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

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

First listed 2026-06-03 | Last updated 2026-06-01

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 ($a \gtrsim 0.1\,μ{\rm m}$) 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 Angstrom bump, 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 AGN generate, process, and redistribute small grains, potentially facilitating rapid ISM grain growth and enriching the circumgalactic medium.

Short digest

This paper reports JWST/NIRSpec identification of UDS-27023, a compact Type I QSO at z=4.556 whose rest-frame UV continuum is dramatically suppressed relative to its optical emission. By comparing the spectrum to intrinsic blue-QSO composites, the authors reconstruct an extreme line-of-sight extinction curve with A1500/AV ≈ 8 that is steeper than standard Milky Way, SMC, average-QSO, or recent JWST galaxy laws, yet shows no 2175 Å bump. They argue that the dust is therefore strongly weighted toward small silicate grains, plausibly produced by grain shattering in AGN-driven shocks or outflows and possibly by in-situ condensation in dense QSO winds. If so, steep-extinction QSOs may mark a short-lived phase in which luminous AGN generate and redistribute the small grains needed for rapid early dust growth and circumgalactic enrichment.

Key figures to inspect

• Figure 1. This is the setup figure that establishes UDS-27023 as a real, compact broad-line quasar rather than a photometric oddity. The NIRCam morphology, PRISM spectrum, and Hα-based fit together show the secure z=4.556 identification and the unusually suppressed rest-UV continuum that motivates the steep-extinction interpretation.
• Figure 2. This is the paper’s central evidence figure. It shows both the template-based dereddening of the full rest-frame UV to NIR SED and the reconstructed extinction curve, making clear that UDS-27023 has a far-UV rise far steeper than Milky Way, SMC, average-QSO, starburst, or recent JWST galaxy curves, while lacking a 2175 Å bump.
• Figure 3. This is the best synthesis figure for the physical claim. The left panel places the source at extreme dust-law steepness relative to SDSS galaxies and z~4-10 JWST attenuation measurements, while the right panel links that phenomenology to grain-size modeling and shows why a small-grain-weighted, silicate-dominated distribution is the natural interpretation.
• Figure 5. This figure captures an important consequence of the result rather than just the measurement itself. It shows that SEQ-like extinction can move UDS-27023 outside standard JWST AGN and QSO color selections and even into a UVJ-quiescent locus under incorrect low-redshift assumptions, explaining why similar objects may be systematically missed.
• Figure 6. This is a strong companion to Figure 5 because it demonstrates the failure mode in a full photometric fit. Using only JWST photometry, Prospector still prefers a quiescent or post-starburst galaxy solution at z~2, underscoring that spectroscopy was essential to recover the true high-redshift quasar nature and revealing a concrete selection bias against steep-extinction QSOs.