# Strong Balmer break objects at z 7-10 uncovered with JWST

A Kuruvanthodi, D Schaerer, R Marques-Chaves, D Korber, A Weibel, P

Oesch, G Roberts-Borsani

► To cite this version:

A Kuruvanthodi, D Schaerer, R Marques-Chaves, D Korber, A Weibel, et al. Strong Balmer break objects at z 7-10 uncovered with JWST. Astronomy & Astrophysics - A&A, 2024, 691, pp.A310. 10.1051/0004-6361/202451622. hal-04679913

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# Strong Balmer break objects at z ~ 7–10 uncovered with JWST

A. Kuruvanthodi<fnref n="1" />,*, D. Schaerer<fnref n="1" />,<fnref n="2" />, R. Marques-Chaves<fnref n="1" />, D. Korber<fnref n="1" />, A. Weibel<fnref n="1" />, P. A. Oesch<fnref n="1" />,<fnref n="3" />,<fnref n="4" />, and G. Roberts-Borsani<fnref n="1" />

Received 23 July 2024 / Accepted 17 September 2024

## ABSTRACT

We report the discovery of robust spectroscopically confirmed Balmer break (BB) galaxies and candidates, with secure spectroscopic redshifts $7.1 \le z \le 9.6$, from publicly available *James Webb Space Telescope (JWST)* extra-galactic photometric and spectroscopic surveys. To achieve this, we used dedicated filters probing the BB and inspected the objects with NIRSpec spectroscopy. We have recovered the previously known objects with strong BBs and here reveal 10–11 new objects with clear BBs, thus tripling the number of spectroscopically confirmed galaxies with a BB at $z > 7$. Approximately half of them show a pure BB and no signs of recent star formation, whereas the other half show BB and emission lines, most likely indicating galaxies whose star formation ceased earlier and has restarted recently. Overall, we find that ~10–20% of all galaxies from our sample show signatures of an evolved stellar population. Furthermore, we find that the strength of the BB does not significantly depend on the rest-UV and rest-optical brightness of these sources. In short, our work confirms that photometry alone has the potential to measure BB strengths and to identify evolved stellar populations at high redshift, and that such objects may be more frequent than previously thought. The presence of galaxies with a range of break strengths and the joint presence of BB and emission lines indicate a bursty nature of the star formation in the early Universe.

**Key words.** galaxies: evolution – galaxies: formation – galaxies: general – galaxies: high-redshift – galaxies: star formation – dark ages, reionization, first stars

## 1. Introduction

The presence of Balmer break (BB) galaxies in the epoch of reionization has been proposed by multiple studies, even in the pre-*James Webb Space Telescope (JWST)* era (Laporte et al. 2017; Roberts-Borsani et al. 2020; Hashimoto et al. 2018). Historically, a strong color excess observed in Spitzer IRAC bands has been interpreted as BB for redshift $z \sim 6–7$ galaxies (e.g. Eyles et al. 2007), which indicated a very high formation redshift for these sources. However, Schaerer & de Barros (2009) showed that this color excess could be due to strong emission lines such as [O III] and thus not reflect a real BB. Multiple attempts were made to disentangle the true BB sources from emission-line-dominated ones, by pushing to the high redshift so that these strong lines will be outside the IRAC bands (Roberts-Borsani et al. 2020; Hashimoto et al. 2018). However, redshift uncertainties, the lack of photometric bands redward of the break, and the lack of spectroscopy made it hard to reveal the true nature of these sources.

This picture is changing the era of JWST, thanks to its remarkable sensitivity in the near-to-mid-infrared regime, availability of multiple medium bands, and spectroscopic capabilities. Today, several BB galaxies are already spectroscopically observed above redshift $z > 5$ (Looser et al. 2024; Strait et al. 2023; Witten et al. 2024), and more candidates have been proposed with photometry (Laporte et al. 2023; Trussler et al. 2024a,b).

