# Confirmation and refutation of very luminous galaxies in the early Universe

Pablo Arrabal Haro, Mark Dickinson, Steven L. Finkelstein, Jeyhan S.

Kartaltepe, Callum T. Donnan, Denis Burgarella, Adam C. Carnall, Fergus

Cullen, James S. Dunlop, Vital Fernández, et al.

► To cite this version:

Pablo Arrabal Haro, Mark Dickinson, Steven L. Finkelstein, Jeyhan S. Kartaltepe, Callum T. Donnan, et al.. Confirmation and refutation of very luminous galaxies in the early Universe. Nature, 2023, 622, pp.707-711. 10.1038/s41586-023-06521-7. insu-04505554

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# Confirmation and refutation of very luminous galaxies in the early Universe

https://doi.org/10.1038/s41586-023-06521-7

Received: 27 March 2023

Accepted: 7 August 2023

Published online: 14 August 2023

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Pablo Arrabal Haro<fnref n="1" />,²,³, Mark Dickinson<fnref n="1" />, Steven L. Finkelstein², Jeyhan S. Kartaltepe³, Callum T. Donnan⁴, Denis Burgarella⁵, Adam C. Carnall⁴, Fergus Cullen⁴, James S. Dunlop⁴, Vital Fernández⁶, Seiji Fujimoto²,⁷,⁸, Intae Jung⁹, Melanie Krips<fnref n="10" />, Rebecca L. Larson²,³, Casey Papovich¹¹,¹², Pablo G. Pérez-González¹³, Ricardo O. Amorín⁶,¹⁴, Micaela B. Bagley², Véronique Buat⁵, Caitlin M. Casey², Katherine Chworowsky², Seth H. Cohen¹⁵, Henry C. Ferguson⁹, Mauro Giavalisco¹⁶, Marc Huertas-Company<fnref n="17" />,¹⁸,¹⁹, Taylor A. Hutchison²⁰, Dale D. Kocevski²¹, Anton M. Koekemoer⁹, Ray A. Lucas⁹, Derek J. McLeod⁴, Ross J. McLure⁴, Norbert Pirzkal⁹, Lise-Marie Seillé⁵, Jonathan R. Trump²², Benjamin J. Weiner²³, Stephen M. Wilkins²⁴,²⁵ & Jorge A. Zavala²⁶

During the first 500 million years of cosmic history, the first stars and galaxies formed, seeding the Universe with heavy elements and eventually reionizing the intergalactic medium<fnref n="1" />⁻³. Observations with the James Webb Space Telescope (JWST) have uncovered a surprisingly high abundance of candidates for early star-forming galaxies, with distances (redshifts, z), estimated from multiband photometry, as large as z ≈ 16, far beyond pre-JWST limits⁴⁻⁹. Although such photometric redshifts are generally robust, they can suffer from degeneracies and occasionally catastrophic errors. Spectroscopic measurements are required to validate these sources and to reliably quantify physical properties that can constrain galaxy formation models and cosmology<fnref n="10" />. Here we present JWST spectroscopy that confirms redshifts for two very luminous galaxies with z > 11, and also demonstrates that another candidate with suggested z ≈ 16 instead has z = 4.9, with an unusual combination of nebular line emission and dust reddening that mimics the colours expected for much more distant objects. These results reinforce evidence for the early, rapid formation of remarkably luminous galaxies while also highlighting the necessity of spectroscopic verification. The large abundance of bright, early galaxies may indicate shortcomings in current galaxy formation models or deviations from physical properties (such as the stellar initial mass function) that are generally believed to hold at later times.

The spectroscopic targets were selected from James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam)¹¹ images obtained through the Cosmic Evolution Early Release Science (CEERS) Survey¹² because their colours at wavelengths of 1–5 µm matched predictions for galaxies with redshifts z > 9 (ref. 9), including the key signature of a ‘break’ in the flux density at wavelengths shorter than that of red-shifted hydrogen Lyman α (Lyα; rest-frame wavelength λ<sub>rest</sub> = 1,216 Å) due to the largely neutral intergalactic medium (IGM) expected at early times. Here we focus attention on three bright extreme-redshift candidates: two with photometric redshifts z ≈ 11 (CEERS2_5429, also known as Maisie’s Galaxy⁵,⁹ and CEERS2_588 (ref. 9)); and another with a

photometric redshift z ≈ 16.4 (CEERS-93316 (ref. 6)). At their estimated redshifts, these three galaxies would have ultraviolet (UV) luminosities approximately 2 to 9 times brighter than any other spectroscopically confirmed galaxy at z > 11 (refs. 13,14). We show these photometric redshift estimates in Fig. 1.

Although independent analyses by different teams⁵⁻⁷,⁹,¹⁵ identified these candidates using seven-band JWST/NIRCam photometry, it is possible for foreground objects to infiltrate high-redshift samples due to unusual colours resulting from older stellar populations, strong nebular emission lines and/or dust reddening⁶,¹⁶⁻¹⁸. A tentative detection at 850 µm of dust emission in the vicinity of CEERS-93316 (ref. 18) could

<footnote n="1">¹NSF's National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA. ²Department of Astronomy, The University of Texas at Austin, Austin, TX, USA. ³Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, Rochester, NY, USA. ⁴Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, UK. ⁵Aix Marseille Univ, CNRS, CNES, LAM Marseille, Marseille, France. ⁶Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, La Serena, Chile. ⁷Cosmic Dawn Center (DAWN), Copenhagen, Denmark. ⁸Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark. ⁹Space Telescope Science Institute, Baltimore, MD, USA.</footnote>

<footnote n="10">¹⁰IRAM, Domaine Universitaire, Saint-Martin-d'Hères, France. ¹¹Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA. ¹²George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA. ¹³Centro de Astrobiología (CAB), CSIC-INTA, Madrid, Spain. ¹⁴Departamento de Astronomía, Universidad de La Serena, La Serena, Chile. ¹⁵School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA. ¹⁶University of Massachusetts Amherst, Amherst, MA, USA.</footnote>

<footnote n="17">¹⁷Instituto de Astrofísica de Canarias, La Laguna, Spain. ¹⁸Universidad de la Laguna, La Laguna, Spain. ¹⁹Université Paris-Cité, LERMA - Observatoire de Paris, PSL, Paris, France. ²⁰Astrophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA. ²¹Department of Physics and Astronomy, Colby College, Waterville, ME, USA. ²²Department of Physics, University of Connecticut, Storrs, CT, USA. ²³MMT/Steward Observatory, University of Arizona, Tucson, AZ, USA. ²⁴Astronomy Centre, University of Sussex, Brighton, UK. ²⁵Institute of Space Sciences and Astronomy, University of Malta, Msida, Malta. ²⁶National Astronomical Observatory of Japan, Tokyo, Japan. ²⁷e-mail: parrabalh@gmail.com</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 1** | Photometric redshift probability density function for the primary high-z candidates. The likelihood (bottom) and $\chi^2$ goodness-of-fit (top) distribution functions for the photometric redshift, derived from the JWST CEERS photometry. Results for Maisie's Galaxy, CEERS2_588 and the $z \approx 16.5$ candidate galaxy CEERS-93316 are shown in blue, green and red, respectively. The solid and dashed lines denote the results using the photometric measurements from each team. The spectroscopic measurements of $z = 11.416$, $z = 11.043$ and $z = 4.912$ from this paper are indicated by the thick vertical lines.</figcaption></figure>

support the dust reddening hypothesis, but we have investigated this using new observations at 1.1 mm with the NOEMA interferometer and localized the dust emission to another galaxy approximately 1.5 arcsec away (see Methods for details). Nonetheless, reddening could still affect the observed colours of CEERS-93316.

We observed these galaxies with JWST/NIRSpec¹⁹ using its multi-object spectroscopy mode and prism disperser, whose low spectral resolution at wavelengths shorter than 2.5 µm affords high sensitivity for detecting faint continuum emission and spectral breaks. Details of the observations and data analysis, as well as results from simultaneous observations of several other candidate $z > 9$ galaxies, are summarized in Methods.

