A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains

[2606.02685] A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains Skip to main content Search Submit Donate Log in Astrophysics > Astrophysics of Galaxies arXiv:2606.02685 (astro-ph) [Submitted on 1 Jun 2026 ( v1 ), last revised 27 Aug 2026 (this version, v2)] Title: A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains Authors: Mingyu Li , Zheng Cai , Roberto Maiolino , Fengwu Sun , Xihan Ji , Qiao Duan , Bjorn H. C. Emonts , Xiaohui Fan , Ignas Juodžbalis , Xiaojing Lin , Yixiao Liu , Sandro Tacchella View PDF HTML (experimental) Abstract: The rapid accumulation of massive dust reservoirs in the early Universe remains a major challenge in astrophysics. While core-collapse supernovae can inject large dust grains ($a \gtrsim 0.1\,\mu{\rm m}$) on short timescales, explaining the total dust budgets in the early Universe likely requires efficient grain growth in the interstellar medium (ISM). Such growth depends critically on an abundant population of small grains, which maximize the surface area available for accretion and may be generated by rapid dust-processing or dust-formation channels. Here, we report the discovery of a QSO, UDS-27023, at $z=4.556\pm0.003$, identified using JWST/NIRSpec spectroscopy. By quantitatively comparing the spectra to QSO composite templates, we find that UDS-27023 displays an exceptionally steep far-UV extinction curve ($A_{1500}/A_V \approx 8$) but notably lacks the 2175 A bump ($A_\mathrm{bump}/A_V<0.34$ at $3\sigma$), indicating a dominance of small silicate dust grains. We interpret this phenomenology as evidence for active small-grain production and processing in the QSO environment. Mechanical shattering of pre-existing large grains by QSO-driven shocks and outflows provides one natural pathway, while in situ condensation of silicate grains inside dense QSO-driven winds may offer an additional route. Such a population of steep-extinction QSOs (SEQs) may therefore reveal a short-lived phase in which luminous active galactic nuclei generate, process, and redistribute small grains, potentially facilitating rapid ISM grain growth and enriching the circumgalactic medium. Comments: 20 pages, 7 figures; published on ApJ Letters; data are published on this https URL Subjects: Astrophysics of Galaxies (astro-ph.GA) ; Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE) Cite as: arXiv:2606.02685 [astro-ph.GA] (or arXiv:2606.02685v2 [astro-ph.GA] for this version) https://doi.org/10.48550/arXiv.2606.02685 Focus to learn more arXiv-issued DOI via DataCite Journal reference: Mingyu Li et al 2026, ApJL, 1008, L22 Related DOI : https://doi.org/10.3847/2041-8213/ae9863 Focus to learn more DOI(s) linking to related resources Submission history From: Mingyu Li [ view email ] [v1] Mon, 1 Jun 2026 18:00:00 UTC (3,624 KB) [v2] Thu, 27 Aug 2026 22:38:05 UTC (3,690 KB) Full-text links: Access Paper: View PDF HTML (experimental) TeX Source view license Current browse context: astro-ph.GA < prev | next > new | recent | 2026-06 Change to browse by: astro-ph astro-ph.CO astro-ph.HE References & Citations INSPIRE HEP SciX Google Scholar Semantic Scholar export BibTeX citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer ( What is the Explorer? ) Connected Papers Toggle Connected Papers ( What is Connected Papers? ) Litmaps Toggle Litmaps ( What is Litmaps? ) scite.ai Toggle scite Smart Citations ( What are Smart Citations? ) Code, Data, Media Code, Data and Media Associated with this Article alphaXiv Toggle alphaXiv ( What is alphaXiv? ) Links to Code Toggle CatalyzeX Code Finder for Papers ( What is CatalyzeX? ) DagsHub Toggle DagsHub ( What is DagsHub? ) GotitPub Toggle Gotit.pub ( What is GotitPub? ) Huggingface Toggle Hugging Face ( What is Huggingface? ) ScienceCast Toggle ScienceCast ( What is ScienceCast? ) Demos Demos Replicate Toggle Replicate ( What is Replicate? ) Spaces Toggle Hugging Face Spaces ( What is Spaces? ) Spaces Toggle TXYZ.AI ( What is TXYZ.AI? ) Related Papers Recommenders and Search Tools Link to Influence Flower Influence Flower ( What are Influence Flowers? ) Core recommender toggle CORE Recommender ( What is CORE? ) IArxiv recommender toggle IArxiv Recommender ( What is IArxiv? ) Author Venue Institution Topic About arXivLabs arXivLabs: experimental projects with community collaborators arXivLabs is a framework that allows collaborators to develop and share new arXiv features directly on our website. 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