Detections of a BB at high-$z$ indicate relatively advanced ages of stellar populations ($\ge 50$ Myr and older), probe the timescale of stellar mass assembly, and in principle allow us to constrain the period of the onset of star-formation further in the past (see e.g. Mawatari et al. 2020; Trussler et al. 2024a). Furthermore, observations of BB galaxies and their statistics yield precious information on ceased or “quenched” star-formation in galaxies and constraints on the burstiness (intermittent star-formation) of galaxies and associated timescales (cf. Looser et al. 2024; Witten et al. 2024). It is therefore of great interest to identify new BB galaxies, determine their properties, and attempt to establish the statistics of these populations.

Here we report the spectroscopic discovery of ~10 new BB galaxies at $z \ge 7$, which almost triples the number of spectroscopically confirmed BB galaxies in the epoch of reionization.

The observational data used for this study is described in Sect. 2. Our results are presented and discussed in Sect. 3, and the main conclusions are summarized in Sect. 4. Magnitudes are reported in the AB system, and we assume classical cosmological parameters ($H_0 = 70 \text{ km s}^{-1} \text{ Mpc}^{-1}$, $\Omega_M = 0.3$, $\Omega_\Lambda = 0.7$).

## 2. Observations and BB measurements

### 2.1. JWST NIRCam and NIRSpec observations

To analyze the spectral energy distributions (SEDs) of high-$z$ galaxies and examine their BBs, we used publicly available multi-band imaging data from large extra-galactic

<footnote n="1">¹ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland</footnote>

<footnote n="2">² CNRS, IRAP, 14 Avenue E. Belin, 31400 Toulouse, France</footnote>

<footnote n="3">³ Cosmic Dawn Center (DAWN), Copenhagen, Denmark</footnote>

<footnote n="4">⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark</footnote>

<footnote>* Corresponding author; adarsh.kuruvanthodi@unige.ch</footnote>
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Balmer break strength measurements versus redshift for galaxies with secure spectroscopic redshifts. The gray area shows $BB \le 1$, often referred to as a Balmer jump. The green circles show the sources in this work, and the dashed green line is the median $BB$ obtained in three redshift intervals used in this work. All $BB$ measurements are from photometry, and the upper limits are $3\sigma$. The $BB$ measurements by Roberts-Borsani et al. (2024) using stacked JWST spectra are shown by the dash-dotted black line. For comparison, the dotted blue line shows our photometric $BB$ measurements based on the same spectra, obtained from synthetic photometry using the same filters as for our sample. $BB$ objects from the literature are shown in magenta (Vikaeus et al. 2024; Looser et al. 2024; Boyett et al. 2024; Witten et al. 2024; Wang et al. 2024a); for these objects, we also show our $BB$ measurements from photometry (green circles with dark border).</figcaption></figure>

surveys undertaken with the JWST (CEERS, FRESCO, JADES, PRIMER; Finkelstein et al. 2023; Oesch et al. 2023; Eisenstein et al. 2023; Dunlop et al., in prep.). In all of these fields, NIRCam observations are available in the following seven filters, which we primarily used: F115W, F150W, F200W, F277W, F356W, F410M, and F444W. Where available, we also used HST filters below 1 µm (i.e., F435W, F606W, and F814W). We used the photometric catalogs constructed by Weibel et al. (2024), to which we refer readers for details regarding data reduction, source extraction, and flux measurements. In short, we selected objects in the stacked F277W+F356W+F444W detection image. The $5\sigma$ depth in the F277W (F444W) band of the surveys considered here are 28.51 (28.21), 28.62 (28.36), 29.16 (28.84), and 29.93 (29.20) for PRIMER-UDS, PRIMER-COSMOS, CEERS (EGS), and FRESCO + JADES (GOODS-NORTH/SOUTH), respectively (see Weibel et al. 2024). Furthermore, we required $\ge 3\sigma$ detections in the filters required for the measurement of the $BB$ (see Sect. 2.2), which allowed us to accurately determine $BBs$ for galaxies down to ~29 magnitude in F444W.

