W\simeq 0.75-1.16$,
suggesting a weak contribution of nebular emission in F444W (e.g., H$\beta$
and [O iii]), thus negligible star formation over the last $\lesssim 10$ Myr.

####  4.6.3 Stellar mass and SFR surface densities

Based on the stellar masses and half-light radii obtained for the individual
clumps (Table 3), we obtained high stellar mass surface densities of
log($\Sigma_{M})=3.86\pm 0.11$, $3.55\pm 0.53$, and $4.14\pm 0.14$ ${\rm
M_{\odot}pc^{-2}}$ for clumps A, B, and C, respectively (defined as
$\Sigma_{M}=M_{\star}/(2\pi r_{\rm eff}^{2})$). It is worth noting that the
inferred values of $\Sigma_{M}$ may even be higher if each substructure
comprises multiple unresolved stellar systems. Nevertheless, these values are
already comparable to the densest systems identified at high redshift by
Claeyssens et al. (2023) or Meštrić et al. (2022), and significantly higher
than the average log($\Sigma_{M})\simeq 2$ ${\rm M_{\odot}pc^{-2}}$ observed
in nearby young clusters (Brown & Gnedin 2021). Similarly, the compactness
index, defined as $C_{5}=(M_{\star}/10^{5}M_{\odot})(r_{\rm eff}$/pc${}^{-1})$
is also high in the case of CEERS-1019. It ranges from $C_{5}\simeq 30-90$
depending on the clump, exceeding the values of old globular clusters and
young massive clusters by at least one order of magnitude (Krause et al.
2016), suggesting high cluster formation efficiencies (Krause et al. 2016;
Kruijssen 2012). The SFR surface density is also found to be very high for
clumps A and B with log($\Sigma_{\rm SFR})=3.27\pm 0.11$ and $2.81\pm 0.21$ M⊙
yr-1kpc-2, respectively. In contrast, clump C does not show significant star
formation over the last 10 Myr, yielding an upper limit of log($\Sigma_{\rm
SFR})<2.27$ M⊙ yr-1kpc-2.

Finally, the derived mass and SFR surface densities in CEERS-1019 are
comparable with those of other prominent N-emitters discussed below, such as
GN-z11 (log($\Sigma_{M})\sim 4.6$ ${\rm M_{\odot}pc^{-2}}$, Tacchella et al.
2023), SMACSJ2031 (log($\Sigma_{M})\sim 4.0$ ${\rm M_{\odot}pc^{-2}}$,
log($\Sigma_{\rm SFR})\sim 1.4$ M⊙ yr-1kpc-2, Patrício et al. 2016), the
Sunburst cluster (log($\Sigma_{M})\sim 4.1$ ${\rm M_{\odot}pc^{-2}}$,
log($\Sigma_{\rm SFR})\sim 3.7$ M⊙ yr-1kpc-2, Vanzella et al. 2022), or Mrk
996 (log($\Sigma_{M})\sim 2.8$ ${\rm M_{\odot}pc^{-2}}$, Thuan et al. 1996).
This suggests a potential connection between compactness and a high production
efficiency of nitrogen.

###  4.7 Mass of the enriched material

The total mass of enriched, ionized gas, which is directly observable, can
easily be estimated assuming ionization equilibrium and a constant ISM density
(see, e.g., Dopita & Sutherland 2003):

$M_{\rm ionized}=\frac{m_{p}Q_{H}}{\alpha_{B}n_{e}}=2.5\times
10^{6}\left(\frac{10^{3}}{n_{e}}\right)\left(\frac{Q_{H}}{10^{54}}\right)M_{\odot},$
(1)

where $Q_{H}$ is the ionizing photon production rate which can be determined
from H recombination lines, $n_{e}$ the electron density, $m_{p}$ the proton
mass, and $\alpha_{B}$ the recombination rate coefficient.

For CEERS-1019 we thus find $M_{\rm ionized}\sim 1.2\times 10^{5}$ M⊙, from
the observed H$\beta$ luminosity and adopting $n_{e}=10^{5}$ cm-3, very
similar to $M_{\rm ionized}\sim 2\times 10^{5}$ M⊙ inferred for GN-z11 by
Charbonnel et al. (2023). Maiolino et al. (2023) argue that the amount of
enriched gas in GN-z11 could be even smaller if the N-emitting gas is found at
higher densities, as they suggest.

Figure 5: Observed chemical abundances of the six N-emitters and comparison
samples from the literature. Top: N/O versus O/H, bottom: C/O versus O/H.
CEERS-1019 is shown by a red star, GN-z11 by a blue circle. The $z\sim 2.6-3$
lensed galaxies (Lynx arc, SMACS2023 and the Sunburst cluster) are shown by
red, black, and orange crosses, the low-$z$ galaxy Mrk 996 with a green cross
(two N/O values from James et al. (2009) are shown: for the central region and
from the total galaxy). The magenta shaded region and outlined box indicate
the range of abundances allowed for GN-z11, according to Cameron et al.
(2023a). Low-$z$ star-forming galaxies and Hii regions from the compilation of
Izotov et al. (2023) are shown by small black symbols. The dash-dotted line
shows the average trend observed in low-$z$ star-forming galaxies, as
parametrized by Vila-Costas & Edmunds (1993) for N/O and C/O by Dopita et al.
(2006) respectively.

##  5 Discussion

###  5.1 Observed heavy element abundances in CEERS-1019 comparison to
“normal” objects

The main elemental abundance ratios derived for CEERS-1019 are shown in Fig.
5, and compared to measurements in other galaxies and Hii regions. To do so we
use in particular the recent CNO abundances determined and compiled by Izotov
et al. (2023), who primarily included data from low-redshift star-forming
galaxies observed with HST/COS, and data on individual Hii regions from the
works of Esteban et al. (2002, 2009, 2014), García-Rojas & Esteban (2007), and
López-Sánchez et al. (2007).

As well known, the majority of galaxies and Hii regions follow a fairly well-
defined sequence of N/O versus O/H and C/O versus O/H (e.g. Garnett et al.
1999; Berg et al. 2019), which can be understood with chemical evolution
models (Henry et al. 2000; Chiappini et al. 2006; Prantzos et al. 2018). In
N/O, for example, only few strong outliers with a large nitrogen excess are
known at low redshift (see e.g. Thuan et al. 1996; Pustilnik et al. 2004;
Stephenson et al. 2023). In comparison, CEERS-1019 clearly stands out by
having an extremely high Nitrogen abundance, $\log({\rm N/O})=-0.13\pm 0.11$,
which is approximately 5.6 times the solar ratio (Asplund et al. 2021) and
more than a factor 10 higher than the N/O values generally observed at similar
metallicities (O/H). This exceptionally high N abundance reflects the very
peculiar UV spectrum of CEERS-1019, showing unusually strong Nitrogen lines.

