Carbonaceous dust grains seen in the first
billion years of cosmic time
Joris Witstok1,2, Irene Shivaei3*, Renske Smit4*, Roberto Maiolino1,2,5, Stefano Carniani6, Emma
Curtis-Lake7, Pierre Ferruit8, Santiago Arribas9, Andrew J. Bunker10, Alex J. Cameron10,
Stephane Charlot11, Jacopo Chevallard10, Mirko Curti1,2,12, Anna de Graaff13, Francesco
D’Eugenio1,2, Giovanna Giardino14, Tobias J. Looser1,2, Tim Rawle15, Bruno Rodríguez del Pino9,
Chris Willott16, Stacey Alberts3, William M. Baker1,2, Kristan Boyett17,18, Eiichi Egami3, Daniel J.
Eisenstein19, Ryan Endsley20, Kevin N. Hainline3, Zhiyuan Ji3, Benjamin D. Johnson19, Nimisha
Kumari21, Jianwei Lyu3, Erica Nelson22, Michele Perna9, Marcia Rieke3, Brant E. Robertson23,
Lester Sandles1,2, Aayush Saxena5,10, Jan Scholtz1,2, Fengwu Sun3, Sandro Tacchella1,2, Christina
C. Williams24 & Christopher N. A. Willmer3
Large dust reservoirs (up to ~108 M⊙) have been detected1-3 in galaxies out to redshift z ∼ 8,
when the age of the Universe was only about 600 Myr. Generating significant amounts of
dust within such a short timescale has proven challenging for theories of dust formation4,5
and has prompted the revision of the modelling of potential sites of dust production6–8 such
as the atmospheres of asymptotic giant branch (AGB) stars in low-metallicity
environments, supernovae (SNe) ejecta, and the accelerated growth of grains in the
interstellar medium (ISM). However, degeneracies between different evolutionary
pathways remain when the total dust mass of galaxies is the only available observable. Here
we report observations of the 2175 Å dust attenuation feature, well known in the Milky
Way (MW) and galaxies at z ≲ 39–11, in the near-infrared spectra of galaxies up to z ∼ 7,
corresponding to the first billion years of cosmic time. The relatively short timescale
implied for the formation of carbonaceous grains giving rise to this feature12 suggests a
rapid production process, likely in Wolf-Rayet (WR) stars or SN ejecta.
As part of the JWST Advanced Deep Extragalactic Survey (JADES), we obtained deep
Near-Infrared Spectrograph (NIRSpec) multi-object spectroscopy taken in the PRISM
configuration (spectral range 0.6 μm to 5.3 μm, resolving power R ∼ 100). Using the NIRSpec
micro-shutter array (MSA), we observed 253 sources across three visits between 21 and 25
October 2022 (JWST programme 1210; PI: Lützgendorf), with exposure times per object ranging
1Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK. 2Cavendish Laboratory, University of
Cambridge, 19 JJ Thomson Avenue, Cambridge, CB3 0HE, UK. 3Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson
AZ 85721, USA. 4Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK. 5Department
of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK. 6Scuola Normale Superiore, Piazza dei Cavalieri
7, I-56126 Pisa, Italy. 7Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire,
Hatfield AL10 9AB, UK. 8European Space Agency, European Space Astronomy Centre, Madrid, Spain. 9Centro de Astrobiología (CAB),
CSIC–INTA, Cra. de Ajalvir Km. 4, 28850- Torrejón de Ardoz, Madrid, Spain. 10Department of Physics, University of Oxford, Denys Wilkinson
Building, Keble Road, Oxford OX1 3RH, UK. 11Sorbonne Université, CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis bd Arago,
75014 Paris, France. 12European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei Muenchen, Germany.
13Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117, Heidelberg, Germany. 14ATG Europe for the European Space Agency, ESTEC,
Noordwijk, The Netherlands. 15European Space Agency, Space Telescope Science Institute, Baltimore, Maryland, USA. 16NRC Herzberg, 5071
West Saanich Rd, Victoria, BC V9E 2E7, Canada. 17School of Physics, University of Melbourne, Parkville 3010, VIC, Australia. 18ARC Centre of
Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia. 19Center for Astrophysics | Harvard & Smithsonian, 60 Garden St.,
Cambridge MA 02138 USA. 20Department of Astronomy, University of Texas, Austin, TX 78712, USA. 21AURA for European Space Agency,
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21210, USA. 22Department for Astrophysical and Planetary Science,
University of Colorado, Boulder, CO 80309, USA. 23Department of Astronomy and Astrophysics University of California, Santa Cruz, 1156 High
Street, Santa Cruz CA 96054, USA. 24NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 North Cherry Avenue, Tucson, AZ
85719, USA.
*These authors contributed equally to this work.
Corresponding authors: Joris Witstok (jnw30@cam.ac.uk), Irene Shivaei (ishivaei@arizona.edu), Renske Smit (r.smit@ljmu.ac.uk).
from 9.3 to 28 hours. The extracted one-dimensional spectra reached a continuum sensitivity
(3σ) of ∼6-40 × 10−22 erg s−1 cm−2 Å−1 (∼27.2-29.1 AB magnitude) at ∼2 μm. Targets were selected
with a specific focus on high-redshift galaxies in imaging with the Hubble Space Telescope
(HST) and JWST/Near-Infrared Camera (NIRCam).
Through visual inspection of all spectra, we find strong evidence of the absorption feature
around a rest-frame wavelength λemit = 2175 Å in the spectrum of a galaxy at z = 6.71
(JADES-GS+53.15138-27.81917; JADES-GS-z6-0 hereafter), revealed via a significant (6σ)
deviation from a smooth power-law continuum, as shown in Fig. 1. This feature, known as the
UV attenuation “bump”, was first discovered by Stecher (1965) along sightlines in the MW9 and
is attributed to carbonaceous dust grains, specifically polycyclic aromatic hydrocarbons (PAHs)
or nano-sized graphitic grains12. We fitted a Drude profile around 2175 Å to the excess
attenuation13, defined as the observed spectrum normalised to a bump-free attenuated spectrum
that is predicted by a power-law function fitted outside of the bump region (see Methods). We
find a bump strength (amplitude) of  mag and a central wavelength λmax =  Å.0. 43−0.07
+0.07 2263
−24
+20
The latter, while within the range expected by models of carbonaceous grains14, is notably higher
than the range typically observed along different sightlines in the MW, potentially suggestive of
a change in grain mixture15. Beyond the local Universe, the feature has previously only been
observed spectroscopically in massive, metal-enriched galaxies at z ≲ 3, suggesting it originates
in dust grains exclusively present in evolved galaxies11,13,16–19. The detection reported here is the
first direct, spectroscopic detection of the UV bump in galaxies at z > 3.
The properties of JADES-GS-z6-0 are summarised in Table 1. This galaxy shows significant dust
obscuration as probed by the ratio of Balmer lines (“Balmer decrement”): here, Hα/Hβ ∼ 3.7
indicates a nebular extinction of E(B − V)neb = 0.25 ± 0.07 mag. In agreement with the trend
between metallicity and bump strength observed at lower redshift, measurements of the
gas-phase and stellar metallicity (Z ∼ 0.2-0.3 Z⊙) further suggest JADES-GS-z6-0 has undergone
substantial metal enrichment relative to galaxies with similar mass at the same redshift20.