The spectrum of Maisie's Galaxy (Fig. 2) shows well-detected continuum emission with a sharp break, below which no significant flux is detected. The most likely interpretations of this feature are the Lyα IGM break or the Balmer break ($\lambda_{rest} = 3,646$ Å); the latter is prominent in galaxies where star formation ceased more than 100 Myr before the time of observation. Integrating the spectrum in wavelength intervals redder and bluer than the observed break, we measure a $3\sigma$ limit on the flux density ratio greater than 2.6, inconsistent with values expected for the Balmer break¹⁴ (Methods). We used several methods²⁰ to measure the wavelength of the spectral break (Methods and Extended Data Table 1). Interpreting this feature as the Lyα break (Fig. 2b) yields a redshift $z = 11.44^{+0.09}_{-0.08}$, consistent with photometric redshift estimates. We also identify an emission line at 4.629 µm as the unresolved singly ionized [O II] doublet ($\lambda_{rest} = 3,727$ Å and 3,730 Å) at $z = 11.416$.

CEERS2_588 exhibits a comparably strong Lyα break and [O II] emission line at $z = 11.043$ (ref. 21). Its break amplitude is greater than 2.2, again inconsistent with a Balmer break. Another, fainter, galaxy (MSA ID 64) also has a strong (greater than 1.9) break and no emission lines, consistent with a Lyα break redshift $z = 10.10^{+0.11}_{-0.26}$ (Methods).

NIRSpec observations of star-forming galaxies at $z < 9.6$ frequently detect strong [O III] ($\lambda_{rest} = 4,960$ Å and 5,008 Å) and Hβ ($\lambda_{rest} = 4,863$ Å) emission lines²²⁻²⁴ that shift beyond the instrument's spectral coverage for redshifts $z > 9.6$. Emission lines at blue optical and UV rest-frame wavelengths are typically weaker and harder to detect in low signal-to-noise ratio (S/N) spectra. The [O II] lines for Maisie's Galaxy and CEERS2_588 are only weakly detected (4.9σ and 6.4σ, respectively)

<table><caption>**Table 1** | Summary of galaxy properties</caption><thead><tr><th>Property</th><th>Maisie's Galaxy (MSA ID 1)</th><th>CEERS2_588 (MSA ID 10)</th><th>CEERS-93316 (MSA ID 0)</th></tr></thead><tbody><tr><td>Right ascension (J2000)</td><td>214.943152</td><td>214.906640</td><td>214.914550</td></tr><tr><td>Declination (J2000)</td><td>52.942442</td><td>52.945504</td><td>52.943023</td></tr><tr><td>Photometric redshift</td><td>11.08<sup>+0.36</sup><sub>-0.39</sub></td><td>11.02<sup>+0.39</sup><sub>-0.27</sub></td><td>16.45<sup>+0.45</sup><sub>-0.18</sub></td></tr><tr><td>Spectroscopic redshift</td><td>11.416 ± 0.005</td><td>11.043 ± 0.003</td><td>4.912 ± 0.001</td></tr><tr><td>T<sub>BigBang</sub> (Myr)</td><td>392</td><td>410</td><td>1,194</td></tr><tr><td>M<sub>UV</sub></td><td>-20.1<sup>+0.1</sup><sub>-0.1</sub></td><td>-20.3<sup>+0.1</sup><sub>-0.2</sub></td><td>-16.2<sup>+0.5</sup><sub>-0.1</sub></td></tr><tr><td>log(M<sub>☉</sub>/M<sub>☉</sub>)</td><td>8.6 ± 0.3</td><td>8.7 ± 0.1</td><td>8.8 ± 0.1</td></tr><tr><td>Range</td><td>8.0–9.0</td><td>8.1–9.1</td><td>8.3–9.2</td></tr><tr><td>A<sub>V</sub> (mag)</td><td>0.1 ± 0.1</td><td>0.2 ± 0.1</td><td>2.3 ± 0.2</td></tr><tr><td>Range</td><td>0.0–0.8</td><td>0.03–0.7</td><td>0.8–2.7</td></tr><tr><td>SFR (M<sub>☉</sub> yr<sup>-1</sup>)</td><td>2 ± 1</td><td>10 ± 4</td><td>60 ± 20</td></tr><tr><td>Range</td><td>1–12</td><td>2–17</td><td>2–80</td></tr><tr><td>log[sSFR(yr<sup>-1</sup>)]</td><td>-8.3<sup>+0.2</sup><sub>-0.3</sub></td><td>-7.7<sup>+0.2</sup><sub>-0.2</sub></td><td>-7.0<sup>+0.1</sup><sub>-0.2</sub></td></tr><tr><td>Range</td><td>-8.6 to -7.6</td><td>-8.2 to -7.5</td><td>-8.1 to -6.9</td></tr></tbody></table>

The photometric redshift calculation is described in Methods and has been updated for this work. Spectroscopic redshifts are derived from emission lines; see Methods for a full discussion. $T_{BigBang}$ denotes the age of the Universe at the spectroscopic redshift. All uncertainties are 68% confidence intervals. Stellar masses ($M_*$), SFRs averaged over the last 100 Myr and $A_V$ values have been derived using three different stellar population modelling techniques (Methods). For each property, the first line gives the best-fit values and 68% confidence intervals from the method in which the spectra and the photometry are fit simultaneously (Synthesizer), while the second line shows the ranges spanned by all three modelling methods including their 68% confidence intervals.

even with moderately long NIRSpec exposure times, highlighting the importance of a robust Lyα break detection.

Maisie's Galaxy and CEERS2_588 are the most UV-luminous galaxies with confirmed spectroscopic redshifts $z > 11$ so far, with rest-frame UV absolute magnitudes $M_{UV} = -20.1$ and $-20.3$, respectively. The physical properties derived through stellar population modelling (see Methods for details) of the observed photometry and spectroscopy of Maisie's Galaxy and CEERS2_588, fixing the redshifts to the spectroscopic values, are presented in Table 1. We find that they have stellar masses ($M_*$) in units of the solar mass ($M_☉$) $log_{10}(M_*/M_☉) = 8.6$ to 8.7 and low dust extinction ($A_V = 0.1$ to 0.2 mag), with high star-formation rates (SFR) for their stellar masses, that is, specific star-formation rates (sSFR ≈ SFR/$M_*$) in the range ($log_{10}(sSFR) = -8.3$ to $-7.7$). These physical properties are consistent with similar values found for the few other spectroscopically confirmed galaxies at $z \ge 11$ (refs. 13,25).

The spectrum of CEERS-93316 (Fig. 3) shows emission lines from the hydrogen Balmer series, [O III], [O II] and [S II] ($\lambda_{rest} = 6,718$ Å and 6,733 Å) at a redshift $z = 4.912 \pm 0.001$. This is the same redshift as for another galaxy, CEERS-DSFG-I¹⁸,²⁶, which lies 26 arcsec away and was previously discussed as a dusty star-forming galaxy with colours that could mimic a high-redshift Lyα break¹⁸. At least two other neighbouring galaxies have very similar redshifts (Methods). The continuum emission of CEERS-93316 fades at wavelengths shorter than 2.5 µm (rest frame $\lesssim 0.4$ µm), although a flux is significantly detected in the spectrum down to $\lambda \approx 1.7$ µm and marginally detected down to $\lambda \approx 1.3$ µm. The strong Hα + [N II] and [O III] + Hβ lines have large rest-frame equivalent widths (EWs; 760 ± 67 Å and 1,116 ± 13 Å, respectively), and hence contribute significantly to the flux measured by NIRCam through the F277W, F356W, F410M and F444W filters. At this precise redshift, Hα emission actually contributes to the flux in the last three of these filters (Fig. 3). The combination of the strong lines and the red continuum leads to the red colours measured between filters F150W, F200W and F277W and the blue colours from F277W to F444W (Fig. 4c).
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**Fig. 2 | Images and spectra of the confirmed z > 11 galaxies.** a,c, Colour images of Maisie's Galaxy (a) and CEERS2_588 (c), showing an approximation of the true rest-frame UV colour (made from the F277W, F356W and F444W images). b,d, Spectra of Maisie's Galaxy (b) and CEERS2_588 (d) highlighting the detected Lyα breaks and [O II] emission lines. The top panel shows the 2D spectrum. The red horizontal lines show the spatial extent over which the 1D

spectrum, shown in the bottom panel, was extracted. The shaded grey area corresponds to 1σ uncertainties in the 1D spectrum and the zero flux level is marked by a dashed black horizontal line. A clear spectral break is seen, below which there is no significant flux. We interpret this as the Lyα break at a redshift of 11.416 and 11.043, which corresponds to a time 392 Myr and 410 Myr after the Big Bang, respectively.