To identify galaxies with secure spectroscopic redshifts, we then used the DAWN JWST Archive (DJA and msaexp Heintz et al. 2024; Brammer 2023)<fnref n="1" />, retaining only galaxies with robust spectroscopic redshifts based on visual inspection of individual spectra (grade = 3). In the redshift interval $7.1 < z < 10$, we find 118 galaxies with matches in our photometric catalog, primarily taken from the above-cited surveys.

Since the spectra data originate from a variety of different programs, each with a priori different selection criteria and essentially unknown selection functions, it is not clear if our selection is representative of the average galaxy population at high-$z$, and rigorous statistical inferences cannot be derived from our work. To the best of our knowledge, none of the JWST programs used here specifically targeted objects with strong $BBs$. In this case, and if the selection criteria used by different programs are diverse enough, it is possible that the analyzed sample somewhat resembles a random selection. In any case, our analysis can at least be used to reveal new, previously unknown galaxies with $BBs$, and to estimate the frequency of these objects among the available data.

## 2.2. BB measurements from photometry

We then selected sources in different redshift bins where we could use NIRCam filters to probe the $BB$. In practice, at redshifts $z = 7.1–8.0, 8.0–9.2, \text{and } 9.2–10$, we used the following colors to measure the $BB$: F277W-F356W, F277W-F410M, and F356W-F444W. With these redshifts and filter choices, the redward filters probed rest-wavelengths >3500 Å and avoided strong emission lines of $H\beta$ and [O III]. The above colors are then a good measure of the SED close to and across the $BB$, as

<footnote n="1">¹ https://dawn-cph.github.io/dja</footnote>
shown by the comparisons with spectroscopic measurements and
simulations presented below. The BB was measured for sources
showing ≥3σ detections in one or two of the BB-probing fil-
ters used, and upper or lower limits were reported for objects
detected only in one of the filters. The rest of the sources with
<3σ in both BB-probing filters were ignored. For 101 objects
(74, 21, and 6 in the three redshift bins), we obtained BB mea-
surements, and for 17 objects, we obtained upper or lower limits
on BB.

The BB strength of our sample is defined here as BB = $F_v^+/F_v$, where the flux densities longward and shortward are taken from photometry, and shown as a function of redshift in Fig. 1. To compare our method with direct measurements from spectroscopy, we have convolved the stacked spectra from Roberts-Borsani et al. (2024) with the NIRCam filters used for our work to derive the corresponding photometric BB measurements. The results are also shown in Fig. 1 together with the BB measurements from the same spectra obtained by these authors. As expected, the photometric BB values are slightly higher than the spectroscopic ones (by ~0.1–0.2), since the filter redward of the break contains several weak emission lines ([O II] λ3727, [Ne III] λ3869, and high-order Balmer lines). This broadly validates our method, showing that objects with BB ≥ 1.1–1.2 should have true BBs.

3. BBs revealed in z ~ 7–10 galaxies

The main result from our work, that is, BB measurements as
a function of redshift, is illustrated in Fig. 1, together with BB
measurements reported in the literature. First, we find a large
number of individual galaxies at *z* > 7 that show a significant
BB. For example, 48 (27) galaxies out of 101 have photomet-
ric BB >1.1 (1.2). The strongest BB we measure is BB ~ 2–2.5
in two galaxies at *z* = 7.76 and *z* = 8.35, which surpass
those reported earlier in the literature at *z* ~ 7–7.5, includ-
ing for instance the (mini)-quenched galaxy from (Looser et al.
2024, hereafter L24). The BB of these objects are comparable
to those reported by (Wang et al. 2024a, hereafter W24) in three
*z_* = 6.9–8.4 galaxies, including one object at *z* = 8.36 for which
they measure BB = 1.96±0.14 from the NIRSpec spectrum. Sec-
ond, at *z* ≥ 8.5 and possibly up to *z* = 9.56, we find several
galaxies with fairly secure BBs, some confirmed by NIRSpec
spectroscopy (see below). Finally, the median BB strength mea-
sured for our sample is in agreement with the measurements
from the stacked JWST spectra of Roberts-Borsani et al. (2024),
showing that on average the BB is close to unity from photom-
etry, which is compatible with the presence of a small Balmer
jump plus weak emission lines longward of the break, as shown
by these authors. However, photometry allows us to reveal the
presence of a BB for individual objects for larger samples, and
for objects beyond the reach of spectroscopy.