To systematically investigate the prevalence of the UV bump and obtain clues on its origin at
such early times, we further selected JADES galaxies on the basis of a confident spectroscopic
redshift above z > 4 with a median signal-to-noise ratio (SNR) of at least 3 per spectral pixel in
the region corresponding to rest-frame wavelengths of 1268 Å < λemit < 2580 Å. This results in a
sample of 49 objects between redshift 4.02 and 11.48. Comparing the continuum slopes on both
sides of the central wavelength at 2175 Å (see Methods), we selected ten galaxies (at
4.02 < z < 7.20) from this parent sample whose spectral shape points towards the presence of a
UV bump.
We constructed a weighted average (“stack”; see Methods) of all 49 objects in our parent sample
as well as our ten selected objects with evidence for a bump signature, as shown in Fig. 2. In
both stacks, we find evidence for emission from the C III λ 1907, 1909 Å nebular lines that are
commonly seen in metal-poor galaxies21. There is no indication of the bump in the parent
sample; the stacked spectrum of the ten selected objects, however, shows a clear depression (5σ)
centred on ∼2175 Å. Though we do not find evidence for significant differences in stellar
properties (i.e. mass or age), these ten galaxies are characterised by a considerable amount of
dust obscuration, comparable to z ∼ 2 galaxies with the bump feature18, and mildly enhanced
metallicities compared to the parent sample (see Methods). We again fitted a Drude profile to the
excess attenuation in the stacked spectrum of these ten objects, finding a bump amplitude of
 mag and a central wavelength λmax =  Å.0. 10−0.01
+0.01 2236
−20
+21
While the UV bump has long been known to exist, its variable presence and strength has been an
open topic of debate in galaxy evolution studies13,22. The feature is commonly attributed to
PAHs12, molecules thought to be susceptible to destruction by hard ionising radiation, and it is
present in the MW and Large Magellanic Cloud (LMC) extinction curves, but very weak or
absent in the Small Magellanic Cloud (SMC) curves23. In the attenuation curve of individual
galaxies, radiative-transfer effects determined by the dust-star geometry can weaken the bump in
the observed integrated spectrum24,25. However, by stacking the photometry of large samples of
galaxies, the bump has been detected to varying degrees at redshifts z ≲ 3, with tentative hints at
z ≲ 611,17,19,26. Spectroscopically, the bump has only been seen in relatively massive and dusty
individual galaxies at z ∼ 213,18. In Fig. 3, the bump amplitude is shown as a function of cosmic
time, including its strength in the extinction curves in MW, LMC, and SMC sightlines23. Our
inferred bump amplitude and central wavelength, especially in the individual spectrum of
JADES-GS-z6-0, are comparatively high, the former defying the trend with stellar mass seen at
lower redshift. This may point towards a different nature of the grains responsible for the
absorption (e.g. graphite instead of PAHs) in addition to a different, likely simpler, dust-star
geometry compared to lower-redshift counterparts – intriguingly, there is tentative evidence for a
colour gradient in JADES-GS-z6-0 (see Methods).
Moreover, a direct detection of the bump at z ∼ 4-7 is striking given that at these redshifts, the
age of the Universe is only around a billion years (∼800 Myr at z = 6.71). Substantial production
of carbon and subsequent formation of carbonaceous grains responsible for the absorption
feature through the standard AGB channel, particularly in the low-metallicity regime
characterising such early galaxies (i.e. Z ∼ 0.1 Z⊙20), would require low-mass (M ≲ 2.5 M⊙) and
hence long-lived stars to reach the AGB at the end of their lives, after more than 300 Myr7. If this
is the dominant channel via which carbonaceous grains are formed, the presence of the UV bump
implies the onset of star formation in these galaxies occurred within the first half billion years of
cosmic time, corresponding to redshift z ≳ 10. Indeed, star formation has been shown to occur at
this early epoch with the confirmation of z > 10 galaxies27. However, in our sample we do not
find evidence for substantial star formation activity that occurred on timescales beyond 300 Myr
(see Methods). The absence of clear signatures from such relatively old stellar populations
suggests that other, faster channels for the production of carbonaceous dust are required in these
early systems, corroborated by the high observed frequency of extremely metal-poor MW stars
that are carbon enhanced28.
One explanation is that these grains are formed on significantly shorter timescales via more
massive and rapidly evolving stars, possibly by SNe or WR stars, which would overhaul some,
and place strong constraints on other, theoretical models of dust production and stellar evolution.
PAH production has indeed been observed in WRs29, and while subsequent SN type-Ib/c
explosions are generally expected to destroy most dust produced in the preceding WR phase,
models have shown that carbonaceous grains produced by binary carbon-rich WRs (WCs) can
survive30. However, for standard initial mass functions (IMFs), WR and in particular WC stars
are rare31. Conversely, isotopic signatures in presolar graphite grains found on primitive
meteorites indicate a type-II SN origin, suggesting the production of these potential carriers of
the UV bump starts at early times32. Indeed, dust production in SN ejecta has been regarded as a
potential rapid channel for significant dust production in the early Universe33, its net efficiency
depending on the grain destruction rate in the subsequent reverse shock34. However, substantial
carbonaceous production in SN ejecta is expected only by some classes of models and for a
certain subclass of scenarios (e.g. non-rotating progenitors), while other models favour the
formation of silicates or other types of dust35–38. In summary, our detection of carbonaceous dust
at z ∼ 4-7 provides crucial constraints on the dust production models and scenarios in the early
Universe.
 Table 1 | Properties of JADES-GS-z6-0.
 
RA (deg) +53.15138
Dec (deg) −27.81917
texp (h) 27.9
zspec 6. 70647
−0.00033
+0.00044
mF115W (mag) 28. 58 ± 0. 12
MUV (mag) –18. 34 ± 0. 12
βUV –2. 13
−0.18
+0.18
γ34 –3. 6
−1.2
+1.5
Zneb (Z⊙) 0. 17
−0.04
+0.05
E(B − V)neb (mag) 0. 25 ± 0. 07
M∗ (108 M⊙) 1. 0
−0.2
+0.3
Z∗ (Z⊙) 0. 34
−0.05
+0.05
SFR30 (M⊙ yr-1) 3. 0
−1.0
+2.0
t∗ (Myr) 18
−7
+11
 
 Error bars represent a 1σ uncertainty. Rows: (1) Right Ascension (RA) in J2000, (2) Declination (Dec) in J2000, (3)
Exposure time (texp) in the NIRSpec PRISM spectra in hours, (4) Spectroscopic redshift (zspec), (5) Apparent AB
magnitude in the NIRCam F115W filter (mF115W), (6) Absolute AB magnitude in the UV (MUV), (7) UV spectral
slope (βUV), (8) Spectral slope change around λemit = 2175 Å (γ34), (9) Gas-phase metallicity (Zneb; from rest-frame
optical emission lines) in units of Solar metallicity, (10) Nebular extinction (E(B − V)neb; from the Balmer
decrement) in magnitudes, (11) Stellar mass (M∗) in 108 Solar masses, (12) Stellar metallicity (Z∗; from SED
modelling) in units of Solar metallicity, (13) Star formation rate in Solar masses per year averaged on a timescale of
30 Myr (SFR30), (14) Mass-weighted stellar age (t∗) in Myr.
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 Fig. 1 | Spectrum taken by JWST/NIRSpec of JADES-GS-z6-0 at redshift z = 6.71. a,
Overview of the spectrum (grey solid line) with a power-law fit to the UV continuum (blue solid
line). Several spectral features used for spectroscopic redshift confirmation are indicated,
including the Lyman-α break, the [O II] λ 3727, 3730 Å doublet, the Hβ, Hγ and
[O III] λ 4960, 5008 Å lines. b, Zoom-in of the UV bump region around λemit = 2175 Å where a
running median (solid black line), representing the attenuated stellar continuum, reveals a deep
localised absorption profile. A Drude profile fit within the vertical dashed lines (purple solid
line) with respect to the smooth power law (blue solid line) yields an amplitude of  mag0. 43
−0.07
+0.07
and a central wavelength λmax =  Å. The hatched region indicates the2263−24
+20
C III λ 1907, 1909 Å doublet. c, The residuals (∆Fλ) show that the power-law (PL) fit alone has a
significant negative flux excess between ∼ 2000 Å and 2400 Å (6.4σ), while the power-law and
Drude profile combined (PL+Drude; purple line) provides a significantly better fit (chi squared
of χ2 = 72.5 versus χ2 = 5.0 for PL and PL+Drude, respectively). All shading represents 1σ
uncertainty.
