We introduce Monte-Carlo-based non-local thermodynamic equilibrium (non-LTE) line radiative transfer calculations in the three-dimensional (3D) dust radiative transfer code SKIRT, which was originally set up as a dust radiative transfer code. By doing so, we developed a generic and powerful 3D radiative transfer code that can self-consistently generate spectra with molecular and atomic lines against the underlying continuum. We tested the accuracy of the non-LTE line radiative transfer module in the extended SKIRT code using standard benchmarks. We find excellent agreement between the S KIRT results, the published benchmark results, and the results obtained using the ray-tracing non-LTE line radiative transfer code MAGRITTE, which validates our implementation. We applied the extended SKIRT code on a 3D hydrodynamic simulation of a dusty active galactic nucleus (AGN) torus model and generated multiwavelength images with CO rotational-line spectra against the underlying dust continuum. We find that the low-J CO emission traces the geometrically thick molecular torus, whereas the higher-J CO lines originate from the gas with high kinetic temperature located in the innermost regions of the torus. Comparing the calculations with and without dust radiative transfer, we find that higher-J CO lines are slightly attenuated by the surrounding cold dust when seen edge-on. This shows that atomic and molecular lines can experience attenuation, an effect that is particularly important for transitions at mid- and near-infrared wavelengths. Therefore, our self-consistent dust and non-LTE line radiative transfer calculations can help the observational data from Herschel, ALMA, and JWST be interpreted.

We studied the absorption features of CO lines against the continuum originating from the heated dust in the obscuring tori around active galactic nuclei (AGNs). We investigated the formation of absorption lines corresponding to the CO rotational transitions using three-dimensional non-LTE line transfer simulations considering the dust thermal emission. As in Papers I–III of this series, we performed post-processed radiative transfer calculations using the “radiation-driven fountain model” (Wada et al. 2016), which yields a geometrically thick obscuring structure around the nucleus. This model is consistent with the spectral energy distribution of the nearest type-2 Seyfert galaxy, the Circinus galaxy. We found that the continuum-subtracted channel maps of J = 4−3 and higher transitions show absorption regions along the disk midplane for an edge-on viewing angle. The spectra consist of multiple absorption and emission features, reflecting the internal inhomogeneous and turbulent structure of the torus. The deepest absorption feature is caused by the gas on the near side of the torus between r = 10 and 15 pc, which is located in front of the AGN-heated dust inside r ≃ 5 pc. We also found that a spatial resolution of 0.5–1.0 pc is necessary to resolve the absorption features. Moreover, the inclination angle must be close to the edge-on angle (i.e., ≳85°) to observe the absorption features. The findings of the present study imply that combining our radiation-hydrodynamic model with high-resolution observations of CO (7–6) by ALMA can provide new information about the internal structure of the molecular tori in nearby AGNs.

A recent hydrodynamic model, the radiation-driven fountain model (Wada et al. 2016), presented a dynamical picture that active galactic nuclei (AGNs) tori sustain their geometrical thickness by gas circulation around AGNs, and previous papers have confirmed that this picture is consistent with multiwavelength observations of nearby Seyfert galaxies. Recent near-infrared observations implied that CO rovibrational absorption lines (ΔJ = ± 1, v = 0 − 1, λ ∼ 4.7 μm) could probe the physical properties of the inside tori. However, the origin of the CO absorption lines has been under debate. In this paper, we investigate the origin of the absorption lines and conditions for detecting them by performing line radiative transfer calculations based on the radiation-driven fountain model. We find that CO rovibrational absorption lines are detected at inclination angles θ _obs = 50°–80°. At the inclination angle θ _obs = 77°, we observe multi-velocity components: inflow (v _LOS = 30 km s⁻¹), systemic (v _LOS = 0 km s⁻¹), and outflows (v _LOS = −75, − 95, and −105 km s⁻¹). The inflow and outflow components (v _LOS = 30 and −95 km s⁻¹) are collisionally excited at the excitation temperatures of 186 and 380 K up to J = 12 and 4, respectively. The inflow and outflow components originate from the accreting gas on the equatorial plane at 1.5 pc from the AGN center and the outflowing gas driven by AGN radiation pressure at 1.0 pc, respectively. These results suggest that CO rovibrational absorption lines can provide us with the velocities and kinetic temperatures of the inflow and outflow in the inner few parsec region of AGN tori, and the observations can probe the gas circulation inside the tori.