3.1. Examination of the strong BB candidates in our sample

Three of our 48 galaxies with BB > 1.1 have previously been
reported as galaxies at *z* > 7 with BBs using spectroscopy. This
includes the mini-quenched galaxy at *z* = 7.29 from L24, object
JADES 8079 from (Vikaeus et al. 2024, hereafter V24)<fnref n="2" />, and an
LRD candidate at *z* = 8.35 from W24. The photometric and
spectroscopic BB measurements of these objects are plotted in
Fig. 1, showing good agreement for the first two and a larger

photometric BB for the W24 object, which is explained by the
presence of lines and the relatively red spectrum longward of the
break.

Although our BB measurements rely purely on photometry
(and spectroscopic redshifts), we can use the available spectra
to examine whether or not our BB candidates are plausible. The
NIRSpec PRISM spectra of selected strong BB candidates with
BB ≥ 1.4 and their photometric fluxes are shown in Figs. 2 and 3,
and compared to the *z* = 7.3 mini-quenched galaxy of L24.
The spectra represent the following three categories found from
inspection of the data: (1) Robust BB galaxies or candidates,
(2) galaxies with a co-existence of BB and emission lines, and
(3) candidate little red dot galaxies (LRD, Labbe et al. 2023;
Matthee et al. 2024; Greene et al. 2024) where a red continuum
makes it difficult to establish the presence of a BB. Finally,
another category includes spectra of insufficient S/N, precluding
an evaluation of the BB.

We consider that all objects from Fig. 2 are in category 1. The
object at *z* = 7.26 shown in the top left panel is object JADES
8079 from V24, which strongly resembles the mini-quenched
galaxy of L24. The origin of the flux excess at 4150 nm could be
due to the [O III] lines, but the photometry is essentially constant
longward of the BB. Object 1181_38684 at *z* = 7.48 (top right)
again shows a clear BB, both in photometry and spectroscopy.
It exhibits a somewhat stronger break and a redder UV slope
than the source of L24, possibly indicating an older age of its
stellar population. For 1181_74111 at *z* = 8.53, spectroscopy
and photometry indicate a clear BB, although our photometric
measurement using F277W and F410M shows a weak BB. The
exact reason for the flux excess in F444W is not clear; it could
be due to the presence of emission lines but the spectrum does
not clearly support this hypothesis. Finally, the bottom left panel
illustrates our highest-*z* BB candidate (1345_80026, *z* = 9.56),
with a well-established break from photometry, but an insuffi-
cient quality spectrum to firmly establish the origin of the break.
Overall, we count 5 objects with BB > 1.2 where both spec-
troscopy and photometry show an essentially flat SED (in *Fv*
units) longward of the break (including two objects from L24
and V24).

To further ensure the reliability of these BB candidates,
we investigated the strengths of rest-optical lines and their
possible contribution to the emission redward of the BB. We
assumed Gaussian profiles to fit simultaneously the Hβ and
[O III] λ4960,5008 emission and a constant for the contin-
uum (assumed here between λ₀ = 4660–5200 Å). Hβ was
not detected in any of our strong BB candidates, with 2σ
upper limits between EW₀(Hβ) ≤ 21.6 Å (for 1210_5115)
and ≤137.8 Å (for 1345_80026). These are substantially lower
than those observed in the averaged stacked spectra from
Roberts-Borsani et al. (2024) (≈140–160 Å for their *z* ≈ 7–9
stacks). Therefore, the BB measurements of these objects should
not be affected by emission lines.