 Fig. 2 | Normalised and stacked spectra around the UV bump of z > 4 JADES galaxies
observed by JWST/NIRSpec. Spectra of all galaxies (small black dots) are shifted to the rest
frame and normalised to the predicted continuum level at a rest-frame wavelength of
λemit = 2175 Å in the absence of a UV bump (see Methods section). The dashed black line
(shading represents 1σ uncertainty) shows a stacked spectrum obtained by combining all 49
objects in wavelength bins of Δλemit = 20 Å, clearly revealing emission from the
C III λ 1907, 1909 Å doublet in the hatched region. The stacked spectrum of ten galaxies
selected to have a bump signature (solid black line, shading as 1σ uncertainty), in addition to
appearing to have a mildly redder UV slope, shows the presence of the UV bump around 2175 Å
having an excess with respect to a power-law continuum (solid blue line; see Methods) at a
significance of 5.4σ. The excess attenuation Aλ, bump (curve at the bottom, corresponding to the
axis on the right) is fitted with a Drude profile (shown in purple with shading as 1σ uncertainty),
where we find an amplitude of  mag and a central wavelength λmax =  Å.0. 10−0.01
+0.01 2236
−20
+21
 
 
 Fig. 3 | Redshift evolution of UV bump strength. The amplitude of the excess attenuation,
Aλ, max, is shown for JADES-GS-z6-0 individually as well as for the stack of ten z ∼ 4-7 JADES
galaxies. Points are coloured according to their (average) stellar mass; error bars along the y-axis
represent 1σ uncertainty. At z ∼ 2, measurements from gamma-ray burst absorbers (Heintz et al.
and references therein) and from stacked spectra in various bins of stellar mass (Shivaei et al.) or
shape of the UV continuum as a whole and in the bump region (Noll et al.) are shown (see
Methods for details)13,18,39. Error bars of the stacked spectra along the x-axis represent the full
redshift range, their central values slightly shifted for visualisation purposes. The bump
amplitudes in the average MW, LMC, and SMC dust extinction curves23,40, converted to an
attenuation for a visual extinction range of 0.1 mag < AV < 0.5 mag, are indicated with light
shadings. The age of the Universe is indicated at the top. A vertical dashed line indicates the
minimum timescale required for carbon production by AGB stars (i.e. 300 Myr) if the galaxy
formed at zform = 10.
 
Methods
Data and parent sample
The observations presented here were taken as part of JADES41, a joint survey conducted by the
JWST42,43 NIRCam44 and NIRSpec45,46 Guaranteed Time Observations (GTO) instrument science
teams. As described in Robertson et al.47 and Curtis-Lake et al.27, deep NIRCam imaging48 over a
wavelength range λobs ≃ 0.8 μm to 5 μm (reaching mAB ∼ 30 mag in F200W) was taken under
JWST programme 1180 (PI: Eisenstein) in an area of 65 arcmin2 over the Great Observatories
Origins Deep Survey – South (GOODS-S), which includes the Hubble Ultra Deep Field
(HUDF). We additionally make use of public medium-band imaging taken as part of the JWST
Extragalactic Medium-band Survey (JEMS49; JWST programme 1963, PI: Williams) and First
Reionization Epoch Spectroscopic COmplete Survey (FRESCO50; JWST programme 1895, PI:
Oesch). Incorporating the wealth of publicly available ancillary data from HST, a catalogue with
photometric redshifts was constructed to identify high-redshift galaxy candidates. NIRSpec
multi-object spectroscopy51 of these NIRCam-selected sources was taken using the MSA52 in the
PRISM/CLEAR spectral configuration, covering a spectral range 0.6 μm to 5.3 μm with
resolving power R ∼ 100. A three-point nodding pattern was implemented for background
subtraction in addition to small dithers with MSA reconfigurations to increase sensitivity and
flux accuracy, improve spatial sampling, mitigate the impact of the detector gaps, and aid the
removal of cosmic rays. Dither pointings consisted of four sequences of three nodded exposures.
Each setup was made up of two integrations of 19 groups, resulting in exposure times of 8403.2
seconds for each sequence and of 33612 seconds (9.3 hours) for each dither pointing27. The main
galaxy considered in this work (JADES-GS-z6-0) was observed in three visits, resulting in an
integration time of 27.9 hours (Table 1), while other targets had exposure times ranging between
9.3 hours and 27.9 hours. The spectral energy distribution (SED) and a false-colour image of
JADES-GS-z6-0 with the location of the NIRSpec MSA shutters overlaid is shown in Extended
Data Fig. 1. We note the imaging reveals a tentative colour gradient, with the shutter capturing
the redder part of the galaxy which may contribute to the strength of the UV bump in the
spectrum of JADES-GS-z6-0.
Flux-calibrated two-dimensional spectra and one-dimensional spectral extractions were obtained
with pipelines developed by the ESA NIRSpec Science Operations Team (SOT) and the
NIRSpec GTO team, which will be discussed in detail in a forthcoming paper. The pipeline
generally adopts the same algorithms included in the official STScI pipeline that generates
MAST archive products. An irregular wavelength grid with 5 spectral pixels per resolution
element was adopted to avoid oversampling of the line spread function at short wavelengths
(λobs ∼ 1 μm). Extraction of the one-dimensional spectra was performed with a 5-pixel aperture
covering the entire shutter size to recover all emission. However, as in Curtis-Lake et al.27, we
considered an additional extraction over a 3-pixel aperture to test the robustness of our findings,
as will be discussed in the Robustness of the UV bump detections section. Given the compact
sizes of the high-redshift galaxies considered here (see Extended Data Fig. 1), slit-loss
corrections were applied under the assumption of a point-like source placed at the relative
intra-shutter position of each galaxy. We note that systematic uncertainties in the slit-loss
correction will be a smooth function of wavelength and will therefore not affect the UV bump
signature, which instead relies on the detection of the UV slope inflection over a relatively small
wavelength range around λemit = 2175 Å (as discussed in the next sections). Extraction was
performed in a shutter-size aperture to recover all emission. Further details regarding the target
selection and data reduction are extensively discussed in preceding JADES works27,41,47,48,51.
