Dust Budget Crisis in Little Red Dots

TL;DR

This study re-evaluates the dust extinction in Little Red Dots (LRDs), finding lower extinction levels than previously thought, which impacts our understanding of their nature and evolution in the early universe.

Contribution

The paper demonstrates that the visual extinction in LRDs is significantly lower than prior estimates, challenging the dust-reddened AGN scenario and proposing a moderate or dust-free environment.

Findings

Visual extinction constrained to A_V ≲ 1.0-1.5 mag for key LRDs

Revised extinction values imply radiative efficiencies around 10%

Moderate or no dust scenario aligns better with observed infrared fluxes

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 So{\l}tan 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.