Context. The UVJ color-color diagram is a widely used diagnostic to separate star-forming and quiescent galaxies. Observational data from photometric surveys reveal a strong stellar mass trend, with higher-mass star-forming galaxies being systematically more dust-reddened. Aims. We analyze the UVJ diagram in the TNG100 cosmological simulation at cosmic noon (z ≈ 2). Specifically, we focus on the trend between UVJ colors and mass, which has not been reproduced in any cosmological simulation thus far. Methods. We applied the SKIRT dust radiative transfer code to the TNG100 simulation to generate rest-frame UVJ fluxes. These UVJ colors were then compared to observational data from several well-studied extragalactic fields from the CANDELS/3D-HST programs, augmented by recent JWST/NIRCam photometry. Results. Quiescent and low-mass (M_⋆ ≲ 10^10.5 M_⊙) galaxies at cosmic noon do not require significant levels of dust reddening, as opposed to massive (M_⋆ ≳ 10¹¹ M_⊙) star-forming galaxies. An extensive range of possible dust models fall short of the required dust reddening in V − J color for massive star-forming galaxies, with the simulated galaxies being too blue by ≈0.9 mag. Conclusions. We find that only variations in the star-to-dust geometries of the simulated galaxies are able to yield V − J colors that are red enough to match the observations. A toy model with isolated dust screens around younger stellar populations (with ages below ∼1 Gyr) can reproduce the observational data, while all “conventional” dust radiative transfer models (where the dust distribution follows the metals in the interstellar medium) fail to achieve the required V − J colors.

Understanding the inner structure of the clumpy molecular torus surrounding the active galactic nucleus is essential in revealing the forming mechanism. However, spatially resolving the torus is difficult because of its size of a few parsecs. Thus, to probe the clump conditions in the torus, we performed the velocity decomposition of the CO rovibrational absorption lines (Δv = 0 →1, ΔJ = ±1) at λ ∼ 4.67 μm observed toward an ultraluminous infrared galaxy IRAS 08572+3915 NW with the high-resolution spectroscopy (R ∼ 10,000) of Subaru Telescope. Consequently, we found that each transition had two outflowing components, i.e., (a) and (b), both at approximately ∼−160 km s⁻¹, but with broad and narrow widths, and an inflowing component, i.e., (c), at approximately ∼+100 km s⁻¹, which were attributed to the torus. The ratios of the velocity dispersions of each component led to those of the rotating radii around the black hole of R _rot,a: R _rot,b: R _rot,c ≈ 1: 5: 17, indicating the torus where clumps are outflowing in the inner regions and inflowing in the outer regions if a hydrostatic disk with σ V ∝ R rot − 0.5 is assumed. Based on the kinetic temperature of components (a) and (b) of ∼720 and ∼25 K, respectively, estimated from the level population, the temperature gradient is T kin ∝ R rot − 2.1 . Magnetohydrodynamic models with large density fluctuations of two orders of magnitude or more are necessary to reproduce this gradient.

The ultraluminous infrared galaxy IRAS 17208−0014 is a late-stage merger that hosts a buried active galactic nucleus (AGN). To investigate its nuclear structure, we performed high-spatial-resolution ( ∼ 0.″04 ∼ 32 pc) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 9 (∼450 μm or ∼660 GHz), along with near-infrared AKARI spectroscopy in 2.5–5.0 μm. The Band 9 dust continuum peaks at the AGN location, and toward this position CO(J = 6 − 5) and CS(J = 14 − 13) are detected in absorption. Comparison with nonlocal thermal equilibrium calculations indicates that, within the central beam (r ∼ 20 pc), there exists a concentrated component that is dense (10⁷ cm⁻³) and warm (>200 K) and has a large column density ( N H 2 > 10 23 cm − 2 ). The AKARI spectrum shows deep and broad CO rovibrational absorption at 4.67 μm. Its band profile is well reproduced with a similarly dense and large column but hotter (∼1000 K) gas. The region observed through absorption in the near-infrared is highly likely in the nuclear direction, as in the submillimeter, but with a narrower beam including a region closer to the nucleus. The central component is considered to possess a hot structure where vibrationally excited HCN emission originates. The most plausible heating source for the gas is X-rays from the AGN. The AKARI spectrum does not show other AGN signs in 2.5–4 μm, but this absence may be usual for AGNs buried in a hot mid-infrared core. Further, based on our ALMA observations, we relate the various nuclear structures of IRAS 17208−0014 that have been proposed in the literature.