Sample selection
After an automated spectral fitting routine with BAGPIPES (Bayesian Analysis of Galaxies for
Physical Inference and Parameter EStimation) code53, spectroscopic redshift estimates were
confirmed by visual inspection independently by at least two team members. The final redshift
values were determined by a subsequent analysis (described in detail in Chevallard et al., in
prep.) with the BEAGLE (BayEsian Analysis of GaLaxy sEds) code54, as described in Curtis-Lake
et al.27 but with a star-formation history (SFH) consisting of a 10 Myr long star-formation burst
combined with a delayed exponential component, and a narrow redshift prior distribution centred
around the visually confirmed redshifts. We selected objects on the basis of a confident
spectroscopic redshift above z > 4 to ensure the rest-frame UV coverage includes the Lyman-α
break. Based on the formal uncertainty, we further selected spectra with median SNR of at least 3
in the region corresponding to rest-frame wavelengths of 1268 Å < λemit < 2580 Å.
We then performed several Bayesian power-law fitting procedures to the rest-frame UV
continuum with a Python implementation55 of the MultiNest56 nested sampling algorithm. To
identify spectra exhibiting a UV bump, we fitted power laws in four adjacent wavelength
windows defined by Noll et al.57 (with corresponding power-law indices γ1 to γ4), excluding the
region 1920 Å < λemit < 1950 Å to avoid contamination by the C III doublet. In the presence of
the UV bump, the spectral shape of the rest-frame UV is characterised by a strong turnover in
power-law slope directly blue- and redwards of 2175 Å covered by regions 3 and 4 respectively,
resulting in a negative γ34 ≡ γ3 − γ4 value. Before fitting these separate wavelength windows in the
individual spectrum, we applied a running median filter over 15 spectral pixels that cover 3× the
spectral resolution. We estimated the uncertainty on the running median with a bootstrapping
procedure where we randomly perturb each of the 15 spectral pixels according to their formal
uncertainty for 100 iterations.
In the fitting algorithm, a likelihood was calculated based on the inverse-variance weighted
squared residuals between a given model and the observed spectrum within the adopted spectral
regions. We chose flat prior distributions for the power-law indices (ranging between −5 < γi < 1)
and normalisation at the centre of each wavelength window (between 0 and twice the maximum
value of the spectrum in the fitting regions). Best-fit values of γ34, whose posterior distribution
was obtained from simultaneously fitting γ3 and γ4, are shown in Extended Data Fig. 2 as the 50th
percentile (i.e. median) with 16th and 84th percentiles as a ±1σ confidence range. A selection of
galaxies with median value of γ34 < −1, in addition to γ34 < 0 within the 1σ uncertainty range, led
to the identification of ten galaxies (including JADES-GS-z6-0) with evidence for a UV bump
(the “bump sample”). The next section will discuss the physical properties of this subsample in
the context of the full sample. Coordinates and other properties of these ten galaxies are reported
in Extended Data Table 1.
Physical properties
We consistently used a flat ΛCDM cosmology based on the results of the Planck collaboration58
(i.e. H0 = 67.4 km s−1 Mpc−1, Ωm = 0.315) throughout. Several of the main physical properties of
the full sample are presented in Extended Data Fig. 2. Extended Data Table 1 lists observed
properties of the ten individual galaxies in the bump sample, while Extended Data Table 2
reports median values for the bump sample, the sample of galaxies not contained in the bump
sample (the “non-bump sample”), and the full sample, as well as values measured from the
stacked spectra.
UV magnitudes and slopes
We derived UV magnitudes directly from NIRCam photometric data points probing a rest-frame
wavelength of ∼1500 Å (F115W in the case of JADES-GS-z6-0; see Table 1), if available (we
note several targets fall outside of the NIRCam footprint). We fitted an overall UV slope βUV to
the rest-frame UV continuum probed by the NIRSpec PRISM measurements using a similar
Bayesian power-law fitting procedure as described in the Sample selection section. We adopted
the spectral windows defined by Calzetti et al.59 which are designed to exclude several UV
emission and absorption features. Indeed, no strong emission lines are observed within these
spectral regions of our low-resolution spectra, and importantly they explicitly exclude the bump
region and C III emission lines. We chose a Gaussian prior distribution for the power-law index
(centred on μβ = −2 with a width of σβ = 0.5) and a flat prior on the normalisation at
λemit = 1500 Å (between 0 and twice the maximum value of the spectrum in the fitting regions).
The resulting UV slope of JADES-GS-z6-0 is reported in Table 1.
Spectroscopic rest-frame optical properties
Emission line fluxes in the NIRSpec PRISM measurements of individual galaxies in our sample
were obtained using the pPXF software60 (for details, we refer to Curti et al.20). We converted
Hα/Hβ line ratios into a nebular extinction E(B − V)neb under the Cardelli et al.61 extinction curve,
assuming an intrinsic ratio of Hα/Hβ = 2.86 appropriate for case-B recombination, Te = 104 K,
and ne = 100 cm−3 (e.g. ref.62). We note that for JADES-GS+53.13423-27.76891 at z = 7.0493 the
Hα line is located precisely on the edge the PRISM spectral coverage, causing the measured
Hα/Hβ ratio to appear significantly below the theoretical value of Hα/Hβ = 2.86 expected in the
absence of dust. Moreover, we caution that potential wavelength-dependent slit-loss effects could
bias the Hα/Hβ measurement (although minimally as the objects in this analysis are not
significantly resolved) and that the stellar and nebular extinction have a non-trivial dependence;
however, despite such systematic uncertainties galaxies strongly obscured by dust are still
expected to be identifiable via elevated Hα/Hβ line ratios.
Gas-phase oxygen abundances in our sample were derived primarily exploiting the detection of
multiple emission lines, where available, in NIRSpec medium-resolution (R ∼ 1000) grating/filter
configurations (G140M/F070LP, G235M/F170LP, G395M/F290LP) taken alongside the PRISM
spectroscopic observations (details are discussed in Curti et al.20). For targets that were not
covered by R ∼ 1000 observations, the PRISM spectra were considered. More specifically, we
required a minimum 3σ detection on [O III] λ 5008 Å, [O II] λ 3727, 3730 Å, [Ne III] λ 3870 Å,
and Hβ to include these lines into the metallicity calculation. On the basis of detected emission
lines, we combined the information from the R3, R23, O32, and Ne3O2 line-ratio diagnostics,
adopting the calibrations described in Nakajima et al.63 In the case where only [O III] λ 5008 Å
and Hβ were detected, and therefore R3 was the only available line ratio, upper limits on
[O II] λ 3727, 3730 Å and [N II] λ 6584 Å were exploited to discriminate between the high- and
low-metallicity solutions of the double-branched R3 calibration. The full procedure is described
in more detail in Curti et al.20. We quote the gas-phase metallicity (Zneb) in units of Solar
metallicity (Z⊙), assuming 12 + log10 (O/H)⊙ = 8.69 as the Solar oxygen abundance64.
We further explored the rest-frame optical properties of our samples by considering composite
spectra around the strong optical emission lines in Extended Data Fig. 3. These stacked spectra
were obtained equivalently as will be described in the Spectral stacking section, but with bins of
∆λemit = 10 Å given the increased spectral resolution of NIRSpec at longer wavelengths45. For the
purpose of studying the Balmer decrement, we only included galaxies where Hα is observable
(i.e. we did not consider objects at z > 7.1, leaving out one source in the bump sample). We
obtained fluxes of the main emission lines (i.e. [O II] λ 3727, 3730 Å, [O III] λ 4960, 5008 Å,
Hβ, and Hα) by fitting Gaussian profiles, shown in Extended Data Fig. 3. Measured line ratios
are reported in Extended Data Table 2.