In contrast to N/O, with $\log({\rm C/O})=-0.75\pm 0.11$, the C/O abundance is
fairly normal for the observed metallicity. The Ne/O abundance, $\log({\rm
Ne/O})=-0.63\pm 0.07$ is somewhat higher (by $\sim 0.15$ dex) than the average
value for normal star-forming galaxies derived by Guseva et al. (2011) at the
same metallicity.

Interestingly, these observed abundance ratios of CEERS-1019 resemble those of
globular cluster stars, similarly to what was pointed out by Senchyna et al.
(2023) and Charbonnel et al. (2023) for GN-z11. The origin of these peculiar
abundances ratios will be discussed below.

###  5.2 Abundances in other N-emitters

Interestingly, the abundance ratios found in CEERS-1019 resemble those found
by Cameron et al. (2023a) for the $z=10.6$ galaxy GN-z11 observed recently
with JWST by Bunker et al. (2023), which are shown by boxes in Fig. 5. As
shown, the abundances in GN-z11 suffer from large uncertainties, which are in
particular due to the fact that the [O iii]$\lambda 5007$ line is shifted
beyond the range accessible with NIRSpec and no direct O/H abundance
determination is possible for this object from the present data. Using
photoionization modeling, Senchyna et al. (2023) have further constrained the
abundances in GN-z11, obtaining total gas abundances of $12+\log({\rm
O/H})=7.84\pm 0.06$ and $\log({\rm N/O})=-0.38\pm 0.05$, which are quite
similar to those obtained here for CEERS-1019. Clearly, both CEERS-1019 and
GN-z11 are significantly enriched in Nitrogen, reaching exceptionally high N/O
values. The carbon abundance cannot be well constrained in GN-z11, since the
electron temperature remains undetermined in this object. The allowed range,
derived by Cameron et al. (2023a), is indicated in Fig. 5.

Very few other galaxies or Hii regions with a high N/O abundance and/or clear
detections of nebular lines of N in the UV can be found in the literature.
Barchiesi et al. (2023) list known AGN and galaxies with O vi, N v, or N iv]
$\lambda$1486 emission lines in the rest-UV. Among the non-AGN in their list
one finds the peculiar galaxy named the Lynx arc (at $z=3.36$), which has been
studied by Fosbury et al. (2003) and Villar-Martín et al. (2004), although
Binette et al. (2003) have argued that this object may be an obscured QSO.
According to the photoionization models of Villar-Martín et al. (2004), both
the N/O and C/O abundance ratios of this object are elevated, as seen in Fig.
5. Although suspected, no direct signs of WR stars have been found in this
object (Villar-Martín et al. 2004) and the inferred abundances are not
explained.

Another object showing nebular N iv] $\lambda$1486 emission is the strongly
lensed galaxy SMACSJ2031.8-4036 at $z=3.5$ studied in detail by Christensen et
al. (2012) and Patrício et al. (2016). The available VLT observations (with
XShooter and MUSE) cover both the rest-UV and optical domain, allowing the
detection of numerous emission lines, and thus electron temperature, density
and abundance determinations. Interestingly, this object shows indications for
high density ($n_{e}\ga 10^{5}$ cm-3) from the N iv] $\lambda$1486 doublet and
lower densities from other diagnostics Patrício et al. (2016). The metallicity
$12+\log({\rm O/H})=7.76\pm 0.03$ is very similar to CEERS-1019 and it shows a
normal C/O abundance ($\log({\rm C/O})=-0.80\pm 0.09$), according to
Christensen et al. (2012). Inspection of their spectra, kindly provided by the
authors, shows a clear detection of both N iv] $\lambda$1486 and N iii]
$\lambda$1750 lines, which allows us to determine N/O from the UV lines and
the reported $T_{e}$ using the same methods described above (see Sect. 4.4).
We find a relatively high N abundance of $\log({\rm N/O})=-0.66\pm 0.1$, which
we also report in Fig. 5. Finally, we also find a normal Neon abundance of
$\log({\rm Ne/O})=-0.82$ from the reported line fluxes.

In the list of Barchiesi et al. (2023) other non-AGN spectra showing UV lines
of Nitrogen show only N v P-Cygni lines, which are most likely due to stellar
emission, or are stacked spectra with weak detections, not suitable for our
purpose.

Another high-redshift object where N iii] $\lambda$1750 emission has recently
been detected is the strongly lensed and multiply imaged stellar cluster at
$z=2.368$ in the Sunburst arc (Meštrić et al. 2023), an exceptional object
studied in depth by various authors (e.g. Rivera-Thorsen et al. 2019; Vanzella
et al. 2020). From a detailed analysis and photoionization modelling, Pascale
et al. (2023) infer in particular a high N/O abundance ratio ($\log{\rm
N/O}=-0.21^{+0.10}_{-0.11}$), and normal C/O and Ne/O ratios for a metallicity
(O/H) of approximately $\sim 0.22$ solar. The N/O ratio of this object fares
thus among the highest values, comparable to CEERS-1019, and C/O is also
similar, as also shown in Fig. 5.

To extend our comparison, we have also examined the low-redshift galaxy Mrk
996, which is a well-known Blue Compact Dwarf (BCD) galaxy with peculiar
properties, such as a high electron density, broad emission line components in
H$\alpha$, [O iii] $\lambda\lambda$4959,5007 and other lines, the presence of
Wolf-Rayet stars of WN and WC type, and a high N/O abundance (see e.g. Thuan
et al. 1996; Pustilnik et al. 2004; James et al. 2009; Telles et al. 2014).
This galaxy also shows N iii] and N iv] emission lines in the UV (Mingozzi et
al. 2022; Senchyna et al. 2023). From integral-field observations James et al.
(2009) have found a normal N abundance ($\log({\rm N/O})\approx-1.43$) across
the galaxy and a N-enhancement by a factor $\sim 20$ ($\log({\rm
N/O})\approx-0.13$) in the broad line component, emitted in the central
region. The two measurements are plotted in Fig. 5. The C/O abundance of Mrk
996 can be derived from the C iii] $\lambda$1909 and O iii] $\lambda$1666 line
ratio, which is taken from the HST/COS observations from the CLASSY survey
(Berg et al. 2022; Mingozzi et al. 2022), and adopting the electron
temperature $T_{e}=10^{4}$ K from James et al. (2009). We find a high Carbon
abundance of $\log({\rm C/O})=-0.22$, close to solar, for this galaxy.
However, for its metallicity ($\sim 0.5\times$ solar, according to James et
al. 2009) the C/O abundance ratio is comparable to that of other galaxies and
Hii regions, hence not unusual.

