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

TL;DR

This work delivers the first infrared spectral predictions from a fully self-consistent, cosmological-conditions quasar simulation (FORGE'd in FIRE) by post-processing with SKIRT radiative transfer. The emergent IR emission arises from a dusty torus composed of a magnetically supported outer accretion disk and tidally stripped ISM dust, with a buried, Compton-thick AGN at early stages and pronounced IR anisotropy driven by cold dust inflows. Sublimation is stratified by dust composition and size, yielding orientation-dependent extinction curves that gray after multiple scattering in the optically thick medium. The study further shows that bipolar outflows clearing polar cavities can reveal a type 1/2 appearance while preserving IR anisotropy, suggesting dusty starburst quasars may pass through a buried, IR-bright phase early in their evolution. These results provide a physically motivated bridge between cosmological quasar formation and observable IR signatures, with implications for interpreting warm ULIRGs and hot DOGs in the early growth of AGN.

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.

Figures (11)

• Figure 1: Images from the post-processing radiation emitted out to the black hole radius of influence (BHROI, $\sim$ 1 pc) from the central black hole at five different inclinations and $L_\mathrm{AGN} = 1 \times 10^{44}~\mathrm{erg}~\mathrm{s}^{-1}$, prior to dust self-absorption. The red color indicates emission from wavelengths $\lambda > 37~\mu\mathrm{m}$ (roughly tracing cold dust emission with temperatures $T \lesssim 80~\mathrm{K}$), green indicates emission from $5~\mu\mathrm{m} < \lambda < 37~\mu\mathrm{m}$ ($80~\mathrm{K} \lesssim T \lesssim 600~\mathrm{K}$), and blue indicates emission from $\lambda < 5~\mu\mathrm{m}$ ($600~\mathrm{K} \lesssim T < T_\mathrm{sub} \simeq 1500~\mathrm{K}$). Broadly, the system appears to fit with a standard smooth dust torus model. However, certain orientations reveal significant polar inhomogeneity in the form of clumps and streams as well as significant azimuthal asymmetry. The torus emitting regions correspond best to the highly magnetized outer accretion disk vertically supported by turbulence, in addition to dusty gas tidally torn from the interstellar medium (ISM) complex.
• Figure 2: Mean spectra measured at a radius of 1 pc from the central supermassive black hole (SMBH) for an active galactic nucleus (AGN) luminosity of $L_\mathrm{AGN} = 5 \times 10^{45}~\mathrm{erg}~\mathrm{s}^{-1}$. The blue curve shows the intrinsic input spectrum and the orange curve shows the emergent spectrum after reprocessing from Compton scattering and multiple anisotropic dust scattering, absorption and re-emission. The dark orange shading indicates the standard deviation and the lighter shading shows the full range (min-max) of the emergent spectra along our $10^3$ isotropic sightlines. Most emergent sightlines are heavily attenuated in the optical/ultraviolet (UV) and feature a prominent hot dust peak, indicating that this early-stage dusty AGN system is mostly buried. Significant anisotropy is evident even in the mid-infrared, where the spectral luminosity can still vary by a factor of a few.
• Figure 3: Mean spectra measured at a radius of 1 pc from the central black hole, decomposed into its components with shaded regions showing the sightline (min-max) range. The blue solid line shows the portion of emission originating from the accretion disk that escaped directly to the observer without scattering (only attenuating through absorption) and the dashed green line shows photons that underwent at least one scattering event. The orange solid line corresponds to the thermal dust re-emission that does not undergo a scattering event whereas the purple dashed line corresponds to the dust emission that does. These four curves sum to the total emergent emission. The optical/UV emission is dominated by scattered light. The dotted orange line corresponds to the total dust re-emission if there were no dust self-absorption. Self-absorption both shifts the dust emission towards cooler dust and significantly weakens silicate emission lines.
• Figure 4: Sightline dependence of escaped bolometric luminosity from post-processing radiative transfer on our fiducial $1$ pc, $L_\mathrm{AGN} = 5 \times 10^{45}~\mathrm{erg}~\mathrm{s}^{-1}$ run, as discussed in Section \ref{['subsec:anisotropy']}. For illustration purposes, we show Cartesian axes aligned such that the angular momentum of the outer accretion disk lies along the z axis, the inflowing stream of dusty gas lies roughly along the x axis, and the polar angle $\theta$ is measured from the z axis. We display the escaping bolometric luminosity as a function of sightline using a Mollweide projection. Most of the extinction in the escaped luminosity is dominated by a dusty stream of cold inflowing gas. We also plot the maximum, minimum and mean $L_\mathrm{bol}$ sightline spectra, showing that both the optical and IR change as a function of sightline.
• Figure 5: Sightline dependence of escaped luminosity in the fiducial run as in Figure \ref{['fig:lum_angle_dependence_1pc']}, but now separated into wavelength bands. We define here the X-ray emission as $\lambda < 100~\text{\AA}$, the UV/optical as $0.01~\mu\mathrm{m} < \lambda < 0.8~\mu\mathrm{m}$, the near infrared (NIR) as $0.8~\mu\mathrm{m} < \lambda < 3~\mu\mathrm{m}$, the mid infrared (MIR) as $3~\mu\mathrm{m} < \lambda < 25~\mu\mathrm{m}$. What little emission escapes in the X-ray and UV/optical is highly anisotropic due to significant dust extinction and Compton scattering. The NIR and MIR, which dominate the escaping emission at this scale and primarily originate from self-absorbed dust emission from the hot/warm dust near the SMBH, but can still vary by a factor of a few due to reprocessing from the dusty stream fueling accretion.