Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Kejian Chen, Zhengrong Li, Kohei Inayoshi, Luis C. Ho,
| Summary:
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
constrained to $A_V lesssim 1.0$ mag for A2744-45924 and $A_V lesssim 1.5$
mag for RUBIES-BLAGN-1 under the SMC extinction laws, with slightly weaker
constraints for those with gray extinction in the UV range. The revised $A_V$
values yield a radiative efficiencies of $10%$ for the LRD population, easing
the tension with the Soltan 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.
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