Taken together we thus conclude that all of the six N-emitters show an
elevated (supersolar) N/O abundance ratio, whereas the C/O abundance is normal
in four of them, and only one of them (the Lynx arc) appears enhanced in C/O.
The observed and other properties of these objects are also summarized in
Table 4. We will now discuss possible scenarios to explain to observed
abundance pattern.

###  5.3 Possible chemical enrichment scenarios

Galactic chemical evolution models are able to reproduce the observed average
trends of the abundance ratios of CNO and H for “normal” galaxies (see e.g.
Henry et al. 2000; Chiappini et al. 2006; Berg et al. 2019; Johnson et al.
2023), although the evolution of Nitrogen has notoriously been more
complicated to explain, since the observations show a behaviour like a primary
element at low (subsolar) metallicity (cf. discussion and references in
Prantzos et al. 2018). To examine the conditions which may be more appropriate
for low metallicity dwarf galaxies and Hii regions, which dominate the current
samples of extra-galactic CNO measurements in galaxies (the samples shown
here), various authors have studied the effects of variable or bursty star-
formation histories, outflows and different star-formation efficiencies.
Again, such models are able to reproduce the average trends of C/O, N/O and
C/N as a function of metallicity and they can also explain the observed
scatter in the data, e.g. by the presence of burst phases (see Berg et al.
2019, for a recent study).

Since the observed abundance ratios of CEERS-1019 and possibly other
N-emitters are, however, clearly more extreme than those of the bulk of
galaxies studied so far, we need to examine the possible nucleosynthetic
sources and the conditions capable to explain them. To do so, we first
consider two quantitative scenarios, the first involving enrichment from
normal massive stars, and the second nucleosynthesis from super-massive stars.
These scenarii were considered in recent studies (e.g. Charbonnel et al. 2023;
Nagele & Umeda 2023; Watanabe et al. 2023).

####  5.3.1 Enrichment from massive stars – “WR-scenario”

It is well-known that the stellar winds of massive stars can carry important
amounts of newly-created elements such as He and N (from H-burning, the latter
resulting at the expense of C and O) or C and O (from He-burning); those
elements appear at the stellar surfaces and are ejected by the winds during
the so-called Wolf-Rayet (WR) phases, with N enhanced in the WN phase and C
enhanced in the subsequent WC phase (Maeder 1983). The stellar wind yields
depend strongly on the initial mass and metallicity of the stars, and also on
other properties such as stellar rotation and the efficiency of mixing in the
stellar interiors, or their evolution in close binary systems (e.g. Georgy et
al. 2012; Szécsi et al. 2015; Pauli et al. 2022).

Using the recent stellar yields from Limongi & Chieffi (2018) we have computed
the cumulative stellar wind yields of a simple stellar population as a
function of time, for a Kroupa (2002) IMF, three different metallicities
([Fe/H]$=-2$, $-1$ and 0, respectively) and three different initial rotational
velocities (V${}_{\rm Rot}=0$, 150 and 300 km/s, respectively). The latter
value of V${}_{\rm Rot}=$300 km/s was adopted in Charbonnel et al. (2023) to
discuss the observations of GN-z11. Assuming that stars more massive than
20–25 M⊙ do not explode but collapse and become black holes (see dicussion in
Prantzos et al. 2018), the stellar ejecta have exclusively a wind composition
for several million years. In the first couple of Myr, that composition is the
original one of the stellar envelope, then it is dominated by H-burning
products and subsequently, by He-burning products. To compare with observed
abundance ratios Charbonnel et al. (2023) assumed a dilution of the wind
ejecta with an equal amount of ISM. Here we assume no such mixing, thus
maximizing the effect of the stellar winds on the composition. Physically,
this may correspond to the situation where the winds of the previous O-star
phase, operating for a few Myr, have opened a cavity in the ISM where the
winds of the subsequent WR phase are expanding. Actually, there is mixture
with pristine ISM material, since we include the winds released by all stars
above 12 M⊙ and in the considered period of 8 Myr the stars less massive than
20 M⊙ do not reach the WR phase.

In Fig. 6 we display the evolution of various quantities of the ”WR scenario”
for stars of [Fe/H]$=-1$, a value reasonably close to the metallicity of the
extragalactic systems studied here. Results are shown up to 8 Myr after the
formation of a stellar population of total mass $10^{8}$ M⊙with a normal IMF
(Kroupa 2002). During that period, stars below 25 M⊙ have not yet ended their
lives (by assumption), so that only the wind ejecta populate the cavity
crafted by the winds and the radiation of the stars. The mass of the wind
ejecta increases steadily, from $\sim$104 M⊙ after the first Myr to $\sim$106
M⊙ at 4 Myr and more slowly after that. In Sec. 4.7 we discussed the amounts
of ionized gas estimated in CEERS-1019 and GN-z11, which are compatible with
the model results for this earliest period after the starburst (horizontal
dashed lines in the top panel).

The evolution of the wind composition differs between the non-rotating and the
rotating stars. The former (solid red curves) have practically no mixing
between their convective core and radiative envelope; in consequence, the
signatures of H-burning (high N/O and N/C) appear abruptly in the wind, once
the mass loss uncovers the former H-burning core. The latter (solid blue
curves) undergo rotational mixing, bringing slowly the H-burning products to
the surface; as a result, the N/O and N/C ratios increase slowly but steadily,
up to the equilibrium value, which is similar to the case of non-rotating
stars. The timescale for the appearance of high N abundance is $\sim 3$ Myr,
in good agreement with the time window inferred by Senchyna et al. (2023) for
GN-z11. About a Myr later, some amounts of He and He-burning products – mainly
C and insignificant O amounts – appear in the wind ejecta of the most massive
rotating stars (from 120 to $\sim 70$ M⊙) while the less massive ones never
reach the WC phase; the combined effect is a strong increase of C/O, a strong
decrease of N/C and a small variation of N/O. In contrast, none of the non-
rotating stars reaches the WC phase at such low metallicity, and all the CNO
ratios remain basically unchanged. After that, the situation is expected to
change drastically, as the first SN from M$<$25 M⊙ stars explode and eject
their core material in the ISM.

Figure 6: Evolution of IMF-weighted time-integrated masses and abundances of
the winds of a stellar population of total mass 108 M⊙created at t=0,
according to the models of Limongi & Chieffi (2018) with metallicity [Fe/H]=-1
and initial rotational velocity VROT=300 km/s (solid blue curves) or 0 km/s
(solid red curves) in all panels; practically no dilution with ambient ISM is
assumed (99% of ejecta and 1% of ISM). Comparison is made to abundance data
from CEERS-1019 (this work, orange shaded), Lynx arc (green shaded), GN-z11
(Senchyna et al. 2023, violet shaded with age dermination). The two horizontal
dashed lines in the top panel indicate the estimated mass of ionized gas
observed in CEERS-1019 and GN-z11, respectively (see Sec. 4.7). The yellow
shaded area indicates the short period ($\sim$0.5 Myr) where all three
abundance ratios for CEERS-1019 are well reproduced by the rotating massive
star winds. In the bottom panel, displaying the evolution of the He mass
fraction, the corresponding lifetimes of the stars are indicated (filled
circles, color-coded for VROT= 0 and 300 km/s) for selected masses (associated
numbers in M⊙).