Stellar population synthesis modelling
We employed the BAGPIPES code53 to model the SED simultaneously probed by the NIRSpec
PRISM measurements and NIRCam photometry, for which we use a conservative 10% error
floor. As underlying stellar models, we used the Binary Population and Spectral Synthesis
(BPASS31) v2.2.1 stellar population synthesis models including binary stars under the default
BPASS IMF, having a slope of −2.35 (for M > 0.5 M⊙) and ranging in stellar mass from 1 M⊙ to
300 M⊙. Aiming for a model that is simple yet able to capture older stellar populations, we
adopted a constant SFH with a minimum age varying between 0 (i.e. ongoing star formation) and
500 Myr, and a maximum age varying between 1 Myr and the age of the Universe. The total
stellar mass formed was varied between 0 and 1015 M⊙, and the stellar metallicity between 0 and
1.5 Z⊙. Nebular emission was included in a self-consistent manner using a grid of Cloudy65
models parametrised by the ionisation parameter (−3 < log10 U < −0.5). We chose a flexible
Charlot & Fall66 dust attenuation prescription with varying visual extinction (0 < AV < 7 mag) and
power-law slope (0.4 < n < 1.5), while we fixed the fraction of attenuation arising from stellar
birth clouds to 60% (the remaining fraction originating in the diffuse ISM; e.g. ref.67). We note
the Calzetti et al.59 dust attenuation curve yields consistent results. A first-order Chebyshev
polynomial (described in Carnall et al.68) was included to account for aperture and
flux-calibration effects in the spectroscopic data. The detailed properties of JADES-GS-z6-0 are
reported in Table 1. Moreover, the resulting stellar masses (M∗), star formation rates (SFRs)
averaged over the last 30 Myr (SFR30), and mass-weighted stellar ages (t∗) inferred from SED
models of the entire sample are presented in Extended Data Fig. 2. Median values of all
properties for the galaxy sample with and without evidence for a UV bump are reported in
Extended Data Table 2.
Stellar population age determination
We further explored whether the apparent absence of a significantly older stellar population (i.e.
t∗ > 300 Myr) could be explained by an “outshining effect” due to a more recent burst of star
formation69. Indeed, there is evidence that a significant fraction (20% to 25%) of reionisation-era
galaxies (z ≳ 6) host such evolved stellar populations70,71. Taking the best-fit parameters in our
BAGPIPES model, we added an instantaneous burst of star formation to the original model with a
single (constant) SFH component. Comparing the reduced chi-squared values between the
original, single-component model and the new two-component model (accounting for an
additional three model parameters, namely stellar mass, metallicity, and age of the burst), we
inferred, from a stellar population synthesis modelling point of view, how large a stellar mass can
be “disguised” in an evolved stellar population. This is illustrated in Extended Data Fig. 4,
showing the age-sensitive 4000 Å (Balmer) break. To avoid systematic uncertainties due to flux
calibration and/or slit losses in the spectrum, we restricted the chi-squared analysis to the
photometry. We determined the difference in reduced chi-squared values with
, where ( ) is the reduced chi-squared metric of∆χ
ν
2 = χ
ν, 𝑒𝑣𝑜𝑙𝑣𝑒𝑑
2 − χ
ν, 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙
2 χ
ν, 𝑜𝑟𝑖𝑔𝑖𝑛𝑎𝑙
2 χ
ν, 𝑒𝑣𝑜𝑙𝑣𝑒𝑑
2
the single-component (two-component) model. From this conservative estimate, we cannot
definitively rule out the existence of an additional population of evolved stars; for example, for
(i.e. at 2σ or 95% confidence) JADES-GS-z6-0 can have up to 5.5 · 107 M⊙∆χν
2 = 4
(9.6 · 107 M⊙), 0.55× (0.95×) its inferred stellar mass, in a 250 Myr (500 Myr) old burst of star
formation. This scenario, however, where a galaxy builds up more than half of its stellar mass
following an extended period (i.e. more than 250 Myr) with little or no star formation, is
physically implausible given the smooth SFH expected for relatively massive galaxies in this
early epoch (M∗ ≳ 108 M⊙)72. Even a more stochastic mode of star formation is not likely to
undergo such a lengthy quiescent period, suggesting that the SEDs should reveal detectable
signatures of stars with intermediate ages (∼100 Myr) if star formation activity can truly be
traced back over a time period required for AGB stars to produce significant amounts of dust.
Instead, we constrain an additional 100 Myr old component to have at most 0.31× the current
stellar mass (∼3 · 107 M⊙; 2σ). This suggests that more than half, if not most, of the stellar mass
in JADES-GS-z6-0 was built up in less than 100 Myr. Finally, we note that stacked rest-frame
optical spectra (discussed in the Spectroscopic rest-frame optical properties section), when
normalised to the continuum at λemit ∼ 3600 Å, equally do not reveal a strong Balmer break either
in the bump sample or in the full sample, further supporting the finding that these galaxies have
relatively young stellar populations.
Ancillary far-infrared observations
To search for additional signatures of dust obscuration, we considered archival Atacama Large
Millimeter/submillimeter Array (ALMA) 1.2 mm and 3 mm continuum imaging taken over
GOODS-S. All sources in our sample are contained within the combined 1.2 mm data of the
ALMA twenty-six arcmin2 survey of GOODS-S at one millimeter (ASAGAO73; ALMA project
code 2015.1.00098.S, PI: K. Kohno) survey, which includes the ALMA HUDF74 (project code
2012.1.00173.S, PI: J. Dunlop) and GOODS-ALMA75 (project code 2015.1.00543.S, PI: D.
Elbaz) surveys and reaches a continuum sensitivity of ∼78 μJy (3σ). A further 15 sources,
including three sources in the bump sample (JADES-GS+53.17022-27.77739,
JADES-GS+53.16743-27.77548, and JADES-GS+53.16660-27.77240), are covered by the
ALMA SPECtroscopic Survey (ASPECS76,77; project code 2013.1.00146.S, PI: F. Walter),
reaching a 3σ continuum sensitivity of ∼38 μJy at 1.2 mm and ∼11.4 μJy at 3 mm. None of the 49
sources in our sample, however, show a significant detection (3.5σ) in either dataset. A stacking
procedure similarly does not yield any detectable continuum emission, neither for the sources in
the bump sample nor for the full sample, indicating that the non-detections can be explained by
the relatively low sensitivity of the ALMA mosaics. Indeed, we have verified that for a typical
SFR of a few solar masses per year (as inferred for JADES-GS-z6-0), even a conservatively high
fraction (50%) of dust-obscured star formation results in an infrared luminosity that requires
several tens of hours to secure a confident detection (LIR ∼ 1010 L⊙, translating into a continuum
flux density of Fν ∼ 5 μJy in band 6).
Bump parametrisation and fitting procedure
Given an observed flux density profile Fλ, we parametrised the UV bump profile by defining the
excess attenuation as in Shivaei et al.13, Aλ, bump = −2.5 log10 (Fλ/Fλ, cont). For the individual
spectrum of JADES-GS-z6-0, we took the power-law fit with UV slope βUV measured outside the
bump region as the attenuated spectrum without a bump, Fλ, cont. When considering the excess
attenuation in the individual spectrum of JADES-GS-z6-0, we again used the running median
and corresponding uncertainty (described in the Sample selection section), which was
additionally used to compute the significance of the negative flux excess of the spectrum with
respect to the power-law fit alone (panel c of Fig. 1). We note the formal uncertainty of each
spectral pixel is scaled upwards to include the effects of covariance between adjacent pixels; we
have verified a similarly high significance is found when bootstrapping a spectrum first rebinned
to match the spectral resolution element (thereby largely negating the effects of correlated noise).