Objects from categories 2 and 3 are shown in Fig. 3
The spectrum of 1345_80372 at *z* = 7.48 shows Hβ and
[O III] λλ4959,5007 emission, which indicates that the F356W
filter and hence our photometric measure of BB could be boosted
by Hγ and other emission lines. On the other hand, a spectral
break is quite clearly present in the spectrum, suggesting that
this is probably an object showing both a BB plus emission lines,
similar to the object recently reported by Witten et al. (2024).
Finally, 2565_15420 at *z* = 7.14 illustrates objects whose pho-
tometry and spectra resemble those of LRDs, with a blue/flat
continuum shortward and a red continuum longward of the BB

<footnote n="2">² These authors also report 8115 which is identical to the galaxy from L24.</footnote>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** NIRSpec PRISM spectra of selected strong BB candidates at $z \sim 7-9.6$, showing objects with clear or plausible BBs. Binned and unbinned spectra are shown for illustration. For comparison, we also overplot the spectrum of the mini-quenched $z = 7.3$ galaxy from Looser et al. (2024), redshifted and rescaled to approximately match the observed flux blueward of the BB. Top left: Clear BB galaxy at $z = 7.26$ (object in Vikaeus et al. 2024). Top right: BB galaxy at $z = 7.48$. Bottom left: BB candidate at $z = 8.53$. Bottom right: BB candidate at $z = 9.56$.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** NIRspec PRISM spectra of selected BB candidates with plausible BBs. The labels are the same as in Fig. 2. Left: Clear BB galaxy at $z = 7.48$ with emission lines. Right: Spectrum of an LRD candidate at $z = 7.14$.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** BB as a function of $M_{UV}$ (left) and $M_V$ (right), in AB magnitude, for all spectroscopically confirmed galaxies with $z \sim 7.1–10$. The apparent magnitude of these sources at redshift 8.5 is also shown in the secondary x-axis.</figcaption></figure>

(and indications for emission lines in some cases), as shown for instance by Kocevski et al. (2024). In these cases, it is difficult to measure the true BB from photometry, but some studies have already reported the presence of a BB in LRDs (e.g. Wang et al. 2024a,b). From our inspection of the data, we consider that 5–7 galaxies show simultaneous signs of a BB and emission lines, which most likely indicate the presence of renewed star-formation after an inactive period, akin to sequences of multiple bursts (Witten et al. 2024). The spectra of these sources and the properties of the whole spectroscopic sample (main sample and the additional sample) are shown and tabulated in the appendix (Fig. A.1 and Table B.1).

### 3.2. BBs are found from bright to faint sources

To examine whether sources showing strong BBs are preferentially found in bright and/or massive galaxies, we show in Fig. 4 our measurements of BB as a function of the rest-UV and V-band absolute magnitudes (probed by F150W and F444W, respectively). The 10–11 sources with BB, that is, either showing BB alone or BB with lines, are highlighted in this figure. No specific trend is seen for BB with either $M_{UV}$ or $M_V$, and objects with strong and/or secure BBs are found over a range of magnitudes, approximately from $M_V$ ($m_{F444W}$) ~ -23 to -18 (24–29), that is, over 2 dex in rest-optical fluxes. This is in agreement with earlier studies which examined the behavior of the BB with rest-UV magnitude/luminosity (Vikaeus et al. 2024; Roberts-Borsani et al. 2024; Wilkins et al. 2024).