As shown in Fig. 6 in the early evolution of a stellar population, there is a
period of several Myr during which the N/O ratio in the stellar winds reaches
the high N/O ratios observed in CEERS-1019 and in the other N-emitters
analyzed here. However, rapidly after reaching the maximum N/O value, the
carbon abundance also increases (very strongly in rotating star or less so
without rotation), implying C/O and N/C ratios that are incompatible with the
observations of CEERS-1019, SMACS2031, and the Sunburst cluster over most of
the time (see also Fig. 8). In the results displayed here, there is thus only
a fairly short period of $\sim$ 0.5 Myr (yellow shaded area in Fig. 6) where
all three ratios N/O, N/C, and C/O are compatible with the observations of
CEERS-1019 for the case of rotating stars. In view of the timescales involved
(several Myr), the probability of such an occurrence is small but certainly
non-negligible. We note that this occurs rather early in the evolution of the
starburst, but well within the time window found by the analysis of Senchyna
et al. (2023) for GN-z11 (violet horizontal segments in the 2nd and 3d
panels). We also note that other stellar models than those used here could
result in more extended periods of high N/O and N/C ratios. This could be the
case, for instance, of stars rotating more rapidly than 300 km/s (e.g. the
fast rotators at nearly break-up velocity of 800 km/s calculated by Hirschi
2007), binary stars, or stars calculated with higher mass loss rates, etc.
(see e.g. Eldridge & Stanway 2022, for a recent review). On the other hand,
for the central region of Mrk 996 which shows both N and C enrichment, we find
that all the abundance ratios are well reproduced by the models. Furthermore,
in this galaxy the WR-scenario is directly supported by the presence of WR
stars both of WN and WC types (Telles et al. 2014). Similarly, N and C
enrichment found in the Lynx arc could also be explained by the WR scenario,
and earlier studies have argued for the presence of WR stars, from emission
line modelling of this peculiar object (see e.g. Villar-Martín et al. 2004).

Figure 7: Observed chemical abundances (N/O versus O/H in the top panel, C/O
versus O/H in the bottom) of the six N-emitters (using the same symbols as in
Fig. 5) and comparison with predictions for enrichment from massive stars
(dotted and dash-dotted lines showing the “WR scenario” for non-rotating and
rotating stars, respectively; see text) and supermassive stars (solid and
dashed). Different colors indicate different metallicities. The predictions
for the WR scenario are shown for a very low dilution (1%) with ISM matter.
The solid lines show the predicted composition using SMS models with 104 M⊙ at
different metallicities ($12+\log({\rm O/H})$$\approx$ 7.0,7.8, 8.3) from
Charbonnel et al. (2023) and for varying amounts of dilution with an ISM of
standard composition. The dashed lines show an SMS model with 105 M⊙ from
Nagele & Umeda (2023).  Figure 8: Observed and predicted abundance ratios of
N/C as a function of O/H for the six N-emitters and comparison samples from
the literature. Observed data are shown using the same symbols as in Fig. 5,
model predictions with the linestyles of Fig. 5.

Is there any direct evidence for WR stars in the N-emitters discussed here? In
short, WR stars have been reported only in the low-$z$ galaxy Mrk 996, as
mentioned earlier. In the spectral range covered by the observations of
CEERS-1019, the strongest WR features could be He ii $\lambda$1640 and C iv
$\lambda$1550 in the rest-UV and the so-called blue WR-bump centered around He
ii $\lambda$4686\. None of these features is detected in the current NIRSpec
spectra and the same holds for GN-z11 (see: Bunker et al. 2023; Maiolino et
al. 2023). However, the JWST spectra of these very high-$z$ objects, and in
particular for CEERS-1019, are of insufficient spectral resolution and S/N to
rule out, e.g., He ii $\lambda$1640 emission with equivalents widths $\la
7-10$ Å (depending on the adopted FWHM of the WR line), and therefore stellar
populations comparable to those of Mrk 996, which has EW(1640)$\approx 3-4$ Å,
cannot be ruled out from the present data. The rest-UV spectrum of SMACS2031
from Patrício et al. (2016) also shows no clear feature of WR stars. He ii
$\lambda$1640 is present with an EW(1640)$=0.99\pm 0.1$ Å, but it is only
marginally broader than nebular emission lines.

The very high-S/N spectrum of the Sunburst cluster, discussed by Meštrić et
al. (2023), also shows no signature of WR stars. Except for the nebular lines,
the Sunburst spectrum resembles in fact strongly the spectrum of the well-
known massive star cluster R136 in the LMC, which is known to be very young
($\sim 1.5$ Myr) and to host very massive stars with masses up to $\sim 200$
M⊙ (Vanzella et al. 2020; Meštrić et al. 2023). The Sunburst cluster also
appears to be too young to host WR stars. Finally, Villar-Martín et al. (2004)
have suggested the presence of WR in the Lynx arc, in particular to explain
the hard observed ionizing spectrum, but no direct signatures are detected in
the relatively low S/N spectra available for this object.

In conclusion, except for Mrk 996 where the presence of important populations
of WR stars (both of WN and WC types) is established, no direct evidence for
WR stars is found in the other N-emitters studied here. However, this does not
necessarily exclude the WR-scenario, since WR stars may be present below the
detection threshold.

####  5.3.2 Enrichment from super-massive stars ($M\ga 1000$ M⊙) – SMS
scenario

An alternate scenario, already invoked by Charbonnel et al. (2023) to explain
the high N-abundance in the compact galaxy GN-z11 at $z=10.6$ , is that of
super-massive stars (SMS), which have previously been proposed to explain the
abundance anomalies of the multiple stellar populations seen in old Galactic
and extra-galactic globular clusters (GC) and in extra-galactic massive star
clusters with ages down to $\sim$ 1.7 Gyr (Gieles et al. 2018). In essence,
this model proposes that gas accretion and collisions of proto-stars in the
densest clusters lead to the runaway formation of one or several SMS, with
masses $M\ga 10^{3}M_{\odot}$ that increase with the cluster mass. During some
time before two-body relaxation heats the cluster, this mostly convective SMS
undergoes accretion (from proto-stars in the cluster and infalling gas) and it
ejects processed matter, whose composition reflects the conditions in its hot
H-burning core. Namely, the ejected material is strongly enriched in N, Na,
and Al, and strongly depleted in O and C as a result of CNO, NeNa, and MgAl
nuclear reactions at high temperature. As initially shown by Denissenkov &
Hartwick (2014), the whole range of abundance anomalies (C-N, O-N, Na-O, Mg-Al
anticorrelations) in GC stars is very well accounted for after dilution of the
SMS ejecta with proto-GC gas.