The significant reddening implied by the faint, red UV continuum emission is corroborated by the large Balmer decrement measured from the spectrum (Hα/Hβ = 5.2 ± 0.5), which implies, despite the large EWs of the lines, a substantial nebular colour excess between blue (B) and visual (V) wavelengths of E(B − V) = 0.60 ± 0.10 (Methods). Indeed this combination of line emission and reddening at z ≈ 5 has been considered previously as an alternative explanation for the colours of CEERS-93316 (refs. 6,16–18). The Hα emission may imply rapid star formation (28$^{+7}_{-6}$M$_{⊙}$ yr$^{-1}$) (Methods), although some other faint red objects at z ≈ 5 show strong line emission with broad Balmer line components that indicate the presence of active galactic nuclei (for example, refs. 27,28).

The example of CEERS-93316 is a reminder that the prevalence of very strong nebular line emission in early galaxies can influence broad-band photometry with instruments such as JWST/NIRCam and mimic the colours predicted for much more distant objects. Exceptional candidates for galaxies at extreme redshifts require spectroscopic confirmation. However, we note that this is a fairly extreme case, as it is only at very specific redshifts that strong emission lines can fully mimic a Lyα break with multiple broad-band filter detections, and reddening is also required to suppress detection in bluer filters (Methods).

Fortunately JWST/NIRSpec is a powerful tool for spectroscopic analysis and redshift measurements, as demonstrated by our verification

<figure><img src="image_8.png" /></figure>

**Fig. 3 | NIRSpec spectrum of z phot ≈ 16 candidate CEERS-93316.** a,b, 2D (a) and 1D (b) spectra, with the red lines denoting the spatial extraction limits. Emission lines labelled in **b** indicate an unambiguous z = 4.912 for this object. The shaded grey area corresponds to 1σ uncertainties in the 1D spectrum. Panel **b** also illustrates the wavelengths covered by the JWST/NIRCam filter set used to select this galaxy, highlighting how [O III] contamination in F277W

(green) and Hα contamination in all of F356W (yellow), F410M (orange) and F444W (red) conspired to mimic the signature of both a very high-redshift Lyα break and a blue continuum at longer wavelengths. This unusual (but educational) situation is detailed further in Fig. 4. Inset in **b**: an F150W + F200W + F277W colour image of this galaxy.
<figure><img src="image_9.png" /></figure>

**Fig. 4 | Spectral energy distributions of the three main sources reported.**
**a,** Maisie's Galaxy. **b,** CEERS2_S88. **c,** CEERS-93316. Each panel shows the observed photometry (coloured points) compared with the best-fitting stellar population model (black line) and the observed spectra (coloured line). In **c,** the model is deliberately shown at *z* = 16.2 to demonstrate the way in which strong emission lines impact the observed photometry in this specific case to replicate the presence of the Lyα break at *z* ≈ 16 and an apparently blue continuum at longer wavelengths. The brown squares show the observed photometry omitting the spectroscopic emission line contribution (darker brown lines in the spectrum). Error bars are 1σ uncertainties and the upper limits correspond to 2σ limits.

of two galaxies with $z > 11$ and three other galaxies at $7.9 < z < 10.1$ (Methods). NIRSpec observations have now confirmed redshifts for 10 galaxies at $z > 10$ (refs. 14,20,21,29,30), including five with $M_{\text{UV}} < -20$, unexpectedly bright compared to most pre-JWST predictions. These confirmations are crucial to validate the methods employed for $z \ge 10$ photometric selection²⁰ and to improve our physical understanding of the first phases of galaxy formation.

Early analyses of candidate galaxies at $z = 9.5–12$ have shown that their observed abundance is significantly higher than that predicted by nearly all theoretical models⁵,⁷,⁹,³¹. This could indicate that specific assumptions in these models need to be modified at very high redshifts, including the possibility of negligible dust attenuation³²,³³ or an increased efficiency of converting gas into stars, or even a shift toward the formation of more massive stars in these early epochs. Such a ‘top-heavy’ initial mass function (IMF) has indeed been predicted to be present in early galaxies, driven

by a combination of their chemically simple makeup³⁴,³⁵ as well as a rising cosmic microwave background temperature floor³⁶. However, before any such unusual physical properties can be robustly inferred for early galaxies, their photometric redshift estimates must be verified. Four out of seven photometric candidates from CEERS data at $9.5 < z < 12$ with $M_{\text{UV}} < -20$ have been spectroscopically confirmed²⁰, including the present examples at $z > 11$, with no confirmed interlopers except the $z \simeq 16$ candidate CEERS-93316. The space density of confirmed objects alone exceeds the predictions of most theoretical models. The spectroscopic observations reported here reinforce the higher-than-expected abundance of such bright galaxies in the very early universe, while also vetting photometric redshift outliers like CEERS-93316 that could be misinterpreted if left unrecognized.

## Online content

Any methods, additional references, Nature Portfolio reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-023-06521-7.

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### Cosmological model

Throughout this work we adopt a Λ cold dark matter cosmological model (where Λ is the cosmological constant) with relative densities of matter ($\Omega_m$) and dark energy ($\Omega_\Lambda$) of $\Omega_m = 0.315$, $\Omega_\Lambda = 0.685$ and a Hubble constant $H_0 = 67.36$ km s$^{-1}$Mpc$^{-1}$ (ref. 37). All magnitudes are presented in the AB system³⁸.

### Near-infrared imaging data and spectroscopic target selection

We select high-redshift galaxy candidates from the first epoch of imaging from the Cosmic Evolution Early Release Science (CEERS; principal investigator (PI): S. Finkelstein⁵) survey. These data consist of four roughly contiguous pointings with the NIRCam instrument on JWST, covering an area of approximately 35 arcmin². The full area is covered by six broad-band filters (F115W, F150W, F200W, F277W, F356W and F444W) and one medium-band filter (F410M). We use the v.0.5 data release from the CEERS team (https://ceers.github.io)¹². These reductions use the STScI JWST pipeline with several custom modifications, correcting for various issues, including 1/f noise, wisps and snowballs (https://jwst-docs.stsci.edu/data-artifacts-and-features/snowballs-and-shower-artifacts), with updated photometric calibration from late 2022. The 5σ photometric depths of these data for compact source detection are approximately 29.0–29.2 AB magnitudes in F115W, F150W, F200W, F277W and F356W, and about 0.7 and 0.5 mag shallower in F410M and F444W, respectively.

Photometry was optimized for accurate measurements of colours, generally following previous CEERS measurements⁹, with a few key updates to improve measured colours, particularly between Hubble Space Telescope (HST) and JWST. Specifically, all bands with point spread functions (PSFs) smaller than that of F277W (that is, Advanced Camera for Surveys (ACS) F606W and F814W, and NIRCam F115W, F150W and F200W) were convolved to match the F277W PSF. For the remaining images, fluxes were measured in the native images, but a correction was applied on a per-source basis as the ratio of the flux in the native F277W image to that of the F277W image convolved to match the PSF in a given larger-PSF image. We then used simulation-based corrections to derive accurate total fluxes. Photometric redshifts were derived using EAZY³⁹, including new custom templates designed to match the expected blue colours of early galaxies⁴⁰.

The NIRSpec spectroscopic observations targeted three primary galaxy candidates: Maisie's Galaxy (refs. 5,9), with photometric redshift $z \approx 11.1$, CEERS-93316 with photometric redshift $z \approx 16.4$ (ref. 6) and CEERS-DSFG-1, a JWST-detected galaxy with millimetre-wavelength emission that is likely to arise from dust-obscured star formation at $z \approx 5$ (ref. 26) but whose near-infrared colours could be interpreted as suggesting $z > 10$ (ref. 18). Many other faint, high-redshift candidates were also observed simultaneously, including CEERS2_588, which has photometric redshift $z \approx 11$ (refs. 6,9) and is comparably as bright as Maisie's Galaxy. These object names come from the original papers that reported these objects rather than from a homogeneous source catalogue.