The presence of significant BBs over a broad range of $M_V$ and $m_{F444W}$ indicates that BB galaxies are found for a variety of UV magnitudes, as also noticed earlier (e.g. Vikaeus et al. 2024), and presumably also over a range of stellar masses. However, detailed SED fits are necessary to translate the observed properties into physical ones, such as stellar masses, which will be presented in a detailed follow-up study (Kuruvanthodi et al., in prep.). In any case, these observations suggest that galaxies with

strong BB can exist over a larger range in masses, from a few times $10^8$ (e.g. L24) $M_{\odot}$ to $>10^9 M_{\odot}$, and even higher, such as for LRD-type objects (see e.g. Wang et al. 2024a).

### 3.3. BB galaxies at $z \sim 7–9$ are more frequent than previously thought

Out of the 101 sources for which secure spectroscopic redshift is available, we found spectra for 62 objects in DJA. Out of these 62 sources, 5 (~8%) show a BB with a flat red continuum akin to that in a mini-quenched galaxy, 5–7 (~10%) sources show a BB with emission lines, and 1 (~2%) is an LRD-like candidate with a possible break. That means that ~16–20% of the sources in our sample show a BB or the presence of an evolved stellar population.

Alternatively, an inspection of the spectra of all objects with photometric BB > 1.2 (16 sources) yields the following: a BB is clearly detected and thus confirmed in 6 of them (~37%), 6 others show emission lines and no clear break (37%), and the remaining spectra are of insufficient quality. This translates to a failure rate of 37% (maximum 63%) in finding true BB candidates from our photometric method.

If we consider the full photometric sample (with secure spectroscopic redshifts), 27 sources show a BB above 1.2 out of 101 (~27%), comparable to or slightly higher than the inference from the spectra. If one assumes the above-mentioned failure rate (and range thereof) for the whole sample, ~10–17% will have a true BB, which is larger than the previously confirmed BB objects but comparable to the fraction of “smoldering” (i.e., with suppressed star-formation) galaxies around at $z \sim 7–8$ determined by Trussler et al. (2024b). In any case, with a total sample of 10–11 spectroscopically confirmed BB objects at $z > 7$, our work essentially triples the number of such objects.

Previously, JWST spectroscopic observations revealed 4 objects with significant BBs at $z > 7$ (Looser et al. 2024; Vikaeus et al. 2024; Wang et al. 2024a; Witten et al. 2024) and
a galaxy with a BB ~ 1 (Boyett et al. 2024), as also shown in Figs. 1 and 4³. These studies reported individual objects selected by different criteria, and which do not result from a systematic search for strong BB objects. Their BB measurements from spectroscopy agree well (within 1−2σ) with ours from photometry, as seen from Fig. 1, and the spectroscopic measurements are not affected by possible emission lines. Our sample not only adds new spectroscopically confirmed BB objects but also extends somewhat the magnitude range over which such objects are found, as shown in Fig. 4.

### 3.4. Origin of the observed BBs

None of the objects reported here show a BB exceeding any fundamental limit. In particular, the observed values of BB are within the range predicted for normal stellar populations and can be reached with ages less than the Hubble time, as shown earlier by Wang et al. (2024a) for the strongest BB object studied here. The BB strengths measured in this study are in agreement with the theoretical predictions of Binggeli et al. (2019) and Wilkins et al. (2024), where some simulations predict BB strength up to 1.5−2.0 in redshift ~10 with extinction. Smaller values are predicted by Steinhardt et al. (2024) who use, however, D4000, which shows a small dynamical range due to chosen wavelength windows. In short, our strong BB candidates do not break standard cosmology, but they might indicate the need for new star formation models or obscured star formation (see Fig. 5 in Steinhardt et al. 2024).