Using the MultiNest56 nested sampling algorithm, we fitted the excess attenuation Aλ, bump with a
Drude profile78, which has been shown to appropriately describe the spectral shape of the
bump13,18,79. Centred on rest-frame wavelength λmax, it is parametrised as
𝐴
λ, 𝖻𝗎𝗆𝗉
= 𝐴
λ, 𝗆𝖺𝗑
γ2/λ2
1/λ2−1/λ
𝗆𝖺𝗑
2( )2+γ2/λ2
where the full width at half maximum (FWHM) is given by FWHM = γλmax2. We fixed
γ = 250 Å/(2175 Å)2 which, if λmax = 2175 Å, corresponds to FWHM = 250 Å in agreement with
what has been found for z ∼ 2 star-forming galaxies13,18. Again motivated by the spectral
windows defined by Calzetti et al.59, we performed the fitting procedure in a region of
1950 Å ≤ λemit ≤ 2580 Å (reflecting the γ3 and γ4 regions discussed in the Sample selection
section), which excludes the C III doublet. As a prior for the bump amplitude Aλ, max, we
conservatively chose a gamma distribution with shape parameter a = 1 and scale θ = 0.2, which
favours the lowest amplitudes (noting a flat prior yields comparable results). For the central
wavelength, we adopted a flat prior in the range 2100 Å < λmax < 2300 Å.
Spectral stacking
To perform a spectral stacking analysis, we shifted each spectrum to rest-frame wavelengths λemit
and normalised it to the value of the power-law fit at λemit = 2175 Å. The individual continuum
spectra and corresponding uncertainties are rebinned to bins of ∆λemit = 20 Å using SpectRes80.
Stacked continuum profiles were created by weighting each binned data point by its inverse
variance, though we note we obtain similar results with an unweighted average. The stacked
continuum profile Fλ of the ten galaxies with evidence for a UV bump is converted to an excess
attenuation as described in the previous section, where for the “bumpless” profile (Fλ, cont), we
refitted a power-law continuum to the stacked continuum profile of the ten galaxies, noting the
difference in slope (measured to be β ∼ −1.95) compared to the stacked spectrum of the full
sample of 49 galaxies (with β ∼ −2.12, see Extended Data Table 2). To ensure good agreement
with the observed continuum outside of the region used in the bump fitting procedure, this
power-law was determined from the Calzetti et al.59 windows bluewards of λemit = 1850 Å
(explicitly excluding the C III doublet region), whereas at wavelengths beyond the bump region,
we consider the windows 2500 Å < λemit < 2600 Å and 2850 Å < λemit < 3000 Å (avoiding
potential Mg II doublet emission at λemit ∼ 2800 Å). Fitting a Drude profile78 yields an amplitude
of  mag and a central wavelength λmax =  Å. We note the amplitude remains0. 10−0.01
+0.01 2236
−20
+21
effectively unchanged if we instead fix the central wavelength to λmax = 2175 Å.
Robustness of the UV bump detections
JADES-GS-z6-0
To test the robustness of the UV bump identification in JADES-GS-z6-0, we extracted
one-dimensional spectra from the three separate observing visits to show that the feature around
2175 Å is not dominated by a single observation. This is illustrated in Extended Data Fig. 5,
which shows measurements from the individual visits normalised to its power-law fit. We
furthermore tested our extraction of the one-dimensional spectra using different apertures on the
reduced two-dimensional spectra (see the Data and parent sample section). This slightly lowers
the average continuum flux level and SNR, but we find no significant changes to the rest-frame
UV spectrum. We also compared NIRCam apodised photometry (the total background-subtracted
NIRCam flux passing through the NIRSpec MSA slit) to synthetic photometry obtained from
convolving the PRISM spectra with NIRCam filters. We verified that for most sources the two
fluxes are offset by a constant factor with offsets smoothly varying as a function of wavelength.
Finally, we note the attenuation feature is a highly localised region in the low-resolution PRISM
spectra (a rest-frame width of 250 Å is sampled by ∼6 independent spectral resolution elements
at a resolution of R(λobs ∼ 1.7 μm) ∼ 50) such that its magnitude will not be significantly affected
by the absolute flux calibration. At the same time, this wavelength range is probed by more than
10 native detector pixels, indicating the chances that this feature is produced by correlated
detector noise or other artefacts are minimal.
Stacked spectra
In this section, we discuss the robustness of the identification of the bump feature in our stacked
spectra. Firstly, randomly splitting our bump sample into two subsamples, we have confirmed the
bump signature remains present in both, implying the stacked spectrum is not dominated by a
single source. Further, we have verified that performing an analogous stacking procedure at a
different wavelength (2475 Å), for a subset of sources selected based on the continuum shape
around 2475 Å in an equivalent manner as the γ34 selection described in the Sample selection
section, does not produce a significant broad absorption feature as in Fig. 2. Instead, this results
in a narrow negative excess with positive excess on the edge of our fitting window, hence
yielding a substantially lower amplitude when fitted with a Drude profile.
We now turn to explore various properties of the different samples measured by NIRSpec and
NIRCam to test whether the bump signature could purely be due to random noise fluctuations, in
which case the ten selected galaxies are expected to simply be a random subset of the parent
sample. As seen in Extended Data Fig. 2, we find a significant correlation (i.e. p < 0.05 for the
null hypothesis that the data is uncorrelated) between on the one hand the UV slope inflection
around 2175 Å, γ34, and on the other hand absolute UV magnitude MUV. Our selected bump
sample is measured to be intrinsically fainter in the rest-frame UV (higher MUV, independent of
the SED modelling). This may be indicative of the absence of young stellar populations, in line
with the theoretically predicted trend of decreasing bump strength with increasing star formation
activity, and hence the intensity and hardness of UV radiation field81. Moreover, several of the
median properties hint at systematically different physical conditions in galaxies part of the
bump sample: in particular, these objects exhibit a significantly enhanced Hα/Hβ ratio, indicating
that on average the nebular emission in these galaxies experiences a higher degree of dust
obscuration, with nebular extinction values comparable to those of z ∼ 2 galaxies with a UV
bump18. Moreover, their slightly elevated gas-phase oxygen abundances indicate that they are
more metal enriched (see Extended Data Table 2). Interestingly, however, the stellar masses of
the bump sample are significantly lower than their z ∼ 2 counterparts, as pointed out in Fig. 3. We
note that other factors such as geometry could play an important role in determining the strength
of the UV bump, though larger samples are needed to confirm these trends.
To avoid potential biases by contaminants in our γ34-selected sample in the correlations based on
individual galaxy properties, we further turn to explore the stacked spectra. For instance, we note
the bump and non-bump samples appear to be characterised by a comparable median UV slope
as measured in the individual spectra, which is confirmed by the agreement of UV slopes in
unweighted stacked spectra. However, the weighted stacked spectrum shown in Fig. 2 reveals
that the bump sample has a significantly redder UV continuum (as discussed in the Spectral
stacking section). From the stacked spectra around the strong optical emission lines in Extended
Data Fig. 3, we again find the Hα/Hβ ratio in the bump sample to be significantly higher,
translating into a nebular extinction E(B − V)neb a factor of ∼2 higher than in the stacked spectrum
of the full sample. This indicates the bump sample preferably contains dustier galaxies, strongly
favouring the interpretation that the observed excess attenuation around 2175 Å is due to dust
absorption. Moreover, we find evidence for a mildly higher metallicity in the bump sample
through an enhanced line ratio of [O III] λ 5008 Å to Hβ. Though this line ratio follows a
double-branched metallicity solution (e.g. ref.20), the low-metallicity solution that monotonically
increases with [O III]/Hβ should be appropriate for the current sample of galaxies, given the
[O III]/[O II] line ratio of approximately 10 (both in the full sample and the subset of sources
selected in the bump sample). We note such differences in the Hα/Hβ and [O III]/Hβ line ratios
are absent in the control sample (selected based on the continuum shape around 2475 Å)
discussed above.