### NOEMA data and analysis

Observations with the James Clerk Maxwell Telescope (JCMT) and its Submillimetre Common-User Bolometer Array 2 (SCUBA-2) detected tentative 850 µm emission in the vicinity of CEERS-93316. Previous analysis¹⁸ showed that this submillimetre emission is unlikely to arise from an object at $z > 10$, and that a $z \approx 5$ galaxy with dust attenuation and strong emission lines could explain the measured JWST photometry.

We observed dust continuum with band 3 of the Institute Radio Astronomie Millimétrique (IRAM) NORthern Extended Millimeter Array (NOEMA) as a Director's Discretionary Time (DDT) programme no. D22AC001 (PI: S. Fujimoto). The observations were carried out between 26 November and 8 December 2022 in several visits with the C configuration using 10–11 antennas. The data were processed in the

standard manner with the pipeline using the latest version of GILDAS. We used CASA v.6.5 for the imaging⁴¹.

The upper and lower side bands of the band 3 receiver were tuned to 256.6 GHz and 271.1 GHz, resulting in a central wavelength of 1.134 mm. The observations of 3C45, 3C273, J1439 + 499 and 3C84 served as band-pass phase calibrators. Additional targets 1418 + 546 and J1439 + 499 were used for the phase and amplitude calibrations. We calibrated the absolute flux scale against MWC349, LkHa101 and 2010 + 723 whose flux densities are regularly monitored at NOEMA. The total integration time on-source was 6.5 hours.

To maximize sensitivity, we used natural weighting for the imaging. The resulting 1-mm continuum map has a synthesized beam full-width at half-maximum of 0.91" × 0.67" with 1σ sensitivities of 28 µJy per beam for the continuum. We do not identify any line features at the target position and thus produce the 1-mm continuum map from all channels.

In Extended Data Fig. 1, we show the NOEMA 1.1-mm intensity contours overlaid on the NIRCam colour image around CEERS-93316. We do not detect emission at the position of CEERS-93316, with a 3σ upper limit of 0.084 mJy per beam.

On the other hand, the NOEMA observations revealed relatively bright continuum emission at 9σ significance from a nearby (approximately 1.5") galaxy with a photometric redshift $z \approx 4.9$, very similar to the spectroscopic redshifts of CEERS-93316, CEERS-DSFG-1 (also detected at millimetre wavelengths) and several other galaxies in the field (see below and Extended Data Table 1). We measure a 1.1 mm flux density $0.56 \pm 0.07$ mJy with an optimized aperture. This is in good agreement with the previous report of SCUBA-2 850 µm emission with $0.64 \pm 0.26$ mJy¹⁸, implying that this neighbouring galaxy is the main contributor to the SCUBA-2 detection.

### Near-infrared spectroscopy and data reduction

JWST/NIRSpec multi-object spectroscopic observations (DDT programme no. 2750; PI: P. Arrabal Haro) were executed on UTC 24–25 March 2023 using the PRISM/CLEAR combination of disperser and filter. The prism disperser covers the wavelength range 0.6 to 5.3 µm with varying spectral resolution $R \equiv \lambda/\Delta\lambda \approx 30$ at $\lambda \approx 1.2$ µm to more than 300 at $\lambda > 5$ µm. The low resolution of the prism at bluer wavelengths aids the detection of a faint UV continuum and the Lyα break, while the higher resolution at red wavelengths facilitates the detection of redshifted optical rest-frame emission lines.

The NIRSpec Micro-Shutter Assembly (MSA)⁴² was configured to observe 160 faint-object targets. MSA ID numbers were assigned to the targets when the spectroscopic observing plan was designed. For most observed targets, three-shutter slitlets were employed, enabling a three-point nodding pattern to facilitate background subtraction. CEERS2_588 was fit into the MSA configuration 'by hand' using a two-shutter slitlet, and therefore received two thirds of the full exposure time. The observations consisted of 3 sequences of 3 nodded integrations, each with 9 to 10 readout groups in NRSIRS2 readout mode, for a total combined exposure time of 18,387 s.

The MSA configurations included a total of seven NIRCam-selected $z \ge 8.5$ galaxy candidates as well as 153 other targets. We report results for ten objects in Extended Data Table 1 including the primary targets described above. Extended Data Fig. 2 shows images for the nine of them with robust spectroscopic redshift, with MSA shutter overlays superimposed. Essential parameters for Maisie's Galaxy, CEERS2_588 and CEERS-93316, including results from the present analysis, are summarized in Table 1. Detailed analysis of CEERS-DSFG-1 will be presented elsewhere.

The NIRSpec data processing followed the methodology employed for other CEERS NIRSpec prism observations²⁰,²². We make use of the STScI Calibration Pipeline (https://jwst-pipeline.readthedocs.io/en/latest/index.html) v.1.8.5 and the Calibration Reference Data System (CRDS) mapping 1088. We use the calwebb-detector1 pipeline module to subtract the bias and the dark current, correct the 1/f noise and
generate count-rate maps (CRMs) from the uncalibrated images. The parameters of the jump step are modified for an improved correction of 'snowball' events associated with high-energy cosmic rays.

The resulting CRMs are then processed with the calwebb_spec2 pipeline module which creates two-dimensional (2D) cutouts of the slitlets, performs the background subtraction making use of the nodding pattern, corrects the flat-fields, implements the wavelength and photometric calibrations and resamples the 2D spectra to correct the distortion of the spectral trace. The pathloss step accounts for the slit loss correction at this stage of the reduction process.

The images of the individual nods are combined at the calwebb_spec3 pipeline stage, making use of customized apertures for the extraction of the one-dimensional (1D) spectrum.

The 2D and 1D spectra are simultaneously inspected with the Mosviz visualization tool (https://jdaviz.readthedocs.io/en/latest/mosviz/index.html)⁴³ to mask possible remaining hot pixels and other artifacts in the images, as well as the detector gap (when present). Data from the three consecutive exposure sequences are combined after masking image artifacts to produce the final 2D and 1D spectral products.

The JWST pipeline uses an instrumental noise model to calculate flux errors for the extracted spectra. We test and rescale these flux errors for the effect of data correlation introduced by the pipeline following procedures described elsewhere²⁰. To further test possible flux discrepancies due to uncorrected slit losses or inaccurate flux calibration, we integrate the spectra through the NIRCam filter bandpasses and compare the resulting synthetic photometry to the measured NIRCam photometry. For the main sources presented in this work, the flux ratios are similar for all NIRCam bands with robust (5σ) detections, and do not show any trend with wavelength. We rescale the spectroscopic fluxes by a constant factor derived from the average of the flux ratios for all bands with detections. These rescaling factors are 1.29, 1.46 and 1.56 for CEERS-93316 (MSA ID 0), Maisie's Galaxy (MSA ID 1) and CEERS2_588 (MSA ID 10), respectively.

## Redshift estimation

For objects with multiple clear emission lines, we were easily able to identify the [O III] and/or Hα emission lines. Spectroscopic redshifts are measured by comparing the observed emission lines with their theoretical vacuum transitions. A Gaussian profile is fitted for every spectral line to establish the transition wavelength. The Gaussian fluxes are used to weigh the mean redshifts in Extended Data Table 1. In those observations where the [O III] doublet lines are merged, the profile fitting assumes the same gas kinematics and their amplitudes fixed by the theoretical emissivity ratio (2.98 (ref. 44)) to isolate them.

We also estimate the redshifts using the detected continuum with two methods. The first method consists of fitting a five-parameter model to describe an object's spectrum and determine the wavelength of the Lyα break²⁰. We first create the intrinsic spectrum based on the redshift, UV absolute magnitude and UV spectral slope. We then add IGM absorption by zeroing out the spectrum below 1,215.67 Å rest-frame wavelength, and add Lyα damping wing absorption by adding two additional parameters: the neutral hydrogen fraction ($x_{\text{HI}}$) and an ionized bubble radius ($R_{\text{bubble}}$)¹⁴,⁴⁵. We derive posterior constraints on these five parameters using an IDL implementation of the EMCEE Python code⁴⁶, restricting the spectrum to wavelengths below the 2,500 Å rest-frame for a given redshift. The second method is full spectral fitting to the NIRSpec data, which detects the continuum for all our galaxies, using EAZY³⁹ and a variety of spectral templates, including UV-bright and dusty galaxies. For Maisie's Galaxy and CEERS2_588 the spectra cover both the Lyman and Balmer breaks. All methods provide results in agreement within $\Delta z \approx 0.1$, consistent within our estimated uncertainties.

