Shocks, Winds, and a Torus: The Large Binocular Telescope Interferometer (LBTI) Resolves the Active Nucleus of NGC 4151

TL;DR

The study presents 8.7 and 10.5 μm LBTI Fizeau MIR images of the Seyfert 1 galaxy NGC 4151, complemented by 3.7 and 4.8 μm AO imaging, achieving ELT-like resolution that resolves a ≈32 pc dusty structure with PA ≈125°. Spatially resolved SEDs across the inner ≈100 pc reveal dust temperatures elevated by both AGN heating and jet-driven shocks, with a western arc co-located with the radio jet and ionized gas showing a temperature rise and deeper silicate absorption. A disk+wind interpretation, with an optically thin, clumpy structure, best matches the data and aligns with a broader disk+wind picture seen in NGC 1068, though jet heating emerges as a crucial secondary heating mechanism needing integration into radiative-transfer models. The results support a unified view of AGN circumnuclear dust that accounts for jet-induced heating and misalignments between the jet, disk, and torus, and they highlight the necessity of including mechanical feedback in AGN models to accurately interpret MIR emission and the inferred dust properties.

Abstract

We present mid-infrared (MIR) observations of the Seyfert 1 galaxy NGC 4151 using the Large Binocular Telescope Interferometer (LBTI). We took open-loop Fizeau images with 66-104 mas (5.8-9.1 pc) resolution in the N-band (at $8.7$ and $10.5~μm$), using the full resolution of the LBTI -- equivalent to that of a 28.8 m telescope. These images were complemented by AO imaging in the LM-bands ($3.7$ and $4.8~μm$), with 50-62 mas (4.4-5.4 pc) resolution. These images bridge the scales between previous Very Large Telescope Interferometer (VLTI)/MIDI and VLT/VISIR data, delivering ELT-like imaging resolution in the N-band. We resolve a dusty torus, (diameter 32 pc, PA$=125^{\circ}$), and detect dusty clouds within the narrow line region. Matching the resolution across four bands, we measured spatially-resolved SEDs of the central $\sim 100$ pc. Modified blackbody fitting revealed dust temperature and extinction profiles, indicating both heating from the accretion disk and additional shock heating due to the radio jet. The spatial coincidence of ionized emission (e.g., [Fe II] and [O III]), extended MIR structures, and radio features further supports the interpretation of shock heating. Comparison with NGC 1068 tests the Unified Model of Active Galactic Nuclei (Unified Model of AGN): Structures are similar, despite differences in orientation and Eddington ratio. NGC 4151's torus is smaller than NGC 1068's following a $r\propto L^{0.5}$ scaling. These thirty-meter-telescope class observations of NGC 4151 and NGC 1068 highlight the need to revise MIR radiative transfer models of AGN to account for jet-related heating.

Figures (13)

• Figure 1: LBTI/NOMIC Fizeau images of NGC 4151 and PSF calibrator HD105140. The left two panels (in blue) show the 8.7 $\mu$m~images of the sources after frame selection, corotation, and stacking. The right two panels (in red) show the same at 10.5 $\mu$m. The PSF calibrator has been artificially rotated to match the observed position angles of the target. Inset in each panel is the median Fizeau PSF without corotation; it shows the characteristic fringe pattern and the quality of the frame selection. Extended emission is immediately visible in the NGC 4151 images; the images are much more filled in with extended flux than the calibrators.
• Figure 2: LBTI/LMIRCam AO images of NGC 4151 and PSF calibrator NGP 39 74. The left two panels (in viridis) show the 3.7 $\mu$m~images of the sources after frame selection, corotation, and stacking. The right two panels (in magma) show the same at 4.8 $\mu$m. The PSF calibrator has been artificially rotated to match the observed position angles of the target. Extended emission is visible near the cores of the NGC 4151 images relative to the calibrator.
• Figure 3: Deconvolved images of the nucleus of NGC 4151. Top row) Stacked, corotated images at each wavelength (as in Figs. \ref{['fig:psfs_fizeau']} and \ref{['fig:psfs_ao']}) with PSF calibrator inset. Bottom row) CLEAN deconvolution results at each wavelength. The restoring beam of each image is given in the lower left. Contours start at $95\%$ of the peak flux and decrease by factors of 2 down to a factor of 512. The LMIRCam images have been rescaled to match the pixel scale of the NOMIC images (18 mas/px).
• Figure 4: Fitted modified blackbody temperatures and extinction values for the MIR emission in NGC 4151. Top row) In the left panel, the 5 GHz radio contours williams2020 are overplotted on the temperature profile. In the right panel, the same contours are plotted over the effective A$_{\rm V}$ profile. Negative values indicate silicate emission rather than absorption. Bottom row) same as the top row but contours from the $\lbrack$FeII$\rbrack$ emission may2020. There is a visible increase in temperature coincident with the $\lbrack$FeII$\rbrack$ cloud, immediately north of a radio knot.
• Figure 5: Comparison of MIR and visible/NIR structures. The $\lbrack$OIII$\rbrack$ image is from hutchings1999 and the other visible/NIR images are from may2020. In the top row we show $[\rm{O}{III}]~\lambda501$ nm, H$\alpha~\lambda656$ nm, $[\rm{Fe}{II}]~\lambda1644$ nm, and H$_2~\lambda1748$ nm, from left to right. In the bottom row we show contours of the same, with the LBTI 10.5 $\mu$m~contours overplotted (same levels as Fig. \ref{['fig:deconv']}). In the $[\rm{Fe}{II}]~\lambda1644$ panel, we also show the radio 5 GHz emission from williams2020. Many of the MIR features are coincident with visible/NIR emission features. The MIR arc in particular exhibits similarities to the $[\rm{O}III]$ and H$\alpha$ emission. The molecular hydrogen emission is nearly anti-coincident with the visible/MIR features.