Finally, we have verified that we are able to perform a blind selection from the parent sample of
sources with the highest Balmer decrements and reddest UV slopes which results in a tentative
detection of the UV bump. Specifically, requiring a Balmer decrement of Hα/Hβ ≳ 4 and UV
slope of βUV ≳ −2.2 yields a sample of four sources all contained within the bump sample (i.e.
JADES-GS+53.16871-27.81516, JADES-GS+53.13284-27.80186,
JADES-GS+53.17022-27.77739, and JADES-GS+53.16743-27.77548; see Extended Data Table
1) but notably excludes JADES-GS-z6-0. Without any pre-selection on the continuum shape
around 2175 Å, the stacked spectrum of these four galaxies produces a tentative (∼4σ) bump
feature.
Bump amplitude comparison with literature results
As discussed in Shivaei et al.13, the adopted parametrisation of bump amplitude in the excess
attenuation (i.e. Aλ, max; see the Bump parametrisation and fitting section) includes the extinction
in the absence of the bump, E(B − V), to avoid propagating the large uncertainties of this
parameter that stem from not well-constrained assumptions on the attenuation curves of
high-redshift galaxies. We note a direct measurement of the Balmer recombination line ratios can
in principle constrain the nebular extinction82, but its relation with stellar extinction carries
uncertainty in addition to suffering from wavelength-dependent slit-loss effects (also discussed in
the Spectroscopic rest-frame optical properties section). In Fig. 3, we directly compare these
excess attenuation strengths, taking into account the underlying extinction E(B − V) for bump
strengths measured in MW, LMC, and SMC extinction curves. In terms of the commonly used
Fitzpatrick & Massa40,78,83 parametrisation, Aλ, max = c3/γ2 E(B − V). We retrieve E(B − V) from the
measured total-to-selective extinction (i.e. RV = AV / E(B − V)) in each extinction curve, assuming
a range of 0.1 mag < AV < 0.5 mag. Data points from Noll et al.18 and Heintz et al.39 (and
references therein) are similarly converted to a consistent bump amplitude Aλ, max using their
measured values of E(B − V). Measurements from Noll et al.18 represent the stacked spectra of
three subsamples that were selected based on their UV slope βUV and bump strength parametrised
by the γ34 parameter (see the Sample selection section): the upwards pointing triangle in Fig. 3
has βUV < −1.5 and γ34 > −2, the diamond has βUV > −1.5 and γ34 > −2, and the downward pointing
triangle has γ34 < −2. We note that the Heintz et al.39 measurements of gamma-ray burst absorbers
are effectively along a line of sight through the galaxies, while the Shivaei et al.13 and Noll et
al.18 measurements, similar to the measurements in this work, are based on the total integrated
light of galaxies. The distribution of dust with respect to the stars within galaxies affects the
latter, integrated observations of the UV bump24,84.
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Data availability
The data that support the findings of this study are publicly available48,51 at
https://archive.stsci.edu/hlsp/jades.
Code availability
The AstroPy85,86 software suite is publicly available, as is BAGPIPES53, MultiNest56,
PyMultiNest57, pPXF60, and SpectRes80. BEAGLE51 is available via a Docker image upon request
at http://www.iap.fr/beagle/.
Acknowledgements
This work is based on observations made with the NASA/ESA/CSA James Webb Space
Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space
Telescope Science Institute, which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are
associated with programmes 1180, 1210, 1895, and 1963. The authors acknowledge the
FRESCO team led by PI Pascal Oesch for developing their observing program with a
zero-exclusive-access period. JW gratefully acknowledges support from the Fondation MERAC.
JW, RM, MC, FDE, TJL, WMB, LS, and JS acknowledge support by the Science and
Technology Facilities Council (STFC), by the European Research Council (ERC) through
Advanced Grant 695671, “QUENCH”, and by the UK Research and Innovation (UKRI) Frontier
Research grant RISEandFALL. RS acknowledges support from an STFC Ernest Rutherford
Fellowship (ST/S004831/1). RM also acknowledges funding from a research professorship from
the Royal Society. SCa acknowledges support by the European Union’s HE ERC Starting Grant
No. 101040227 – WINGS. ECL acknowledges support of an STFC Webb Fellowship
(ST/W001438/1). SAr, BRDP, and MP acknowledge support from the research project
PID2021-127718NB-I00 of the Spanish Ministry of Science and Innovation/State Agency of
Research (MICIN/AEI). AJB, AJC, JC, and AS acknowledge funding from the “FirstGalaxies”
Advanced Grant from the European Research Council (ERC) under the European Union’s
Horizon 2020 research and innovation programme (Grant agreement No. 789056). SAl
acknowledges support from the JWST Mid-Infrared Instrument (MIRI) Science Team Lead, grant
80NSSC18K0555, from NASA Goddard Space Flight Center to the University of Arizona. EE,
DJE, BDJ, MR, BER, FS, and CNAW acknowledge the JWST/NIRCam contract to the
University of Arizona, NAS5-02015. DJE is supported as a Simons Investigator. MP
acknowledges support from the Programa Atracción de Talento de la Comunidad de Madrid via
grant 2018-T2/TIC-11715. CCW is supported by NOIRLab, which is managed by the
Association of Universities for Research in Astronomy (AURA) under a cooperative agreement
with the National Science Foundation. This research is supported in part by the Australian
Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D),
through project number CE170100013. This study made use of the Prospero high performance
computing facility at Liverpool John Moores University. The authors acknowledge use of the lux
supercomputer at UC Santa Cruz, funded by NSF MRI grant AST 1828315.
Author contributions
JW, IS, RS, and RM led the writing of this paper. MR, EE, FS, KNH, and CCW contributed to
the design, construction, and commissioning of NIRCam. AJB, CNAW, CW, DJE, MR, PF, RM,
SAl, and SAr contributed to the design of the JADES survey. BER, ST, BDJ, CNAW, DJE, IS,
RE, SAl, and ZJ contributed to the JADES imaging data reduction. BER contributed to the
JADES imaging data visualisation. BDJ, ST, RE, EN, and WMB contributed to the modelling of
galaxy photometry. KNH, JL and RE contributed the photometric redshift determination and
target selection. BER, CNAW, CCW, KNH, and MR contributed to the JADES pre-flight
imaging data challenges. SCa, MC, JW, PF, GG, SAr, MP, and BRdP contributed to the NIRSpec
data reduction and to the development of the NIRSpec pipeline. SAr contributed to the design
and optimisation of the MSA configurations. AJC, AJB, CNAW, ECL, and KB contributed to the
selection, prioritisation and visual inspection of the targets. SCh, JC, ECL, RM, JW, RS, FDE,
TJL, MC, AdG, AS, and LS contributed to analysis of the spectroscopic data, including redshift
determination and spectral modelling. PF, TR, GG, NK, and BRdP contributed to the design,
construction and commissioning of NIRSpec. FDE, TJL, MC, BRdP, RM, SA, and JS
contributed to the development of the tools for the spectroscopic data analysis, visualisation and
fitting. CW contributed to the design of the spectroscopic observations and MSA configurations.
