Detection of the 2175 Å UV bump at z > 7: Evidence for rapid dust evolution in a merging reionization-era galaxy

Ormerod, K ORCID: 0000-0003-2000-3420, Witstok, J, Smit, R ORCID: 0000-0001-8034-7802, De Graaff, A, Helton, JM, Maseda, MV, Shivaei, I, Bunker, AJ, Carniani, S, D'Eugenio, F, Bhatawdekar, R, Chevallard, J, Franx, M, Kumari, N, Maiolino, R, Rinaldi, P, Robertson, B and Tacchella, S (2025) Detection of the 2175 Å UV bump at z > 7: Evidence for rapid dust evolution in a merging reionization-era galaxy. Monthly Notices of the Royal Astronomical Society, 542 (2). pp. 1136-1154. ISSN 0035-8711

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Abstract

Dust is a fundamental component of the interstellar medium within galaxies, as dust grains are highly efficient absorbers of ultraviolet (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 exhibit a strong feature known as the 2175 Å 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 polycyclic aromatic hydrocarbon 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.

Item Type:	Article
Uncontrolled Keywords:	5109 Space Sciences; 51 Physical Sciences; 5101 Astronomical Sciences; Climate-Related Exposures and Conditions; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	23 July 2025
Date of first compliant Open Access:	27 February 2026
Date Deposited:	27 Feb 2026 14:09
Last Modified:	27 Feb 2026 14:09
DOI or ID number:	10.1093/mnras/staf1228
URI:	https://researchonline.ljmu.ac.uk/id/eprint/28157

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