Detection of the 2175Å UV Bump at <i>z</i> &amp;gt; 7: Evidence for Rapid Dust Evolution in a Merging Reionisation-Era Galaxy

Abstract

Abstract Dust is a fundamental component of the interstellar medium (ISM) within galaxies, as dust grains are highly efficient absorbers of UV and optical photons. Accurately quantifying this obscuration is crucial for interpreting galaxy spectral energy distributions (SEDs). The extinction curves in the Milky Way (MW) and Large Magellanic Cloud (LMC) exhibit a strong feature known as the 2175Å UV bump, most often attributed to small carbonaceous dust grains. This feature was recently detected in faint galaxies out to z = 7.55, suggesting rapid formation channels. Here we report the detection of a strong UV bump in a luminous Lyman-break galaxy at zprism = 7.11235, GNWY-7379420231, through observations taken as part of the NIRSpec Wide GTO survey. We fit a dust attenuation curve that is consistent with the MW extinction curve within 1σ, in a galaxy just ∼700 Myr after the Big Bang. From the integrated spectrum, we infer a young mass-weighted age (t⋆ ∼ 22 − 59 Myr) for this galaxy, however spatially resolved SED fitting unveils the presence of an older stellar population (t⋆ ∼ 252 Myr). Furthermore, morphological analysis provides evidence for a potential merger. The underlying older stellar population suggests the merging system could be pre-enriched, with the dust illuminated by a merger-induced starburst. Moreover, turbulence driven by stellar feedback in this bursty region may be driving PAH formation through top-down shattering. The presence of a UV bump in GNWY-7379420231 solidifies growing evidence for the rapid evolution of dust properties within the first billion years of cosmic time.