The existence of substantial dust reservoirs in galaxies at high redshifts (z ≈ 8), corresponding to when the universe was only around 600 million years old, poses a significant challenge to current models of dust formation. These models struggle to explain the rapid accumulation of dust within such a short timeframe, given the generally accepted mechanisms of dust production, such as the evolution of asymptotic giant branch (AGB) stars, supernovae (SNe) ejecta, and grain growth in the interstellar medium (ISM). While previous studies have focused on the total dust mass in galaxies, this approach often presents ambiguities and degeneracies in identifying the specific dust production processes involved. The presence of significant dust at these early epochs suggests either exceptionally early onset of star formation or the existence of more efficient dust production pathways than those currently understood. This research aims to address this knowledge gap by investigating the presence and characteristics of a specific spectral feature associated with carbonaceous dust: the 2175 Å ultraviolet (UV) bump. This feature, well-established in the Milky Way and nearby galaxies, offers a unique spectroscopic signature that can provide insights into the type and formation processes of carbonaceous dust grains. The detection of this UV bump at high redshift would not only confirm the presence of carbonaceous dust but also provide critical constraints on the timescales for its formation and the likely stellar sources involved.

Previous studies have reported the detection of large dust reservoirs in high-redshift galaxies, prompting revisions of dust formation models. These models have focused on various sites of dust production, including AGB stars in low-metallicity environments, SN ejecta, and accelerated grain growth in the ISM. However, relying solely on the total dust mass leads to considerable uncertainties in pinpointing the dominant formation pathways. The 2175 Å UV attenuation bump, a well-known feature in the Milky Way and galaxies at lower redshifts (z ≤ 3), is predominantly attributed to carbonaceous dust grains, such as polycyclic aromatic hydrocarbons (PAHs) or nano-sized graphitic grains. Its presence and strength in galaxies have been linked to metallicity and dust-to-star geometry. However, spectroscopic observations of this feature at high redshifts (z > 3) have been scarce, limiting our understanding of its early universe evolution. This study uses the unique capabilities of the James Webb Space Telescope (JWST) to probe this spectral feature in high redshift galaxies.

This research utilized deep near-infrared spectroscopic data from the JWST Advanced Deep Extragalactic Survey (JADES). The data were obtained using the JWST Near-Infrared Spectrograph (NIRSpec) in the PRISM configuration, covering a spectral range of 0.6 to 5.3 μm with a resolving power of R ≈ 100. A total of 253 sources were observed across three visits, with exposure times ranging from 9.3 to 28 h. The targets were high-redshift galaxies selected based on Hubble Space Telescope and JWST/Near-Infrared Camera (NIRCam) imaging data. Data reduction involved standard procedures for flux calibration and background subtraction using pipelines developed by the European Space Agency’s NIRSpec Science Operations Team and the NIRSpec Guaranteed Time Observations team. The one-dimensional spectra were extracted using both 5-pixel and 3-pixel apertures to assess the robustness of the findings. Slit-loss corrections were applied assuming point-like sources. To identify galaxies exhibiting the UV bump, a Bayesian power-law fitting procedure was applied to the rest-frame UV continuum, analyzing power-law indices in four separate wavelength windows. Galaxies with a negative difference between power-law indices in two specific regions surrounding the expected 2175 Å bump were selected as candidates. A Drude profile was fitted to the excess attenuation in both an individual galaxy spectrum and stacked spectra to quantify the bump’s amplitude and central wavelength. The physical properties (stellar mass, age, metallicity, dust obscuration) of the selected galaxies were then determined using Bayesian analysis of their spectral energy distributions and emission lines. Stacked spectra of the entire sample and the subset of galaxies with detected bumps were created for comparative analysis.

The study detected the 2175 Å UV attenuation bump in the spectrum of a galaxy at redshift z = 6.71 (JADES-GS-z6-0), representing a direct spectroscopic detection of this feature at z > 3. This detection was confirmed through a significant (6σ) deviation from a smooth power-law continuum. A Drude profile fit yielded a bump amplitude of 0.43+0.07−0.07 mag and a central wavelength λmax = 2,263 ± 20 Å. The galaxy exhibited significant dust obscuration, indicated by a Balmer decrement of Hα/Hβ ≈ 3.7, corresponding to a nebular extinction of E(B−V)neb = 0.25 ± 0.07 mag. The observed gas-phase and stellar metallicities (Z ≈ 0.2–0.3 Z⊙) suggest substantial metal enrichment. Further analysis of a sample of 49 galaxies with z > 4 revealed ten galaxies showing evidence for the UV bump. These galaxies, while showing no significant difference in stellar properties (mass or age) compared to the parent sample, display considerable dust obscuration and enhanced metallicities. The stacked spectrum of these ten galaxies showed a clear, significant (5σ) depression centered around 2,175 Å, with a fitted bump amplitude of 0.10+0.03−0.01 mag and a central wavelength λmax = 2,236+30−20 Å. The relatively high bump amplitude in JADES-GS-z6-0, compared to lower redshift galaxies, may indicate a different grain composition or a simpler dust-star geometry. The detection of carbonaceous dust at z = 4–7 challenges standard dust production models, which typically rely on AGB stars with longer lifetimes to produce carbonaceous grains. This suggests that faster production mechanisms, like SN or Wolf-Rayet (WR) stars, are likely major contributors to carbonaceous dust formation in the early universe.

The detection of the 2175 Å UV bump in high-redshift galaxies (z = 4–7) provides strong evidence for the presence of carbonaceous dust in the early universe. The observed high bump amplitudes and central wavelengths, particularly in JADES-GS-z6-0, deviate from the trends observed in lower-redshift galaxies, suggesting a possible difference in the composition or spatial distribution of dust grains. The short timescale implied for dust formation at these redshifts contradicts traditional models that rely on AGB stars as the primary source of carbonaceous dust. The observed metal enrichments in the galaxies, combined with the absence of significant old stellar populations, points towards more rapid formation channels such as SN or WR stars. This conclusion supports models of dust production in SN ejecta and binary carbon-rich WR stars that are able to produce and preserve carbonaceous grains. The findings challenge existing theoretical models of dust production and stellar evolution, highlighting the need for revised models that incorporate faster dust formation mechanisms prevalent in the early universe.

This study presents the first direct spectroscopic detection of the 2175 Å UV bump in galaxies at z > 3. The detection of carbonaceous dust at such high redshifts implies faster dust formation mechanisms than previously considered, possibly involving SN or WR stars. This result refines our understanding of dust production in the early universe and highlights the crucial role of JWST in addressing open questions in galaxy evolution. Future research should focus on expanding the sample size of high-redshift galaxies observed with NIRSpec to confirm the prevalence of the UV bump and further constrain the dust formation pathways. Detailed modelling of dust production in various stellar environments, incorporating different grain compositions and dust-star geometries, is also needed to fully understand the observed features.

The sample size of galaxies with detected UV bumps is relatively small, limiting the statistical power of some analyses. The uncertainties associated with dust attenuation models and radiative transfer effects could affect the interpretation of bump strengths. The study primarily focuses on carbonaceous dust, and neglecting other dust types might influence the overall dust mass estimation. The assumption of point-like sources in the slit-loss correction may introduce some systematic uncertainties.