The constant supply of unprocessed material to the SMS “freezes” its evolution
close to the zero-age main sequence, preventing strong He-enrichment of the
SMS yields, in agreement with GC multiple band photometry (Milone 2015; Milone
et al. 2018). This also solves the so-called “mass budget” problem encountered
by all the other scenarios that try to explain the presence and properties of
multiple stellar populations in globular clusters (Prantzos & Charbonnel 2006;
Schaerer & Charbonnel 2011; Krause et al. 2012; Renzini et al. 2015; Krause et
al. 2016; Bastian & Lardo 2018). For example, Gieles et al. (2018) find that a
SMS forming into a dense cluster hosting $10^{7}$ proto-stars can reach and
process respectively $\sim 5\%$ and $\sim 45\%$ of the cluster mass. This is
significantly higher than the $\sim 2$% of wind mass ejected in the massive
star scenario (cf. Fig. 6). In particular, the super-linear scaling predicted
between the amount of material nuclearly processed by the SMS and the cluster
mass explains the observed increase of the fraction of second population stars
with GC mass (Carretta et al. 2010; Milone et al. 2017). This picture is
dubbed the “conveyor-belt” SMS model. The high amount of processed matter also
implies that any additional matter ejected by the SMS during its final phase
(once the conveyer-belt stops) will have very little impact on the final
abundance ratios.

In Figs. 7 and 8 the solid lines show, for three different initial
metallicities (0.34 Z⊙, 0.12 Z⊙, and 0.018 Z⊙), the predicted chemical
abundance ratios resulting from the mixture of ejecta of $10^{4}$ M⊙ SMS in
the conveyer-belt scenario with different amounts of ISM gas with a normal,
initial abundance (stellar models from Charbonnel et al. 2023). The
composition of the SMS ejecta reflects the yields from H-burning via the CNO-
cycle. It is very strongly enriched in Nitrogen, with N/O $>10$, i.e. nearly
100 times super-solar, and very strongly depleted in Oxygen and Carbon. With
an increasing fraction of matter from the SMS mixed into the ISM, the
predicted N/O and N/C ratios increase strongly. The resulting mixture also
shows a decreasing O/H abundance (metallicity) while C/O remains relatively
constant.

The observed N/O ratio of GN-z11 and CEERS-1019 can be explained by mixing
approximately equal amounts of SMS ejecta with ISM gas, as already shown by
Charbonnel et al. (2023). The N/O abundance of all other N emitters considered
here could also be explained with the SMS scenario. The same is also true for
the C/O and N/C abundance ratios, except for the two objects which show a high
C/O ratio, Mrk 996 and the Lynx arc. As already mentioned before, C/O in these
galaxies reveals the presence of both H- and He-burning products, which, in
the case of Mrk 996, is compatible with its observed WR star population. In
short, the comparison of the observed N/O, C/O, and N/C ratios suggests that
CEERS-1019, SMACS2031, and the Sunburst cluster might be explained by the SMS
conveyor-belt scenario, implying that they should contain one or several
proto-GC, and Mrk 996 and the Lynx arc by the WR scenario. From the available
data and the lack of accurate C/O measurements, the case of GN-z11 remains
inconclusive.

Nagele & Umeda (2023) have computed the composition of the material ejected
through winds along the entire evolution of SMS with masses between $10^{3}$
and 105 M⊙ for 0.1 Z⊙, neglecting the conveyor belt rejuvenation of the star
discussed above (they assume that SMS form through gravitational collapse
during the merger of gas-rich galaxies at high-$z$, see Mayer et al. 2015). In
addition, they estimate if and when the SMS become GR unstable as they evolve,
as well as the modifications of the composition of the material that can be
ejected at the end of the life of the stars in the case they eventually
explode due to the CNO cycle and the rp (rapid proton capture) process (for
details see Nagele et al. 2023a). Their $10^{3}$ and $10^{4}$ M⊙ models – not
shown here – predict strong N-enrichment on the main sequence, confirming the
results of Denissenkov & Hartwick (2014) and Charbonnel et al. (2023).
However, these two models do not become GR unstable and make it until carbon-
oxygen burns. As a consequence, their winds reach super-solar C and O
abundances because of the dredge-up of C and O from the core during central
He-burning, and they are strongly enriched in He. This implies that without
undergoing the conveyor-belt episode that is required to solve the mass budget
and the photometric constraints for the GC case, the total yields of such
models cannot explain the GC abundance anomalies, nor can they explain the N/O
and C/O ratios in CEERS-1019 and in GN-z11 as discussed by Nagele & Umeda
(2023).

On the other hand, Nagele & Umeda (2023) find that their $5\times 10^{4}$ and
$10^{5}$ M⊙ models at 0.1 Z⊙ become GR unstable close to or at the end of the
main sequence, implying that their winds contain super-solar N and sub-solar C
and O before the stars eventually collapse to a black hole or are disrupted by
a thermonuclear explosion. The dashed lines in Figs. 7 and 8 show the range of
abundances expected when the ejecta of their $10^{5}$ M⊙ model is diluted to
various degrees with ISM of initial composition. In addition to the
N-enrichment along the main sequence, this includes their estimate of the
additional N that is produced during the expected CNO-powered explosion
(Nagele et al. 2023a). This model accounts well for the observed abundance N/O
ratios in CEERS-1019, GN-z11, and SMACS2031. And it also produces enough
enriched material to be able to pollute sufficient ionized gas, i.e. masses in
the observed range (see Sect. 4.7), as shown by Nagele & Umeda (2023).

From this, we conclude that SMS over a wide range of masses can simultaneously
explain the GC abundance anomalies and the N/O and C/O ratios in CEERS-1019,
GN-z11, and SMACS2031, if they eject large quantities of H-processed material
early on the main sequence, as predicted by the conveyor-belt SMS scenario
(Gieles et al. 2018), or if the SMS sheds large amounts of processed material
due to instabilities and an explosion during the CNO-cycle (cf. Nagele & Umeda
2023).

In Sect. 5.4 we will further argue whether the N-emitters are proto-GCs, and
discuss possible implications of the SMS scenario, including the possible
formation of an intermediate-mass black hole (IMBH).