## Redshift results

We derive redshifts from multiple emission lines for galaxies with MSA IDs 0 (via [O II], [O III] and Hα), 2 (via [O III], Hα), 28 (via [O II] and

[O III]) and 355 (via [O III] and Hβ). We measure Lyα break redshifts for MSA IDs 1, 10, 28, 64 and 355. For Maisie's Galaxy and CEERS2_588 (MSA IDs 1 and 10) we also detect single emission lines that we identify as the unresolved [O II] doublet at redshifts that are consistent with (but more precise than) the Lyα break redshifts, and we adopt those values as our fiducial redshifts for those galaxies. MSA ID 69 shows a weak break at approximately 1.3 µm, and this methodology does find a maximum likelihood of $z = 9.744$. However, the result has a large uncertainty, with a significant probability of the redshift being lower. We thus do not consider MSA ID 69 confirmed. We present our redshift measurements in Extended Data Table 1, and the spectra of the confirmed high-z objects are shown in Extended Data Fig. 3.

For Maisie's Galaxy, CEERS2_588 and MSA ID 64, we considered the alternative interpretation of a Balmer break at redshifts $z_{\text{BB}} = 3.14$, 3.02 and 2.70, respectively. Averaging the observed spectrum in $f_i$ units over wavelength intervals $(1+z_{\text{BB}}) \times 3,145-3,563$ Å and $3,751-4,198$ Å (ref. 14) for Maisie's Galaxy we measure a S/N of 0.0 and 9.7 below and above the break, and a $3\sigma$ limit on the flux ratio greater than 2.6, which is considerably larger than the maximum value of 1.8 that would be predicted from stellar population synthesis models¹⁴. This supports the higher redshift interpretation of the break resulting from the Lyα opacity of the IGM. For CEERS2_588 the spectrum is negative over the bluer window; the background may be slightly oversubtracted due to the presence of a large galaxy near the offset shutter in this two-shutter slitlet (Extended Data Fig. 2). The $3\sigma$ limits on the break amplitudes for CEERS2_588 and MSA ID 64 are greater than 2.2 and 1.9, respectively, also inconsistent with the Balmer break interpretation.

The derived spectroscopic redshifts are in broad agreement with the photometric redshift. For Maisie's Galaxy, in particular, the photometric redshift has been modified since the galaxy was discovered as data processing improved. The galaxy was originally thought to be at $z \approx 14$, but the photometric redshift was revised to $z = 11.8^{+0.2}_{-0.3}$ after improved astrometric registration recovered weak flux in F150W⁵. Improved NIRCam zero-point flux calibration revised this further to $z = 11.5$ (ref. 9), while the latest photometric redshift from the photometry described in this Article (which is updated relative to earlier work⁹ to better match HST and JWST photometry) suggests $z = 11.08^{+0.36}_{-0.39}$, consistent within the errors with the spectroscopic redshift $z = 11.416$. For CEERS2_588 the photometric ($z = 11.02^{+0.39}_{-0.27}$) and spectroscopic ($z = 11.043$) redshifts also agree very well.

The Lyα break redshift we measure for CEERS2_588 ($z = 11.43^{+0.01}_{-0.07}$) is significantly larger than the [O II] emission line redshift. A smaller offset with the same sense is found for Maisie's Galaxy. The spectral breaks in both galaxies are more gradual or 'rounded' than is typical for Lyα breaks observed in galaxies at lower redshifts, which may indicate the presence of a strong Lyα damping wing due to high column densities of neutral gas either intrinsic to (or immediately surrounding) the galaxies⁴⁷ or from a highly neutral IGM⁴⁸.

CEERS-93316 and the millimetre-detected galaxy CEERS-DSFG-1 both have redshifts $z = 4.91$. We identify two more galaxies (MSA IDs 2763 and 3149) at nearly the same redshift in the NIRSpec observations (Extended Data Fig. 4 and Extended Data Table 1), supporting the hypothesis of a physical overdensity¹⁶. Moreover, the NOEMA-detected galaxy 1.5 arcsec from CEERS-93316 (Extended Data Fig. 1) has a very similar photometric redshift and may well also be associated, but still requires spectroscopy.

## Emission lines

The spectra of Maisie's Galaxy and CEERS2_588 exhibit [O II] emission lines with S/N = 4.9 and 6.4, respectively. While not highly significant, the emission lines are spatially well-aligned with the galaxies' continuum emission, and careful inspection of subsets of data from the nodded exposures show no indication that these features are data artifacts²¹. At the expected wavelengths of the [Ne III] 3870 line, CEERS2_588 shows a feature with S/N = 2.0, and Maisie's Galaxy shows
a positive signal with still lower significance. Weak positive features are seen near the expected wavelengths for [O III] 1660 and [C III] 1909, but these cannot be considered significant.

The emission lines in CEERS-93316 were fitted manually from the spectrum by first subtracting a local continuum and fitting Gaussian profiles to the visible emission lines. In the case of the blended but partially-resolved [O III] 4,960 Å and 5,008 Å lines, a double Gaussian function was fitted. Fluxes and upper limits for the main emission lines present for the sources in this work are presented in Extended Data Table 2. The Balmer lines (Hα and Hβ) are corrected for underlying stellar absorption using the best-fitting stellar continuum from the Bagpipes fits (corrections < 1%; see below for a description of the Bagpipes fits). Hα is blended with [N II] 6,550 Å and 6,585 Å lines in the NIRSpec data. We apply a correction [N II]/Hα ≈ 2% based on an oxygen abundance of 12 + log₁₀ (O/H) = 8.0 estimated from the [O II], [O III], Hβ and [Ne III] lines. The Hα flux, corrected for [N II] emission and for slit losses, is (5.27 ± 0.11) × 10⁻¹⁸ erg cm⁻² s⁻¹, and the Balmer decrement line flux ratio is f(Hα)/f(Hβ) = 5.2 ± 0.5. Assuming Case B recombination at T = 10⁴ K and Galactic extinction⁴⁹ with total-to-selective extinction ratio at visual wavelengths R<sub>V</sub> = 3.1, this corresponds to nebular extinction E(B - V) = 0.60 ± 0.10 and a factor of 4.1<sup>+1.1</sup><sub>-0.9</sub> attenuation at Hα. Assuming the nebular emission is produced by young star formation, this implies an extinction-corrected SFR Hα = 28<sup>+7</sup><sub>-6</sub> M<sub>☉</sub> yr⁻¹ (ref. 50) for a Chabrier IMF⁵¹.

## Spectral energy distribution modelling

We use three spectral energy distribution (SED) fitting codes to derive physical properties for Maisie’s Galaxy, CEERS2_588 and CEERS-93316. The SED fitting results are summarized in Extended Data Table 3.

For the two galaxies with $z > 11$ the three methods yield results for the stellar masses that are consistent within the uncertainties, and within a factor of about two for the best-fitting values. Dust attenuation values ($A_v$) are also consistent within the uncertainties, although Cigale finds a larger best-fit value with larger uncertainties. The SFRs span larger ranges (factors of 3 to 4 between methods), likely due to different assumptions on allowable star-formation histories (SFHs) and, for Cigale, the larger derived value for dust attenuation.

For CEERS-93316 the parameters derived by the three methods span larger ranges, most likely reflecting the complexity of this galaxy with its unusual combination of strong nebular line emission and large dust reddening. Bagpipes finds the lowest values for $M_*$ and $A_v$, while Cigale and Synthesizer find higher values that are consistent within their uncertainties. The large reddening implied for both the stars and the ionized gas is atypical for low mass ($M_* < 10^9 M_☉$) galaxies at high redshifts<sup>52,53</sup>. Bagpipes and Synthesizer find that the lowest and highest values for the SFR, respectively, differ by a factor of 20, while Cigale’s best-fit SFR is intermediate between the other methods, with a larger fractional uncertainty. The larger reddening and SFR values from Cigale and Synthesizer may arise because their fits are additionally constrained by the emission line EWs (and, with Synthesizer, the spectral continuum slope), whereas line emission influences Bagpipes only through its contribution to the NIRCam photometry. Bagpipes also permits greater flexibility in the wavelength dependence of dust attenuation than is allowed for the other methods.

