A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains
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 ≳ 0.1 μm) 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±0.003, 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 (A1500/AV ≈ 8) but notably lacks the 2175 A bump (Abump/AV<0.34 at 3σ), 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.
Versions
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BibTeX
@misc{li2026steepextinction,
title = {{A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains}},
author = {Mingyu Li and Zheng Cai and Roberto Maiolino and Fengwu Sun and Xihan Ji and Qiao Duan and Bjorn H. C. Emonts and Xiaohui Fan and Ignas Juodžbalis and Xiaojing Lin and Yixiao Liu and Sandro Tacchella},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2606.02685},
eprint = {2606.02685},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.02685},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Steep-Extinction Quasi-stellar Object at z=4.6: JWST Evidence for Abundant Small Dust Grains
AU - Mingyu Li
AU - Zheng Cai
AU - Roberto Maiolino
AU - Fengwu Sun
AU - Xihan Ji
AU - Qiao Duan
AU - Bjorn H. C. Emonts
AU - Xiaohui Fan
AU - Ignas Juodžbalis
AU - Xiaojing Lin
AU - Yixiao Liu
AU - Sandro Tacchella
PY - 2026
DA - 2026/08/27
PB - arXiv
DO - 10.48550/arxiv.2606.02685
UR - https://arxiv.org/abs/2606.02685
AB - 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 ≳ 0.1 μm) 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±0.003, 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 ≈ 8) but notably lacks the 2175 A bump (A_bump/A_V<0.34 at 3σ), 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.
ER -