Most likely, the sample presented here represents the bursty nature of $z \ge 7$ galaxies. Indeed, many theoretical and observational studies have already proposed that star formation in the early Universe might be bursty and/or stochastic (e.g. Caputi et al. 2017; Faisst et al. 2019; Rinaldi et al. 2022; Endsley et al. 2024; Looser et al. 2023, 2024; Sun et al. 2023; Witten et al. 2024). These starburst phases are accompanied by temporary or permanent phases of quiescence, presumably due to various feedback mechanisms arising after the burst phase, which then allows the older stellar population (and the BB) to become visible in the integrated spectra. The co-existence of BBs with emission lines probably indicates the restart of star formation after a temporary quiescence (Witten et al. 2024). In summary, the observed range of BB strengths and the existence of BB galaxies with emission lines indicate the galaxies at $z \ge 7$ are transiting between on and off phases of star formation (cf. Witten et al. 2024; Looser et al. 2024; Trussler et al. 2024b). The presence of these galaxies provides constraints on the star-formation histories in the early Universe, and their detailed modeling will help us to constrain the onset of galaxy formation.

A more detailed study of the evolution of the BB in large galaxy samples and over a wider redshift range will be presented in a follow-up study (Kuruvanthodi et al., in prep.).

## 4. Conclusions

To progress towards establishing the occurrence and properties of galaxies showing BBs at $z > 7$, we have selected all galaxies with secure spectroscopic redshifts from the main extragalactic surveys (CEERS, JADES, PRIMER, and FRESCO) observed with JWST and measured the strength of the BB from

photometry. We have validated our method by comparison with spectroscopic measurements of the BB from the stacked spectra of Roberts-Borsani et al. (2024), and we have inspected the individual spectra to validate our measurements and assess the nature of the galaxies.

Our work has revealed three new galaxies from $z \sim 7.1–9.6$ with strong BBs and no signs of emission lines, which have most likely stopped star formation recently (so-called mini-quenched objects). We have also found 5–7 galaxies showing the co-existence of strong BBs and emission lines (Hβ, [O III]), which we interpret as signs of renewed star formation after a previously interrupted (quenched) phase. This approximately triples the number of spectroscopically confirmed galaxies at $z > 7$ showing clear signs of quenched or interrupted star formation, with respect to previous studies (Looser et al. 2024; Vikaeus et al. 2024; Wang et al. 2024a; Witten et al. 2024).

Overall, from our sample of 118 galaxies, 101 yielded BB measurements, with approximately 48% showing a net BB and ~27% a “strong” BB $> 1.2$. From a detailed inspection of the available spectra (for a subsample of 62 objects), we estimate that ~20% show a clear BB, out of which approximately half show no signs of ongoing star formation, and the rest show BB plus emission lines. The remaining objects with apparent strong BBs resemble LRDs or cannot well be classified. Our photometric BB measurements show that objects with strong BBs can be found over a wide range of magnitudes and stellar masses. From our analysis, we conclude that the fraction of mini-quenched or reactivated (bursty) galaxies at $z > 7$ is on the order of ~10–20%, at least in the sample studied here.

Our sample, showing a range of BB strength and line properties, highlights the bursty and stochastic nature of star formation in the early Universe and the existence of significant galaxies in a temporary phase of quiescence in the high-z Universe. Future work will be needed to understand the possible selection biases, establish how these trends evolved toward lower redshift, quantify the timescales involved in the observed star formation and intermittent phases, and work out more general implications.

**Acknowledgements.** We thank the anonymous referee for her/his insightful comments and suggestions. The data products presented herein were retrieved from the Dawn JWST Archive (DJA). DJA is an initiative of the Cosmic Dawn Center (DAWN), which is funded by the Danish National Research Foundation under grant DNRF140. We thank Gabe Brammer for making this tool available. We thank Prof. Corinne Charbonnel for her useful comments and suggestions. This work has received funding from the Swiss State Secretariat for Education, Research, and Innovation (SERI) under contract number MB22.00072, as well as from the Swiss National Science Foundation (SNSF) through project grant 200020_207349.