####  5.3.3 Other scenarios to explain strong N emission

Cameron et al. (2023a) have discussed different processes or sources which
could explain the observed N-enhancement in GN-z11, including enrichment from
AGB stars, pollution from Pop III star-formation, stellar encounters in dense
star clusters, or tidal disruption of stars from encounters with black holes.
The main conclusions of their qualitative discussion is that these
explanations would need very fine-tuned conditions and that the origin of
N-enrichment is currently largely unknown.

The predictions of classical chemical evolution models including AGB stars are
shown e.g. in the studies of Johnson et al. (2023). Watanabe et al. (2023)
also show predictions of such models in comparison with GN-z11. Indeed, as
well known from earlier works, such models cannot produce high N/O abundance
ratios at low metallicity (as observed in the N-emitters discussed here),
since these models include also the yields of massive stars and core-collapse
supernovae, which produce large amounts of oxygen, and hence no extreme N/O
ratios. The pure WR-wind models of Watanabe et al. (2023) are essentially the
same as our massive star models (WR-scenario).

Maiolino et al. (2023) have recently argued that GN-z11 shows signs of a type
1 AGN, with emission from very high density and a Broad Line Region (BLR).
They further argue that the exceptionally high nitrogen abundance “becomes
much less problematic” in the AGN scenario, for several reasons. First, they
point out that several “nitrogen-loud” AGN have been found, making GN-z11 less
peculiar. And second, they mention that only small amounts of enriched gas are
needed if the observed gas is at very high densities. Finally, they mention
supernovae from supermassive stellar progenitors, rapidly recycled secondary
nitrogen production, or bloated atmospheres of giant/supergiant stars as
possible sources of the observed enrichment, without providing quantitative
estimates.

Clearly, the spectra of CEERS-1019 and the other N-emitters discussed here are
very different from nitrogen-loud AGN, as discussed in Sect. 3. Furthermore,
except for GN-z11 for which Maiolino et al. (2023) show indications of
densities $n_{H}\ga 10^{10}$ cm-3 typical of BLR, the densities inferred here
are much lower, typically $n\sim 10^{4-5}$ cm-3, and all observed emission
line properties are compatible with photoionization from star-formation (Sect.
3). The qualitative scenarios sketched by Maiolino et al. (2023) for GN-z11
may therefore not be applicable to the other N-emitters discussed here. In any
case, more quantitative studies on the detailed chemical abundances of
nitrogen-loud AGN and their source of enrichment could be of interest to
better understand the common points and differences with other N-emitters.

For the Sunburst cluster, Pascale et al. (2023) proposed a model where the
super star cluster is surrounded by low- and high-density photoionized clouds
and regions (channels) through which ionizing radiation can escape, and they
argue that only the high-density clouds in the vicinity of the star cluster
are N-enriched and confined by strong external pressure. They estimate that
$\sim$ 500 M⊙ of nitrogen is needed – an amount which can be produced by the
star cluster with a mass $M_{\star}\sim$ few $\times 10^{7}$ M⊙– and suggest
that it originates from young massive stars, ejected, e.g., in dense LBV winds
or non-conservative binary mass transfer. SN ejecta are not favored, since the
Sunburst is not enriched in C, and the inferred age ($\la 4$ Myr) is
consistent with this explanation.

The model of Pascale et al. (2023) is essentially the same as our massive star
scenario, although they do not use a specific model to predict the chemical
yields of the cluster and its temporal evolution, and our massive star
scenario does not include ejecta from mass transfer in binary systems. As
already discussed above, such a scenario requires some specific “tuning”, in
particular the selection of a fairly specific age at which the composition of
ejecta matches the observed abundances. For the Sunburst cluster this seems
very plausible; however, it is not clear if this could be generalized to
CEERS-1019 and the other N-emitters.

Table 4: Properties, proposed scenarii, and nature of the N-emitters  Object | redshift | N/O | C/O | WR features | enrichment | proto-GC | BH formation | nature
---|---|---|---|---|---|---|---|---
CEERS-1019 | 8.678 | super-solar | normal | ? | SMS | yes | no | SF-galaxy
GN-z11 | 10.6 | super-solar | uncertain | ? | SMS or other | no | yes? | AGN?
SMACS2031 | 3.506 | super-solar | normal | ? | SMS | yes | no | SF-galaxy
Sunburst cluster | 2.368 | super-solar | normal |  | SMS | yes | no | SF-galaxy
Lynx arc | 3.36 | super-solar |  $\sim$ solar | ? | WR? | no | no | WR galaxy?
Mrk 996 | 0.00544 | super-solar |  $\sim$ solar | WC+WN | WR | no | no | WR galaxy

###  5.4 Are CEERS-1019 and other N-emitters proto-GC in formation or related
to the formation of intermediate-mass black holes ?

The unusually high N/O abundances derived for GN-z11 and the Sunburst arc and
similarities with the abundance pattern of stars in globular clusters have led
several authors to suggest a link between these peculiar objects and GC
formation (Senchyna et al. 2023; Charbonnel et al. 2023; Nagele & Umeda 2023;
Pascale et al. 2023). With the finding of a highly supersolar N/O ratio and
normal C/O in CEERS-1019 and similar results for other objects from the
literature (in total six N-emitters analyzed here), the question of the nature
of the N-emitters must be rediscussed in light of new and additional evidence.
We summarize basic observational evidence and our favourite
scenarii/explanations in Table 4.

First, the observed abundance ratios of N/O and C/O, which are accurately
measured for five objects, suggest that two of them (the low-$z$ galaxy Mrk
996 and the Lynx arc) are probably explained by pollution from WR stars, as
discussed above. If correct, it implies that the cluster(s) dominating
presumably these objects cannot be progenitors of GCs. This is due to the fact
that massive star wind scenario suffers from the so-called mass budget problem
of GCs (Prantzos & Charbonnel 2006; Decressin et al. 2007), which basically
means that the massive stars cannot produce sufficient amounts of enriched
material to explain the observed population of “polluted” (second population)
stars in GCs without this first population being much more massive than the
second one, in contradiction with observations. In Mrk 996 WR features are
detected, and the presence of WR stars is suspected in the Lynx arc. We
therefore suggest that they are somewhat peculiar star-forming galaxies (WR
galaxies), although we note that Binette et al. (2003) have also considered a
hidden AGN to explain the emission line properties of the Lynx arc.

