Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Zhengrong Li, Kohei Inayoshi, Kejian Chen, Kohei Ichikawa, Luis C. Ho
| Summary:
The James Webb Space Telescope (JWST) observations have revolutionized
extragalactic research, particularly with the discovery of little red dots
(LRD), which we propose are dust-reddened broad-line active galactic nuclei
(AGNs). Their unique v-shape spectral feature observed through JWST/NIRCam
challenges us to discern the relative contributions of the galaxy and AGN. We
study a spectral energy distribution (SED) model for LRDs from rest-frame UV to
infrared bands. We hypothesize that the incident radiation from an AGN,
characterized by a typical SED, is embedded in an extended dusty medium with an
extinction law similar to those seen in dense regions such as Orion Nebula or
certain AGN environments. The UV-optical spectrum is described by
dust-attenuated AGN emission, featuring a red optical continuum at
$lambda>4000$ A and a flat UV spectral shape established through a gray
extinction curve at $lambda<3000$ A, due to the absence of small-size grains.
There is no need for additional stellar emission or AGN scattered light. In the
infrared, the SED is shaped by an extended dust and gas distribution
($gamma<1$; $rhopropto r^{-gamma}$) with a characteristic gas density of
$simeq 10-10^3~{rm cm}^{-3}$, which allows relatively cool dust temperatures
to dominate the radiation, thereby shifting the energy peak from near- to
mid-infrared bands. This model, unlike the typical AGN hot torus models, can
produce an infrared SED flattening that is consistent with LRD observations
through JWST MIRI. Such a density structure can arise from the coexistence of
inflows and outflows during the early assembly of galactic nuclei. This might
be the reason why LRDs emerge preferentially in the high-redshift universe
younger than one billion years.
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