The SFR derived from the extinction-corrected Hα emission from CEERS-93316 (28<sup>+7</sup><sub>-6</sub> M<sub>☉</sub> yr⁻¹) is on the high end of the range derived by Cigale, and below the range derived by Synthesizer. The evidence for strong reddening suggests the possibility that some star formation takes place in regions that are optically thick at UV-optical rest-frame wavelengths, and hence that SFRs from SED fitting and/or Hα emission may be underestimated. The 3σ upper limit at 1.1 mm from NOEMA sets an upper limit to the dust-obscured SFR of approximately 20–200 M<sub>☉</sub> yr⁻¹ assuming modified black body emission with dust temperatures in the range $T_d = 40–70$ K and a typical spectral index of $\beta_d = 1.8$. These limits are reasonably consistent with the SFR estimates derived from SED

modelling and Hα, although the Hα and Synthesizer SFRs would imply dust temperatures $T_d \ge 41$ K and $\ge 48$ K, respectively.

This analysis assumes that the light from CEERS-93316 is dominated by emission from stars and from gas ionized by star formation. Active galactic nuclei (AGN) can also be responsible for strong line emission, and indeed JWST observations have identified a significant population of reddened, broad line AGN at $z \approx 5$ (refs. 27,28). Most of these AGN are very compact or unresolved, whereas CEERS-93316 is spatially extended in NIRCam imaging, indicating that galaxy starlight is present, but this does not exclude a possible contribution from AGN emission as well. There is a tentative emission feature at the wavelength of [Mg II] 2,796 Å and 2,804 Å, a doublet frequently detected in AGN, but its significance is marginal ($S/N = 1.8$).

**Bagpipes.** The Bagpipes⁵⁴ fits make use of stellar population synthesis models⁵⁵ assuming a Kroupa IMF⁵⁶ with a mass range 0.1–100 M<sub>☉</sub>, which results in masses and SFRs very similar to those from the Chabrier IMF⁵¹ used for the Cigale and Synthesizer fits. We consider stellar metallicities (Z) relative to the solar metallicity (Z<sub>☉</sub>) in the range 0.01 ≤ Z/Z<sub>☉</sub> ≤ 3.5. Gas metallicity is set equal to that of the stars. Nebular emission lines and continuum are included in the model using results from the Cloudy photoionization code⁵⁷. Dust attenuation is included using a flexible-slope model, which also includes a Drude profile to model the 2,175 Å dust bump and an additional factor multiplying the attenuation applied to star-forming regions (which we take to include all stellar emission arising from stars younger than 10 Myr and all resulting nebular emission)⁵⁸. The details of further Bagpipes model assumptions are identical to those used in previous work⁶,⁵⁹. In particular, we use a constant SFR model, shown to produce stellar ages and stellar masses at the lower end of the plausible range for objects of interest. We apply a Gaussian prior to the redshift of each object based on the new spectroscopic data, with a standard deviation of 0.01 in each case. The median value for the Gaussian prior is set to z = 11.42 for Maisie’s Galaxy, z = 11.04 for CEERS2_588 and z = 4.91 for CEERS-93316. For the Bagpipes fits, we use only galaxy photometry and spectroscopic redshifts, and do not include additional constraints from the spectra, such as emission line strengths.

**Cigale.** We use the latest version of Cigale<sup>60,61</sup>, using stellar population synthesis models<sup>62</sup> assuming a Chabrier IMF<sup>51</sup> with mass range 0.1–100 M<sub>☉</sub>. Stellar metallicities are allowed to vary from 0.0007 ≤ Z/Z<sub>☉</sub> < 1 for the z > 11 galaxies and 0.007 ≤ Z/Z<sub>☉</sub> ≤ 1 for CEERS-93316. Gas metallicities are allowed to vary independently from those of the stars, with a minimum value 10× larger than the stellar minimum metallicity. We use updated emission line models computed from Cloudy<sup>57</sup>. A Calzetti extinction law<sup>63</sup> is adopted for the dust attenuation of the stellar continuum. However, the nebular emission (continuum and lines) is attenuated with a screen model and a Small Magellanic Cloud (SMC) extinction curve<sup>64</sup>. No 2,175 Å dust bump is added. For the z > 11 galaxies we show that nebular extinction exceeds stellar extinction by a constant factor<sup>63</sup>, while for CEERS-93316 we assume that young stars (less than 10 Myr) and ionized gas are more heavily attenuated than older stars with a variable scaling factor<sup>65</sup>. We select a ‘delayed + burst’ SFH. This form includes a delayed exponential term with SFR $\propto t \times \exp(-t/\tau)$, with $\tau = 200$ Myr for the z > 11 galaxies and 500 and 1,000 Myr for CEERS-93316. On top of this, a short burst is allowed just prior to the time of observation. For the z > 11 galaxies the burst duration is 1 Myr and the burst mass fractions considered are 0%, 0.5% and 10%, whereas for CEERS-93316 we consider burst durations 5, 10 and 20 Myr and burst mass fractions 5%, 7.5%, 10%, 12.5% and 15%. For CEERS-93316, the best-fitting mass fraction is 10 ± 3.5%; the bursts are smaller for the z > 11 galaxies. The SFR reported here is the value averaged over 10 Myr prior to the time of observation. The values for Gaussian priors on redshift were the same as for the Bagpipes fitting. Fitting with Cigale is constrained by the galaxy photometry and redshift.
Additionally, for CEERS-93316, we use constraints from the EWs measured from the NIRSpec spectrum for Hα, Hβ and the two [O III] lines. The EWs from the best-fitting models match the observed values within 30%. The EW of the blended [O II] doublet was included when fitting Maisie's Galaxy and CEERS2_588.

**Synthesizer.** With Synthesizer<sup>66,67</sup>, we use stellar population synthesis models<sup>55</sup> assuming a Chabrier IMF<sup>51</sup> with mass range 0.1–100 M<sub>☉</sub>. We sample stellar metallicities in the range 0.005 ≤ Z/Z<sub>☉</sub> ≤ 1. Nebular continuum and hydrogen and helium emission lines are included in the modelling, assuming Case B recombination, 10<sup>4</sup> K gas temperature, 10<sup>2</sup> cm<sup>-2</sup> density, 15% Lyman continuum photon escape fraction and 10% Lyα forest transmission<sup>67</sup>. Conventional assumptions about the attenuation law and differential nebular-stellar dust effects are assumed<sup>63</sup>, implying that the stellar continuum is less reddened than the nebular emission by a factor of approximately 2. Attenuations are allowed to vary between A<sub>V</sub> = 0 and 1 mag for the z > 11 galaxies, and up to 5 mag for CEERS-93316. We explore star-formation histories described by a delayed exponential form (as also used for Cigale) with timescales ranging from 1 Myr to 100 Myr in 0.1 dex steps. The SFR reported here is the value averaged over 100 Myr prior to the time of observation. Redshifts are fixed to the values given in Extended Data Table 1. Fitting with Synthesizer is constrained using both the galaxy photometry and the spectral fluxes, as well as the measured redshift. Additionally, for CEERS-93316, Hα and Hβ EWs measured from the spectra are included as a fitting constraint, but all models return lower values than those measured by a factor of approximately 4. For Maisie's Galaxy, a relatively strong degeneracy in the age is observed depending on the strength of the Balmer break, which is constrained by the spectroscopic data with relatively low S/N. Fitting spectral data points bluewards of the Balmer break yields younger mass-weighted ages and smaller stellar masses (30 ± 10 Myr, log<sub>10</sub>(M<sub>☉</sub>/M<sub>☉</sub>) = 8.3 ± 0.1). Including NIRSpec spectroscopic data points redwards of the Balmer break increases the derived ages and masses (120 ± 30 Myr, log<sub>10</sub>(M<sub>☉</sub>/M<sub>☉</sub>) = 8.8 ± 0.1).

