Dust Budget Crisis in Little Red Dots

Kejian Chen, Zhengrong Li, Kohei Inayoshi, Luis C. Ho

First listed 2025-05-28 | Last updated 2025-11-26

Abstract

Little red dots (LRDs), a population of active galactic nuclei (AGNs) recently identified by JWST, are characterized by their compact morphology and red optical continuum emission, which is often interpreted as evidence for significant dust extinction of $A_V \gtrsim 3$ mag. However, the dust-reddened AGN scenario is increasingly challenged by their faint near-to-far infrared emission and a potential "dust budget crisis" in cases when the host galaxy is either undetectably low-mass or absent. In this study, we re-evaluate the dust extinction level in LRDs by modeling the UV-to-infrared spectra for various extinction laws and a broad range of dusty distribution parameters. Comparing the predicted infrared fluxes with observational data from the JWST MIRI, Herschel, and ALMA, our analysis finds that the visual extinction is tightly and consistently constrained to $A_V \lesssim 1.0-1.5$ mag for A2744-45924, RUBIES-BLAGN-1, and stacked SEDs from a large sample of LRDs under the SMC extinction law, with slightly weaker constraints for those with gray extinction in the UV range. The revised $A_V$ values yield radiative efficiencies of $\sim 10\%$ for the LRD population, easing the tension with the Sołtan argument for the bulk AGN population at lower redshifts. Moreover, this moderate extinction (or dust-free) scenario, with reprocessed emission spectra testable by future far-infrared observatories, provides a paradigm shift in understanding their natures, environments, and evolutionary pathways of massive black holes in the early universe.

Short digest

Re-assessing LRD dust, the authors forward-model UV–IR SEDs across multiple extinction laws and dust geometries, then demand consistency with JWST/MIRI, Herschel, and ALMA fluxes and limits. They find A_V ≲ 1.0–1.5 mag under an SMC law for A2744-45924, RUBIES-BLAGN-1, and stacked LRD SEDs, with only slightly weaker bounds for gray UV extinction. These moderate extinctions alleviate the “dust budget crisis” for hostless/low-mass systems and imply ≈10% radiative efficiencies, easing Sołtan-argument tension. The work predicts testable far-IR reprocessed spectra, reframing early black-hole growth pathways for LRDs.

Key figures to inspect

• Extinction-law comparison (Fig. 1): inspect SMC, MW (R_V=3.1), Orion, and steep-UV (modified Calzetti) curves to see why SMC-like slopes most strongly cap A_V while gray UV attenuation relaxes the limit.
• A2744-45924 SED fit: compare predicted reprocessed IR (for each extinction law/geometry) with MIRI, Herschel, and ALMA points/upper limits to see the A_V ≲ 1–1.5 mag ceiling emerge.
• RUBIES-BLAGN-1 SED fit: same UV–IR reconstruction versus MIRI/ALMA constraints; note how gray UV attenuation allows marginally higher A_V than SMC while still failing heavy-obscuration scenarios.
• Stacked LRD SED: check how stacking tightens mid/far-IR limits and fixes a low A_V envelope; note the modest far-IR peak the authors propose as a target for future FIR facilities.
• Geometry parameter grid: look for maps varying inner radius, density slope p, and covering fraction showing that even extended/clumpy dust cannot hide A_V ≳ 3 without violating IR limits.