Dust attenuation law in JWST galaxies at z ∼ 7–8

Context. Understanding the wavelength dependence of dust attenuation is vital for inferring the properties of galaxies from their spectral energy distribution (SED) fitting. The dust attenuation curves in star-forming galaxies depend on the complex interplay between the intrinsic physical dust properties and dust-to-star geometry. Due to the lack of observational constraints at high redshift, dust attenuation and extinction laws measured in the local Universe (e.g., the Calzetti attenuation law and the Small Magellanic Cloud and Milky Way extinction laws) have been employed to describe the dust attenuation at early epochs. Aims. We exploit the high sensitivity and spectral resolution of the James Webb Space Telescope (JWST) to constrain dust attenuation laws in z ∼ 7–8 galaxies. Our goals are to: i) check whether dust attenuation curves at high-z differ from the ones measured in the local Universe and ii) quantify the dependence of the inferred galaxy properties on the assumed dust attenuation law. Methods. We developed a modified version of the SED fitting code BAGPIPES by including a detailed dust attenuation curve parameterization. We applied our method to the JWST Near Infrared Spectrograph (NIRSpec) spectra in the ∼0.6–5.3 μm range to probe the nebular line (Hα, Hβ, Hγ, [O II] λ3727, [O III] λλ4959, 5007, [Ne III] λ3869) and continuum emissions of three star-forming galaxies at z = 7–8. Dust attenuation parameters and global galaxy properties are derived from the fit to the data. Results. We find that the attenuation curves of the analyzed high-z galaxies differ from local templates. One out of the three galaxies shows a characteristic 2175Å bump, typically associated with the presence of small carbonaceous dust grains such as polycyclic aromatic hydrocarbons (PAHs). This is among the first pieces of evidence suggesting the presence of PAHs in early galaxies. Galaxy properties such as the stellar mass (M∗) and star formation rate (SFR) inferred from the SED fitting are affected by the assumed attenuation curve (with deviations of up to ∼0.35 dex), however, the adopted star formation history plays the dominant role (up to ∼0.4 dex for the same galaxy properties). Conclusions. Our results highlight the importance of accounting for the potential diversity among dust attenuation laws when analyzing the spectra of high-z galaxies, whose dust properties and dust-to-star geometry are still poorly understood. The application of our method to a larger sample of galaxies observed with JWST can provide important insights into the properties of dust and galaxies in the early Universe.