BER, CW, DJE, MR, and RM serve on the JADES Steering Committee.
Competing interests
The authors declare no competing interests.
Additional Information
Correspondence and requests for materials should be addressed to J. Witstok.
Reprints and permissions information is available at www.nature.com/reprints.
Extended Data Table 1 | Properties of the galaxies in the bump sample.
Error bars on measurements represent 1σ uncertainty. Rows: (1) Source name (including the J2000 RA and Dec in
deg), (2) Spectroscopic redshift (zspec), (3) Apparent AB magnitude in the NIRCam F444W filter (mF444W), (4)
Absolute AB magnitude in the UV (MUV), (5) UV spectral slope (βUV), (6) Balmer decrement (Hα/Hβ).
† JADES-GS-z6-0.
‡Contained in the blind selection discussed in the Robustness of the UV bump detections section.
∗This source falls outside the main NIRCam footprint.
∗∗At this redshift, Hα (partially) shifts outside of the NIRSpec coverage.
Extended Data Table 2 | Properties of the samples of galaxies with and/or without
indication of a UV bump.
Properties of the samples are presented as median values; error bars represent a 1σ uncertainty (obtained with
bootstrapping for the median properties). Rows of individual properties: (1) Spectroscopic redshift (zspec), (2)
Absolute AB magnitude in the UV (MUV), (3) UV spectral slope (βUV), (4) Spectral slope change around
λemit = 2175 Å (γ34), (5) Gas-phase metallicity (Zneb; from rest-frame optical emission lines) in units of Solar
metallicity, (6) Nebular extinction (E(B − V)neb; from the Balmer decrement) in magnitudes, (7) Stellar mass (M∗) in
108 Solar masses, (8) Stellar metallicity (Z∗; from SED modelling) in units of Solar metallicity, (9) Star formation
rate in Solar masses per year averaged on a timescale of 30 Myr (SFR30), (10) Mass-weighted stellar age (t∗) in Myr.
Rows of properties measured in stacked spectra: (1) Number of galaxies contained within the sample (Nsample), (2)
Power-law slope (β), (3) Balmer decrement Hα/Hβ, (4) Nebular extinction (E(B − V)neb) in magnitudes, (5)
[O III] λ 5008 Å/Hβ line ratio, (6) [O III] λ 5008 Å/[O II] line ratio.
Extended Data Fig. 1 | SED modelling and false-colour image of JADES-GS-z6-0. a,
Spectrum observed with NIRSpec (grey solid line and light-grey shading as 1σ uncertainty),
overlaid with NIRCam photometry (grey points; error bars show the filter widths along the
wavelength direction and 1σ uncertainty along the y-axis) of JADES-GS-z6-0. A calibration
correction is applied to the spectrum to match the photometry (see the Stellar population
synthesis modelling section). The running median around 2175 Å is shown as a solid black line.
The best-fit bagpipes SED model (Stellar population synthesis modelling section) is shown as a
light green solid line (predicted spectrum; darker and lighter shading represents the 1σ and 2σ
uncertainty, respectively) and points (predicted photometry; error bars show 1σ uncertainty). b,
Colour-composite 1′′ × 1′′ image constructed from inverse-variance weighted combinations of
NIRCam filters, with the F115W and F150W filters as the blue channel, the F200W filter as
green, and three medium-band filters not contaminated by strong emission lines (F430M,
F460M, and F480M) as red. The position of the NIRSpec MSA shutters in the central nodding
position (shown for all three separate observing visits, which however are nearly identical) are
overlaid in white. A scale of 1 kpc is indicated in the bottom right.
Extended Data Fig. 2 | Sample characteristics. Top row (panels a to f): spectral slope change
around λemit = 2175 Å (γ34) as a function of the physical properties of the galaxy sample.
JADES-GS-z6-0 is highlighted as an enlarged point. Error bars represent 1σ uncertainty. The
median values of galaxies belonging to the sample with (coloured points) and without (black
points) a UV bump signature (see the Sample selection section) are indicated respectively with
coloured and black dashed lines. The Spearman’s rank correlation coefficient, ρS, is reported at
the top of each panel along with its p-value and the completeness (i.e. for which percentage of
the sample the metric was measured). Bottom row (panels g to l): median of the full sample
(large black square), the sample with (coloured square) and without (small black square) bump
signatures. The error bars show uncertainties on the median obtained with bootstrapping.
Quantities shown are the absolute UV magnitude (MUV; panels a and g), UV spectral slope (βUV;
b and h), Balmer decrement Hα/Hβ (c and i), stellar mass (M∗ ; d and j), star formation rate
averaged over the last 30 Myr (SFR30; e and k), and mass-weighted stellar age (t∗; f and l).
Extended Data Fig. 3 | Normalised and stacked rest-frame optical spectra of z > 4 JADES
galaxies observed by JWST/NIRSpec. Similar to Fig. 2, spectra of all galaxies (small black
dots) are shifted to the rest frame but normalised to the flux density at the rest-frame wavelength
of Hβ, λHβ = 4862.7 Å. The dashed black line (shading represents 1σ uncertainty) shows a
stacked spectrum obtained by combining all 49 objects in wavelength bins of ∆λemit = 10 Å. The
main rest-frame optical emission lines are labelled. The stacked spectrum of ten galaxies selected
to have a bump signature (solid black line, shading as 1σ uncertainty) exhibits stronger [O III]
and Hα emission relative to the full sample, while having a consistent [O III]/[O II] line ratio.
This is quantified by the integrated flux ratios of the fitted Gaussian line profiles (respectively
shown as red and blue solid lines with shading as 1σ uncertainty), indicating differences in
oxygen abundance as well as dust obscuration.
Extended Data Fig. 4 | Exploration of signatures from significantly evolved stellar
populations. a, The age-sensitive spectral region around the 4000 Å (Balmer) break of
JADES-GS-z6-0. Similar to Extended Data Fig. 1, the (photometry-corrected) NIRSpec
spectrum is shown as the grey solid line (light-grey shading as 1σ uncertainty), NIRCam
photometry as grey points (error bars show the filter widths along the wavelength direction and
1σ uncertainty along the y-axis). The BAGPIPES SED model with best-fit parameters (see Stellar
population synthesis modelling) is shown as a black solid line (spectrum) and points
(photometry). Two-component models are shown by the blue (age of 100 Myr), green (250 Myr),
and red (500 Myr) solid (107.5 M⊙) and dashed (108.5 M⊙) lines. b, The difference in reduced
chi-squared values, , is shown as a function of the stellar mass of the evolved population,∆χ
ν
2
Mevolved. Dashed horizontal lines indicate the value above which the new model is in 2σ tension
with respect to the single-component model ( ).∆χ
ν
2 = 4
Extended Data Fig. 5 | Rest-frame UV continuum of JADES-GS-z6-0. a, The
one-dimensional spectrum of JADES-GS-z6-0 (smoothed with a 15-pixel median filter as in Fig.
1) is normalised to the predicted continuum level in the absence of a UV bump, modelled as a
power law with index βUV (purple line; shading as 1σ uncertainty). The median SNR and γ34 are
reported in the bottom-left corner. Coloured lines show data from individual observing visits,
while the solid black line and grey shading represent the combined spectra and their 1σ
uncertainty, respectively. A dashed black line indicates the spectrum from a 3-pixel aperture
extraction. b, Two-dimensional spectrum of JADES-GS-z6-0 (not scaled to the predicted
continuum level).