## Photometric redshift mimicry

The combination of very high EW nebular line emission, strongly reddened stellar continuum and a very specific redshift value is responsible for CEERS-93316 mimicking the colours of a galaxy at z ≈ 16. At 4.88 ≲ z ≲ 5.09, Hα falls in the 'triple-overlap zone' for all three of the reddest NIRCam bandpasses used in CEERS (F444W, F410M and F356W), while [O III] + Hβ lies within the next bluest filter (F277W), as shown in Fig. 3. The emission lines boost the signal in those four bands to emulate a roughly constant flux density per unit frequency (F<sub>v</sub>) colour. The highly reddened galaxy continuum fades at rest-frame UV wavelengths (λ<sub>obs</sub> < 2.4 µm), without significant contribution from other emission lines, leading to a faint detection in F200W and no significant detections (at CEERS exposure times) in the bluer NIRCam filters or by HST. This leads to the mistaken impression of a very distant Lyα break 'dropout' (Fig. 4). Galaxies with very strong line emission are common at high redshift, but here the combination of strong line emission with a very red continuum is key to fooling the photometric redshift codes. A similar phenomenon was recognized for the submillimetre galaxy CEERS-DSFG-1 (ref. 18), which has possible (if not necessarily favoured) photometric redshift solutions at z<sub>phot</sub> > 12, although the weak [O III] emission in that galaxy softens the effect compared to that for CEERS-93316. The other two galaxies with NIRSpec-confirmed redshifts z ≈ 4.9 (MSA IDs 2763 and 3149) have bluer colours and are well-detected at NIRCam F115W and HST F814W, and therefore do not have viable photometric redshift solutions at z<sub>phot</sub> > 6. Galaxies like CEERS-93316 at other redshifts could produce similar photometric aliasing effects, depending on where their emission lines fall with respect to photometric bandpasses. For example, at 6.71 < z < 6.99 the [O III] 5,008 line would fall in the same NIRCam triple-overlap zone

for F356W + F410M + F444W, mimicking the colours expected at z ≳ 20. Observations through additional intermediate band filters could help to further distinguish low and high photometric redshift solutions for galaxies like these, but spectroscopy is the ultimate arbiter.

## Data availability

The JWST NIRSpec data are available from the Mikulski Archive for Space Telescopes (MAST; http://archive.stsci.edu), under programme ID 2750. The CEERS JWST imaging data are available from MAST under programme ID 1345. Reduced NIRCam data products from the CEERS team are available at https://ceers.github.io. The NOEMA data is part of programme D22AC, which will be available at http://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=B/iram/noema.

## Code availability

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**Acknowledgements** P.A.H., M.D., S.L.F., J.S.K. and C.J.P. acknowledge support from NASA through the STScI ERS award JWST-ERS-1345 and the JWST-GO-2750 award. S.L.F. acknowledges support from the University of Texas at Austin. P.G.P.-G. acknowledges support from the Spanish Ministerio de Ciencia e Innovación MCIN/AEI/10.13039/501100011033 through grant PGC2018-093499-B-100. A.C.C. acknowledges support from the Leverhulme Trust via a Leverhulme Early Career Fellowship. C.T.D., D.J.M., R.J.M. and J.S.D. acknowledge the support of the Science and Technology Facilities Council. F.C. acknowledges support from a UKRI Frontier Research Guarantee Grant (grant reference EP/X021025/1). M.H.C. acknowledges financial support from the State Research Agency (AEIMCINN) of the Spanish Ministry of Science and Innovation under the grants 'Galaxy Evolution with Artificial Intelligence' with reference

PGC2018-100852-A-100 and 'BASALT' with reference PID2021-126838NB-100. V.F. acknowledges support from ANID through FONDECYT Postdoc 2020 project 3200340. R.A. acknowledges support from ANID through FONDECYT Regular 1202007. This work is based on observations with the NASA/ESA/CSA James Webb Space Telescope obtained from the Mikulski Archive for Space Telescopes at the STScI, which is operated by the Association of Universities for Research in Astronomy (AURA), Incorporated, under NASA contract NAS5-03127. This work is partially based on observations carried out under project number D22AC with the IRAM NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). R.L.L. and K.C. are both NSF Graduate Fellows. T.A.H. is a NASA Postdoctoral Fellow.

**Author contributions** P.A.H. led the JWST NIRSpec DDT observing proposal, the observation design, the data reduction and contributed extensively to text and figures. M.D., S.L.F. and J.S.K. contributed extensively to spectroscopic target selection and prioritization, with extensive input from many authors including C.T.D., A.C.C., F.C., J.S.D., S.F., C.P., P.G.P.-G., D.D.K., D.J.M., R.J.M. and J.A.Z. Galaxy redshifts were measured primarily by P.A.H., V.F., S.L.F., P.G.P.-G., S.F., I.J. and R.L.L. R.L.L. and V.F. measured emission line properties. F.C. and J.R.T. analysed implications from the emission lines. D.B., A.C.C., P.G.P.-G. and L.-M.S. modelled and interpreted the spectral energy distributions of the galaxies. S.F. led the NOEMA observing programme with contributions to planning from several coauthors including J.A.Z. and C.M.C., and M.K. contributed to NOEMA execution and data analysis. M.D. and S.L.F. led the drafting of the paper text with contributions to text, figures, editing and formatting by P.A.H., S.F., C.P., P.G.P.-G., H.C.F., M.G., J.A.Z., T.A.H., K.C. and other authors. M.B.B. led data reduction for the CEERS NIRCam observations with extensive contributions by A.M.K. and H.C.F., S.H.C. and N.P. contributed to the NIRSpec proposal, and R.O.A., V.B., M.H.C., D.D.K., R.A.L. and B.J.W. (as well as many authors above) contributed to the discussion, analysis and interpretation.

**Competing interests** The authors declare no competing interests.

**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41586-023-06521-7.

**Correspondence and requests for materials** should be addressed to Pablo Arrabal Haro.

**Peer review information** Nature thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

**Reprints and permissions information** is available at http://www.nature.com/reprints.
<figure><img src="image_10.png" /></figure>

**Extended Data Fig. 1** | NOEMA 1.1-mm contours overlaid on the NIRCam 6" × 6" RGB colour image (R: F444W, G: F356W, B: F200W) around CEERS-93316 (MSA ID 0) pointed by the white arrow. North is up and east is left.

The magenta contours are drawn at 1σ intervals from ±2σ to ±9σ. CEERS-93316 is undetected at 1.1 mm, while robust emission from a nearby source is likely responsible for a previously reported tentative SCUBA2 detection¹⁸.
<figure><img src="image_11.png" /><figcaption>**Extended Data Fig. 2** | Montage of images of the galaxies targeted for MSA spectroscopy presented here. Each frame shows a 3" × 3" stamp centred on the location of each galaxy, with the ID labelled. The red-green-blue image for</figcaption></figure>

each galaxy corresponds to the JWST/NIRCam F444W, F356W and F277W, respectively, rotated with north up and east to the left. The rectangles show the approximate location of the JWST/NIRSpec MSA slit positions.
<figure><img src="image_12.png" /><figcaption>**Extended Data Fig. 3** | 2D and 1D NIRSpec spectra of six high-z candidates. Features and spectroscopic redshifts are indicated, except for MSA ID 69 where we cannot determine a redshift with confidence. The shaded grey area corresponds to 1σ uncertainties.</figcaption></figure>
<figure><img src="image_13.png" /></figure>

**Extended Data Fig. 4** | 2D and 1D NIRSpec spectra of CEERS-93316, CEERS-DSFG-1 and two other galaxies at similar redshifts *z* = 4.9, highlighting a potential overdensity in the field at this redshift. The two horizontal yellow lines in the 2D spectra indicate the extraction aperture used to measure the 1D

spectra. Dotted purple vertical lines indicate the location of several emission lines when present. The shaded grey area corresponds to 1σ uncertainties in the 1D spectra and the zero flux level is marked by a dashed black horizontal line.
Extended Data Table 1 | Summary of spectroscopically confirmed galaxies

