Early Stages of Dusty Tori: The First Infrared Spectra from a Highly Multiscale Quasar Simulation

Jaeden Bardati, Philip F. Hopkins, Gordon T. Richards

First listed 2025-09-11 | Last updated 2025-12-05

Abstract

We present the first infrared spectral predictions from a self-consistent simulation of the formation of a quasar in a starburst galaxy, spanning the cosmological environment to scales well below the dust sublimation region. The infrared (IR) emission is dominated by a torus-like dust structure composed of the highly magnetized, turbulence-supported outer accretion disk and of accreting gas tidally torn from the interstellar medium (ISM). At these early stages, the active galactic nuclei (AGN) is buried and Compton-thick. The near- to mid-IR escaping luminosity varies by almost an order of magnitude across sightlines, largely due to extinction from the inflowing stream of cold dust. Self-absorption within the torus suppresses silicate emission features, and further reprocessing by the ambient ISM leads to prominent silicate absorption and colder IR emission. The sublimation structure is stratified by composition and size, producing sightline-dependent extinction curves that intrinsically vary in shape. However, after repeated scattering in the optically thick dusty medium, these curves emerge substantially grayed. We also demonstrate that bipolar outflows from the central black hole that carve biconical cavities and reveal the central engine in later stages can preserve IR anisotropy and silicate features. These results suggest that dusty starburst quasars can undergo a buried, IR-bright phase early in their evolution.

Short digest

First IR spectra from a fully self-consistent, cosmological-to-sub-sublimation quasar simulation (post-processed with SKIRT) show that the emission is dominated by a torus-like structure built from a highly magnetized, turbulence-supported outer accretion disk plus dusty gas tidally stripped from the ISM. In this early phase the nucleus is buried and Compton-thick, and the escaping near–mid-IR luminosity varies by nearly an order of magnitude with sightline, driven largely by extinction in an inflowing cold dusty stream. Self-absorption within the torus cools the SED and suppresses silicate emission, while reprocessing by the surrounding ISM imprints prominent silicate absorption; sublimation zones stratified by grain size/composition yield intrinsically varied extinction curves that emerge grayed after multiple scatterings. Clearing biconical cavities in later stages preserves MIR anisotropy and silicate features, implying a buried, IR-bright phase early in dusty starburst quasar evolution.

Key figures to inspect

• Figure 1: Inspect the RGB wavelength maps within ~1 pc (pre–self-absorption) to see the torus-like emitter formed by the magnetized outer disk plus tidally torn ISM, and note polar clumps/streams and strong azimuthal asymmetry that break a smooth-torus picture.
• Figure 2: Compare intrinsic vs emergent spectra at 1 pc and the sightline spread to confirm a hot-dust peak, heavy optical/UV attenuation, and persistent MIR anisotropy (factor-of-few), consistent with a buried Compton-thick phase.
• Figure 3: Use the component decomposition (direct, scattered, dust re-emission with/without scattering) to verify that optical/UV is scattering-dominated and that dust self-absorption both cools the IR SED and weakens silicate emission relative to the no–self-absorption case.
• Figure 4: Examine the escaped bolometric luminosity vs orientation (defined by the 0.1 pc angular-momentum axis) and the max/min/mean spectra to identify the inflowing cold dusty stream as the primary source of sightline-dependent extinction affecting both optical and IR.