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<footnote n="3">³ One additional galaxy with spectroscopic redshift $z = 7.10$ and BB ~1.4 is reported in the work of Trussler et al. (2024b) mentioned above.</footnote>
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**Appendix A: Spectra of other Balmer break candidates**

Spectra of other BB candidates with emission lines are shown
here.

<figure><img src="image_6.png" /><figcaption>**Fig. A.1.** NIRSpec PRISM spectra of additional BB + emission line candidates at *z* ~ 7–9, showing objects with clear or plausible Balmer breaks. Binned and unbinned spectra are shown for illustration. For comparison, we also over-plot the spectrum of the mini-quenched *z* = 7.3 galaxy from Looser et al. (2024), redshifted and rescaled to approximately match the observed flux blueward of the BB.</figcaption></figure>
## Appendix B: Coordinates and properties of the Balmer break sources/candidates

<table><caption>**Table B.1.** Properties of BB candidates. The top panel shows the 6 BB galaxies discussed in the main text, and the bottom panel shows the additional BB candidates with emission lines.</caption><thead><tr><th>ID</th><th>z<sub>spec</sub></th><th>RA<br>(deg)</th><th>Dec<br>(deg)</th><th>BB</th><th>m<sub>F444W</sub><br>(mag)</th><th>EW(H<sub>β</sub>)<br>(Å)</th></tr></thead><tbody><tr><td>1210_5115</td><td>7.26</td><td>53.1528407</td><td>-27.8019455</td><td>1.42 ± 0.16</td><td>28.64 ± 0.08</td><td>&lt;21.55</td></tr><tr><td>1181_38684</td><td>7.48</td><td>189.1211092</td><td>62.2778222</td><td>1.41 ± 0.17</td><td>27.05 ± 0.09</td><td>&lt;59.36</td></tr><tr><td>1181_74111</td><td>8.53</td><td>189.1807727</td><td>62.1805329</td><td>1.02 ± 0.13</td><td>26.56 ± 0.05</td><td>&lt;36.52</td></tr><tr><td>1345_80026</td><td>9.56</td><td>214.8118531</td><td>52.7371100</td><td>1.39 ± 0.14</td><td>26.74 ± 0.07</td><td>&lt;137.80</td></tr><tr><td>1345_80372</td><td>7.48</td><td>214.9278186</td><td>52.8500015</td><td>1.52 ± 0.17</td><td>26.59 ± 0.05</td><td>130.39 ± 21.56</td></tr><tr><td>2565_15420</td><td>7.14</td><td>150.0990078</td><td>2.3436228</td><td>1.71 ± 0.12</td><td>23.86 ± 0.05</td><td>67.43 ± 14.96</td></tr><tr><td>1286_20085619</td><td>7.27</td><td>53.1910569</td><td>-27.7973172</td><td>1.17 ± 0.08</td><td>27.03 ± 0.05</td><td>47.70 ± 16.05</td></tr><tr><td>1286_20086025</td><td>7.27</td><td>53.1837525</td><td>-27.7938918</td><td>1.16 ± 0.08</td><td>26.52 ± 0.05</td><td>&lt;148.60</td></tr><tr><td>1345_80432</td><td>7.48</td><td>214.8120544</td><td>52.7467436</td><td>1.11 ± 0.09</td><td>26.22 ± 0.05</td><td>229.54 ± 59.55</td></tr><tr><td>1345_80025</td><td>7.65</td><td>214.8060747</td><td>52.7508628</td><td>1.46 ± 0.19</td><td>26.95 ± 0.08</td><td>70.58 ± 29.90</td></tr><tr><td>1181_54165</td><td>8.66</td><td>189.2718353</td><td>62.1951783</td><td>1.07 ± 0.15</td><td>27.23 ± 0.06</td><td>&lt;31.61</td></tr><tr><td>1286_36424</td><td>8.68</td><td>53.1760306</td><td>-27.8173945</td><td>1.21 ± 0.14</td><td>27.71 ± 0.05</td><td>&lt;72.98</td></tr></tbody></table>