<table><thead><tr><th>MSA ID</th><th>R.A. (J2000)</th><th>Dec (J2000)</th><th>F277W (AB)</th><th>z<sub>phot</sub> 68% C.L.</th><th>z<sub>spec</sub><sup>lines</sup></th><th>z<sub>spec</sub><sup>break</sup></th><th>z<sub>spec</sub><sup>cont</sup></th></tr></thead><tbody><tr><td>1</td><td>214.943152</td><td>52.942442</td><td>27.9</td><td>10.7–11.5</td><td>11.416 ± 0.005</td><td>11.44<sup>+0.09</sup><sub>-0.08</sub></td><td>11.38<sup>+0.14</sup><sub>-0.08</sub></td></tr><tr><td>10</td><td>214.906640</td><td>52.945504</td><td>27.3</td><td>10.6–11.3</td><td>11.043 ± 0.003</td><td>11.43<sup>+0.01</sup><sub>-0.07</sub></td><td>11.03<sup>+0.02</sup><sub>-0.02</sub></td></tr><tr><td>64</td><td>214.922787</td><td>52.911529</td><td>28.3</td><td>10.4–11.6</td><td>—</td><td>10.10<sup>+0.13</sup><sub>-0.26</sub></td><td>9.73<sup>+0.21</sup><sub>-0.13</sub></td></tr><tr><td>28</td><td>214.938642</td><td>52.911749</td><td>26.9</td><td>8.90–9.00</td><td>8.763 ± 0.001</td><td>8.90<sup>+0.04</sup><sub>-0.02</sub></td><td>8.72<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>355</td><td>214.944766</td><td>52.931450</td><td>28.7</td><td>8.0–8.5</td><td>7.925 ± 0.001</td><td>7.71<sup>+0.12</sup><sub>-0.10</sub></td><td>7.88<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>0</td><td>214.914550</td><td>52.943023</td><td>26.5</td><td>16.0–16.6</td><td>4.912 ± 0.001</td><td>—</td><td>4.89<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>2</td><td>214.909116</td><td>52.937205</td><td>26.2</td><td>3.4–13.6</td><td>4.910 ± 0.003</td><td>—</td><td>4.88<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>2763</td><td>214.927789</td><td>52.935859</td><td>25.1</td><td>4.87–5.44</td><td>4.902 ± 0.001</td><td>—</td><td>4.87<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>3149</td><td>214.914917</td><td>52.943622</td><td>24.7</td><td>0.88–4.81</td><td>4.901 ± 0.001</td><td>—</td><td>4.87<sup>+0.01</sup><sub>-0.01</sub></td></tr><tr><td>69</td><td>214.861602</td><td>52.904604</td><td>28.2</td><td>9.5–10.3</td><td>—</td><td>—</td><td>—</td></tr></tbody></table>

The photometric redshift limits correspond to the 68% confidence intervals. Spectroscopic redshifts presented are based on the fitting of emission lines, the Lyα break, and the complete continuum measured with the NIRSpec prism. Maisie's Galaxy, CEERS2_588 and CEERS-93316 correspond to MSA IDs 1, 10 and 0, respectively.
Extended Data Table 2 | Emission line fluxes and limits

<table><thead><tr><th>MSA</th><th>C III]</th><th>[O II]</th><th>[Ne III]</th><th>Hγ</th><th>Hβ</th><th>[O III]</th><th>[O III]</th><th>Hα+[N II]</th><th>[S II]</th></tr><tr><th>ID</th><th>1909</th><th>3727, 3730</th><th>3870</th><th></th><th></th><th>4960</th><th>5008</th><th></th><th>6718, 6733</th></tr></thead><tbody><tr><td>1</td><td>&lt; 42</td><td>28.5 ± 6.7</td><td>&lt; 21</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>10</td><td>&lt; 66</td><td>45.2 ± 5.9</td><td>&lt; 21</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>64</td><td>&lt; 46</td><td>&lt; 17</td><td>&lt; 17</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>28</td><td>79 ± 15</td><td>56.6 ± 5.0</td><td>55.7 ± 7.7</td><td>64.6 ± 7.8</td><td>103.2 ± 6.4</td><td>204 ± 28</td><td>601.7 ± 8.8</td><td>—</td><td>—</td></tr><tr><td>355</td><td>&lt; 63</td><td>&lt; 13</td><td>&lt; 13</td><td>7.0 ± 2.2</td><td>25.6 ± 4.5</td><td>56.6 ± 1.3</td><td>166.3 ± 4.0</td><td>—</td><td>—</td></tr><tr><td>0</td><td>&lt; 189</td><td>91 ± 17</td><td>&lt; 52</td><td>54 ± 12</td><td>101 ± 10</td><td>256.9 ± 4.0</td><td>755 ± 12</td><td>538 ± 11</td><td>60.0 ± 8.3</td></tr><tr><td>2</td><td>&lt; 768</td><td>&lt; 227</td><td>&lt; 210</td><td>&lt; 163</td><td>&lt; 129</td><td>&lt; 126</td><td>&lt; 125</td><td>667 ± 30</td><td>&lt; 92</td></tr><tr><td>2763</td><td>&lt; 417</td><td>396 ± 34</td><td>&lt; 141</td><td>&lt; 106</td><td>198 ± 24</td><td>485 ± 11</td><td>1425 ± 31</td><td>818 ± 27</td><td>51 ± 11</td></tr><tr><td>3149</td><td>&lt; 339</td><td>668 ± 49</td><td>286 ± 37</td><td>221 ± 26</td><td>438 ± 28</td><td>1023 ± 13</td><td>3006 ± 37</td><td>1674 ± 27</td><td>181 ± 10</td></tr></tbody></table>

Integrated flux and limits for the main emission lines detected in the nine spectroscopically confirmed sources presented in this work, sorted by decreasing redshift as in Extended Data Table 1. All values are in units of $10^{-20}$ erg s$^{-1}$ cm$^{-2}$ and all uncertainties listed are 68% confidence intervals. Upper limits are presented at 3σ. The fluxes presented in this table have been measured after rescaling the spectra to the NIRCam photometry but note that small uncertainties due to imperfect photometric calibration and wavelength dependant slit losses may still affect these measurements and so they should be taken with caution.
Extended Data Table 3 | Summary of galaxy physical properties

<table><thead><tr><th></th><th>MSA ID</th><th>Stellar Mass<br>log<sub>10</sub>(M<sub>*</sub>/M<sub>☉</sub>)</th><th>SFR<br>(M<sub>☉</sub> yr<sup>-1</sup>)</th><th>Stellar A<sub>V</sub><br>(mag)</th></tr></thead><tbody><tr><td rowspan="3">Bagpipes [54]</td><td>1</td><td>8.4<sup>+0.3</sup><sub>-0.4</sub></td><td>3 ± 2</td><td>0.07<sup>+0.09</sup><sub>-0.05</sub></td></tr><tr><td>10</td><td>8.5<sup>+0.3</sup><sub>-0.4</sub></td><td>4 ± 2</td><td>0.10<sup>+0.11</sup><sub>-0.07</sub></td></tr><tr><td>0</td><td>8.4 ± 0.1</td><td>3 ± 1</td><td>1.2 ± 0.4</td></tr><tr><td rowspan="3">Cigale [60, 61]</td><td>1</td><td>8.7 ± 0.3</td><td>8 ± 4</td><td>0.4 ± 0.4</td></tr><tr><td>10</td><td>8.8 ± 0.3</td><td>12 ± 5</td><td>0.4 ± 0.3</td></tr><tr><td>0</td><td>9.0 ± 0.2</td><td>18 ± 11</td><td>2.1 ± 0.6</td></tr><tr><td rowspan="3">Synthesizer [66, 67]</td><td>1</td><td>8.6 ± 0.3</td><td>2 ± 1</td><td>0.1 ± 0.1</td></tr><tr><td>10</td><td>8.7 ± 0.1</td><td>10 ± 4</td><td>0.2 ± 0.1</td></tr><tr><td>0</td><td>8.8 ± 0.1</td><td>60 ± 20</td><td>2.3 ± 0.2</td></tr></tbody></table>

Stellar population properties for Maisie's Galaxy (MSA ID 1, z=11.416), CEERS2_588 (MSA ID 10, z=11.043) and CEERS-93316 (MSA ID 0, z=4.912) from the three different codes described in the Methods section.
All uncertainties listed are 68% confidence intervals.