For CEERS-1019, GN-z11, SMACS2031, and the Sunburst cluster, the N/O, C/O, and
N/C ratios could be explained by the two scenarii discussed earlier, with the
enriched matter originating from normal massive stars or from supermassive
stars. We favour the SMS scenario for several reasons. First, the WR scenario
requires a very special and shorter timing than the SMS scenario. Second,
these galaxies contain at least one sufficiently massive and compact region
(the Sunburst cluster is of course a cluster) with extreme conditions (very
high SFR and mass surface density), and unusually high ISM densities. Such
rare conditions may be necessary for the formation of supermassive stars
through runaway collisions and for the conveyer-belt model, as proposed by
Gieles et al. (2018). This would also naturally explain why N-emitters are
rare. We therefore propose that CEERS-1019, SMACS2031, and the Sunburst
cluster have been enriched by SMS and that they host (or are) proto-GCs in
star-forming galaxies. Finally, the finding of such objects at look-back times
between 11.2–13.3 Gyr is also compatible with them hosting proto-GCs.

The case of GN-z11 may be somewhat different as it may host an AGN, as
suggested by Maiolino et al. (2023). If the high density of the ionized gas
($n_{e}\ga 10^{10}$ cm-3) inferred by these authors is confirmed, it would
significantly reduce the amount of ionized gas which needs to be polluted, but
it still leaves the source of chemical enrichment unexplained (cf. Maiolino et
al. 2023). However, this does not exclude pollution from one or several SMS,
which might even have seeded the “small” massive black hole (with $\log(M_{\rm
BH}/M_{\odot})\sim 6.2\pm 0.3$) or contributed to its growth. Indeed, the
final fate of SMS is difficult to predict since in addition to metallicity and
mass, other input parameters of the stellar models (mass loss, convection,
overshooting, rotation, etc.) may impact the occurrence of the GR instability,
its timing, and whether the collapse would trigger an explosion through the
CNO-cycle (Fuller et al. 1986; Montero et al. 2012; Haemmerlé et al. 2019;
Haemmerlé 2021; Nagele et al. 2023a). In any case, the formation of IMBH with
masses $\sim 10^{4}$ to 105 M⊙ from SMS seems possible at metallicities
comparable to that of GN-z11, as shown e.g. by Nagele et al. (2023b). We
therefore propose that N-emitters could also be an indication of black hole
seed formation from SMS. And these objects could evolve to N-loud quasars, a
rare sub-population of quasars showing strong N lines in the UV (Jiang et al.
2008), and which have been suggested to be objects with high N/O and sub-solar
metallicities in a rapid growth phase (Araki et al. 2012; Matsuoka et al.
2017). We therefore mark GN-z11 as a possible AGN with BH-formation related to
SMS in Table 4. Finally, we also consider that the formation of an IMBH with
mass $\ga 1000$ M⊙ from an SMS is incompatible with the proto-GC scenario, as
the presence of such a BH in old GCs seems to be ruled out observationally
(Baumgardt et al. 2019, and references therein). This is also reflected in
Table 4.

Finally, we wish to remind the reader that Larson et al. (2023) suggested that
CEERS-1019 also hosts a black hole, although our analysis does not show
significant evidence for this and suggests that the object is dominated by
star-formation (see Sect. 3). If CEERS-1019 harbours an AGN, the situation
could be similar to that of GN-z11, just discussed and point to a possible
link between SMS and black hole formation. Also, we note that Binette et al.
(2003) have considered a hidden AGN to explain the emission line properties of
the Lynx arc, although the nature of this source remains unclear. To conclude,
we also recall that none of the four other N-emitters discussed here show any
AGN indication. We are therefore probably left with three good candidates for
SMS and proto-GCs, CEERS-1019, SMACS2031, and the Sunburst cluster.

###  5.5 Future steps and improvements

Clearly, better data and more N-emitters would be helpful to better understand
the origin of the strong N emission lines, to further test the proposed
enrichment scenarios and the possible presence of SMS, and thus to understand
the nature of these rare objects.

An important test for the massive star scenario would be to detect direct
spectral signatures of WR stars. Deeper, very high S/N spectra, in the rest-
optical domain would be ideal for this. The massive star scenario also
predicts important amounts of helium in the ejecta, which might be measurable
from the analysis of nebular He and H emission lines in rest-optical spectra
of sufficient quality. In the SMS scenario, a strong enrichment of aluminum,
originating from H-burning from the MgAl chain (Prantzos et al. 2007), is
predicted (Ramirez-Galeano, in prep.), as observed in GC stars (Carretta et
al. 2009; Pancino et al. 2017; Masseron et al. 2019). In contrast, massive
stars should produce less aluminum (Decressin et al. 2007; Gormaz-Matamala et
al. 2023). Aluminum has spectral signatures in the rest-UV (Al ii
$\lambda$1670, Al iii $\lambda\lambda$1855,1863), which are often seen in
absorption in high-$z$ galaxy spectra (Shapley et al. 2003; Le Fèvre et al.
2019), and which are in emission in some AGNs (see e.g. Alexandroff et al.
2013). These features might be an interesting test of the relation between
N-emitters and proto-GCs, and to distinguish between the WR and SMS scenarii.

To examine if the strong N lines could be related to large density variations
and found preferentially in pockets of high density, it will be of interest to
obtain multiple density measurements probing the widest possible range of
density, regions of different ionization, and possibly also spatial
variations. Both high S/N and high-resolution spectra are needed for this, and
measurements of fine-structure lines of oxygen and nitrogen with ALMA could
also provide insights into this question.

Future studies may reveal new N-emitters, improving the statistics and
providing more test cases. If strongly enhanced N-emitters are found at
significantly lower metallicities (say $12+\log({\rm O/H})$$\la 7$) the SMS
scenario might be favored, since WR stars should be less present at low O/H.
Also, objects with even higher N/O abundances could exist, if the SMS scenario
is correct.

##  6 Conclusion

In this work, we have presented the detailed analysis of CEERS-1019 at
$z=8.678$ using deep spectroscopy and imaging with NIRSpec and NIRCam obtained
from the JWST CEERS program. Low- and medium-resolution NIRSpec spectra
covering $1-5\mu$m reveal a wealth of rest-frame UV and optical nebular
emission lines of various transitions and ionizing states from H, He, C, N, O,
and Ne. In particular, CEERS-1019 shows remarkably intense Nitrogen emission
of N iii and N iv, with N iv] $\lambda 1486$ emerging as the strongest line
within the rest-frame UV spectrum. These emission lines are very rarely seen
in galaxy spectra, and CEERS-1019 – which shows some resemblance with the
peculiar object GN-z11 revealed recently by JWST (Bunker et al. 2023) – is
thus the second “N-emitter” found at $z>8$. From the analysis of these data,
we arrive at the following main results:

  * •

Using the well-detected auroral [O iii] $\lambda 4363$ line we determined the
O/H abundance using the direct method, resulting in $12+\log({\rm
O/H})=7.70\pm 0.18$. We derived the electron temperature from both rest-frame
UV and optical [O iii] lines, yielding consistent values of $T_{e}\approx
18000$ K. The density-sensitive lines of N iv] $1483/1487=0.50\pm 0.22$ and [O
ii] $3727/3729=0.98\pm 0.23$ suggest a relatively high electron density of
$n_{e}\approx 10^{3-5}$ cm-3. These values are consistent with those reported
by other studies for this object (Tang et al. 2023; Nakajima et al. 2023;
Larson et al. 2023).

