Silicate emission in a type-2 quasar: JWST/MIRI constraints on torus geometry and radiative feedback

TL;DR

The paper uses JWST/MIRI mid-infrared spectra of the type-2 quasar J1010 to test torus geometries with BayesClumpy2, comparing CLUMPY and disc+wind CAT3D-WIND models. It finds that the disc+wind torus better fits the nuclear spectrum, yielding a moderate-to-high covering factor and a near-face-on inclination, which exposes hot dust in emission while still obscuring the BLR. Across the QSO2 sample, silicate emission correlates with bolometric luminosity and Eddington ratio, and four of five sources occupy the radiative-blowout region in the $\lambda_{\rm Edd}$–$N_{\rm H}$ diagram, consistent with AGN feedback clearing nuclear dust. Together, these results support a scenario where radiation pressure on dusty gas reduces torus covering factors in luminous AGN, distinguishing QSO2s from Seyfert 2 galaxies and highlighting a potentially transient blowout phase. The work demonstrates the power of high-angular-resolution JWST data to constrain nuclear dust structure and AGN feedback effects on parsec-to-kiloparsec scales.

Abstract

Type-2 quasars (QSO2s) are AGN seen through a significant amount of dust and gas that obscures the central supermassive black hole and the broad line region. Despite this, recent mid-infrared spectra of the central 0.5-1.1 kpc of five QSO2s at z~0.1, obtained with the MRS module of JWST/MIRI, revealed 9.7, 18, and 23 micron silicate features in emission in two of them. This indicates that the high angular resolution of JWST/MIRI now allows us to peer into their nuclear region, exposing some of the directly illuminated dusty clouds that produce silicate emission. To test this, we fitted the nuclear mid-infrared spectrum of the QSO2 with the strongest silicate features, J1010, with two different sets of torus models implemented in an updated version of the Bayesian tool {\tt BayesClumpy}. These are the CLUMPY and the CAT3D-WIND models. The CAT3D-WIND model is preferred by the observations based on the marginal likelihood and fit residuals, although the two torus models successfully reproduce the spectrum by means of intermediate covering factors ($\rm C_T=0.45\pm^{0.26}_{0.18}$ and $\rm C_T=0.66\pm^{0.16}_{0.17}$ for the CLUMPY and CAT3D-WIND models) and low inclinations ($\rm i=50^\circ\pm^{8^\circ}_{9^\circ}$ and $\rm i=13^\circ\pm^{7^\circ}_{6^\circ}$). Indeed, four of the five QSO2s with JWST/MIRI observations, including J1010, are in the blowout or ''forbidden'' region of the Eddington ratio-column density diagram, indicating that they are actively clearing gas and dust from their nuclear regions, leading to reduced covering factors. This is in contrast with Seyfert 2 galaxies observed with JWST, which are in the ''permitted'' regions of the diagram and show 9.7 micron silicate features in absorption. This supports a scenario where the more luminous the AGN and the higher their Eddington ratio, the lower the torus covering factor, driven by radiation pressure on dusty gas.

Figures (7)

• Figure 1: Nuclear mid-infrared spectrum of J1010 from Ramos25 (in black), and continuum spectrum (in orange) obtained from subtracting the PAH bands and the most prominent emission lines (in green) from the nuclear spectrum. The inset shows the 9.7 $\mu$m silicate feature in logarithmic scale.
• Figure 2: Smoothed and interpolated nuclear spectrum of J1010, with the emission lines and the PAH features removed (solid blue line), and best fits with torus models (shaded orange area, which corresponds to model SEDs between the 16 and 84 percentiles of the posterior and solid black line corresponding to the 50 percentil of the posterior). The left panels correspond to the fit with the clumpy torus models of Nenkova08a, and the right panel to the disc+wind models of Hoenig17. Vertical dotted lines are the peak wavelengths measured for the silicate features in the nuclear spectrum (see Table \ref{['tab:silicates']}). The bottom panels show the residuals as a percentage of the continuum flux, with the horizontal dotted lines indicating 0 and 15% error for guidance. The blue and black upper limits are the J, H, and Ks fluxes from the 2MASS Point Source Catalog (1.69, 3.18, and 6.55 mJy, respectively) and corresponding residuals. These upper limits are shown for comparison only and were not included in the fit.
• Figure 3: Model images of the clumpy (top row) and disc+wind (bottom rows) models fitted to the nuclear continuum spectrum of J1010 as they would look projected on the sky. The models correspond to the ones described by the parameters of the 50 percentile given in Tables \ref{['tab:nenkova']} and \ref{['tab:honig']}. In the case of the clumpy model, $\tau_V$=160 and Y=20 were used instead of the values shown in Table \ref{['tab:nenkova']} because the latter are outside the parameter range from which images are computed in HyperCat, but this have negligible impact of torus emission and cloud distribution. The last row is a zoom-in of the disc+wind model showing the same physical region as the clumpy model shown in the top row (30$\times$30 pc$^2$), since the middle row covers a FOV of 300$\times$300 pc$^2$. The physical sizes were computed by assuming a bolometric luminosity of 10$^{\rm 45}$ erg s$^{-1}$, and in the case of the disc+wind model, a pixel scale = 0.19$\times$r$_{\rm sub}$, with r$_{\rm sub}$=0.288 pc for the largest graphite grains. From left to right, the panels correspond to torus model emission at 5, 8, 10, 20, and 1000 $\mu$m, and to the cloud distribution considering all the mass. All the panels are shown in logarithmic color scale and they are normalized to the peak of emission. The black cross in the top and bottom panels indicates the position of the AGN.
• Figure 4: Strength of the 9.7 $\mu$m silicate feature versus bolometric luminosity, Eddington ratio, gas column density, and galaxy inclination. Yellow circles are the five QSOFEED QSO2s studied in Ramos25 and purple squares the six GATOS Seyfert 2 galaxies from GarciaBernete24. The horizontal dashed line in all panels indicates the separation between silicate features in absorption (negative values) and in emission (positive values).
• Figure 5: Column density measured from CO versus Eddington ratio. The blowout region is where radiation pressure pushes away the obscuring material ($\lambda_{\rm Edd}$>$\lambda_{\rm Edd}^{\rm eff}$(N$_H$)), from Ricci17. The dot-dashed line from Venanzi20 indicates the limit where the AGN radiation acceleration balances gravity and IR radiation pressure dominates, giving rise to polar dusty outflows. The dotted line is an extrapolation of this limit to lower column densities, and the horizontal dashed line is the approximate upper limit for absorption due to dust lanes Ricci17. Yellow circles are the five QSO2s from Ramos25 and purple squares are the Seyfert galaxies from GarciaBernete24.