  * •

Metal abundances were derived for different ions of C, N, O, and Ne. Notably,
we found an exceptionally high N/O abundance of log(N/O)$=-0.13\pm 0.11$,
approximately $5.6$ times higher than the solar ratio. Conversely, CEERS-1019
exhibits relatively normal C/O and Ne/O ratios for its metallicity (O/H), with
log(C/O)$=-0.75\pm 0.11$ and log(Ne/O)$=-0.63\pm 0.07$, respectively. This
translates to high N/O and N/C, and normal C/O ratios, typically found in
globular cluster stars, and which reflect the abundance ratios from H-burning
via the CNO-cycle at very high temperature (Prantzos et al. 2017; Gratton et
al. 2019).

  * •

We have discussed possible chemical enrichment scenarios to explain these
peculiar C, N, and O abundance ratios observed in CEERS-1019. Enrichment from
massive star winds through the WR phase can explain the observed ratios but
requires a very short and specific time window (and the presence of WN stars
only); it would also come with a very strong He enrichment. Furthermore, no
signatures of WR stars are detected in CEERS-1019, although their presence
cannot be ruled out from the available data. Alternatively, models of super-
massive stars ($>1000M_{\odot}$) mixed with ISM with a normal composition can
explain the abundance ratios of CEERS-1019. In this scenario, the ejected
processed material via SMS will exhibit H-burning products only, strong
enriched in N and possibly some depletion in O and C, and a normal He content.

  * •

We have investigated the possibility of an AGN in CEERS-1019, a scenario
recently suggested by Larson et al. (2023) due to the detection of a broad
component in H$\beta$. Our own reduction of the NIRSpec spectrum shows a
tentative, broad component in H$\beta$ ($\rm FWHM\simeq 1150$ km s-1) but
detected with a fairly low significance ($\simeq 2.2\sigma$). Line ratios
using rest-UV lines (N v, N iv], C iv, C iii], O iii], and He ii) suggest that
the gas is primarily photoionized by star formation, and any contribution from
an AGN would likely be residual. The non-detection of the high-ionization
lines of N v $\lambda 1240$ and He ii $\lambda 1640$ further support this
scenario.

  * •

CEERS-1019 shows a complex morphology with three resolved clumps. By analyzing
the light distribution of these substructures, we found very compact
morphologies with characteristic half-light radii of $\simeq 100-150$ pc.
Multi-wavelength SED fits for each individual clump predict stellar masses of
log($M_{\star}/M_{\odot})\simeq 8.66-8.94$, resulting in very high stellar
mass surface densities log($\Sigma_{M_{\star}}/(M_{\odot}\rm\,pc^{-2})\simeq
3.55-4.14$. The star formation rate appears very intense in two clumps ($\rm
SFR\simeq 80-150$ $M_{\odot}$ yr-1), while the remaining clump displays a
negligible level of ongoing star formation.

CEERS-1019 represents thus the second example of a rare population of strong
N-emitting galaxies at $z>8$ with highly super-solar N/O abundances, very
compact regions, and a high-density ISM. To put this object into context and
better understand these N-emitters, we have (re-)analyzed other known
N-emitting star-forming galaxies from the literature. This includes three
lensed objects, two galaxies (SMACS2031 and the Lynx arc), and one star-
cluster (the Sunburst cluster) at $z\sim 2.3-3.5$, plus a nearby blue compact
dwarf galaxy (Mrk 996), all of them without any clear indications of AGN
activity. Similar to CEERS-1019, these sources show peculiar abundance ratios
with a supersolar N/O ratio along with very dense clustered mass and star
formation (log($\Sigma_{M_{\star}}/(M_{\odot}\rm\,pc^{-2}))\gtrsim 3.5$) and
high ISM densities ($n_{e}\sim 10^{4}-10^{5}$ cm-3). Two galaxies, Mrk 996 and
the Lynx arc, show an enhanced C/O ratio compared to normal galaxies at the
same metallicity (O/H), indicative of enrichment from WR stars.

We have also presented quantitative predictions for the chemical enrichment in
two different scenarios, including enrichment from winds of massive stars
(called the WR-scenario) or from ejecta of supermassive stars (SMS) with
masses $10^{3}-10^{5}$ M⊙, which have been invoked to explain the abundance
anomalies observed in present-day globular clusters (Denissenkov & Hartwick
2014; Gieles et al. 2018). The WR scenario explains well the two galaxies with
enhanced C/O and is supported by direct evidence of WN and WC stars in Mrk
996. As already found by Charbonnel et al. (2023) for GN-z11, we found that
the SMS scenario reproduced well the observed abundance ratios in CEERS-1019,
SMACS2031, and the Sunburst cluster. These observations probably provide the
best indirect evidence so far for the possible existence of SMS in galaxies.

Finally, considering the preferred enrichment scenarii and other physical
properties, we have also examined which of the N-emitters could host proto-GCs
and what their nature is. From our analysis we concluded that CEERS-1019,
SMACS2031, and the Sunburst cluster host most likely proto-GCs. We also
suggested that the peculiar abundances of GN-z11 could be due to SMS, even if
this object was confirmed to host an AGN, as proposed by (see Maiolino et al.
2023). This could also point to the formation of intermediate-mass black holes
from SMS and suggest a link between the N-emitters and N-loud quasars.

In short, the newly discovered N-emitter CEERS-1019 and other N-emitters show
tantalizing similarities with stars in GCs and the conditions expected during
the formation of GCs. They may also offer a unique window into the formation
of SMS, their role during the formation of GCs, and also their possible
importance as seeds for the formation of massive black holes. More detailed
studies and further discoveries of these rare objects will shed further light
on these exciting topics and questions.

###### Acknowledgements.

We thank Lise Christensen and Johan Richard for sharing spectra from their VLT
observations of SMACS2031. We also thank Mark Gieles, Eros Vanzella, Laura
Ramirez Galeano, Anastasios Fragos, Holger Baumgardt, Montse Villar-Martin and
other colleagues for stimulating discussions. CC acknowledges support from the
Swiss National Science Foundation (SNF; Project 200020-192039). M.M.
acknowledges the support of the Swedish Research Council, Vetenskapsrådet
(internationell postdok grant 2019-00502). Y.I. acknowledges support from the
National Academy of Sciences of Ukraine (Project No. 0123U102248) and from the
Simons Foundation.

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