GATOS XI : Excess dust heating in the Narrow Line Regions of nearby AGN revealed with JWST/MIRI

TL;DR

This paper investigates extended dust in the narrow-line regions of eight nearby AGN using JWST/MIRI imaging from the GATOS/Dust in the Wind program. By combining JWST imaging with VLT/SINFONI spectroscopy and VLA radio data, the authors map dust morphology, SEDs, and heating sources, finding that four galaxies host dust structures up to ~100–200 pc that are strongly coupled to the NLR, radio jets, and coronal emission, while the others show weaker coupling. Dust temperatures in shocked, NLR regions are typically $T_{ m dust}\nobreak hickspace hickspace oughly 150$–$200$ K, higher than what AGN illumination alone can produce for plausible grain sizes and luminosities, implying in-situ heating by shocks with $v_{ m shock} obreak hickspace hickspace ange 200$–$400~ m km~s^{-1}$ in dense gas; simple shock-heating models can reproduce the observed temperatures. The results indicate that shocks associated with AGN outflows may play a crucial role in heating and shaping extended dust in the central hundreds of parsecs, with implications for dust survival, molecular gas processing, and AGN feedback in multi-phase environments.

Abstract

We present JWST/MIRI imaging of eight nearby Active Galactic Nuclei (AGN) from the GATOS survey to investigate the physical conditions of extended dust in their narrow line regions (NLRs). In four galaxies (ESO 428-G14, NGC 4388, NGC 3081, and NGC 5728), we detect spatially resolved dust structures extending ~100-200 pc along the NLR. In these systems, we find a strong link between the morphology of the dust, the radio ejecta, and the coronal [Si VI] emission, implying that dust carries imprints of the processes shaping the NLR. Using spatially resolved spectral energy distributions, we show that dust in the NLR has systematically steeper slopes than star forming clumps. This dust emits at temperatures in the range 150 - 220 K, at a distance of ~150 pc from the nucleus. Using simple models, we show that, even under optimistic assumptions of grain size and AGN luminosity, the excess MIR emission cannot be explained by AGN illumination alone. We interpret this excess heating as in-situ. We show that shocks with velocities of $v_{\rm shock} \sim 200- 400 \, \rm km/s$ in dense gas can close this gap, and in some cases even account for the total observed emission. This, combined with multiple lines of evidence for shocks in these regions, supports a scenario in which shocks not only coexist with dust but may be playing a key role in heating it. Our findings reveal shocks may be an important and previously overlooked driver of extended dust emission in the central hundreds of parsecs in AGN.

Figures (16)

• Figure 1: JWST/MIRI F1000W images of the central regions of four galaxies (ESO 428$-$G14, NGC 4388, NGC 3081, and NGC 5728). North is up and East is to the left. The images have been PSF subtracted and decontaminated from emission lines. Each main panel shows the pure dust continuum emission in logarithmic scale, highlighting extended structures. The zoom-ins display the flux maps of [SiVI] emission observed with SINFONI, matched to a $\sim 4" \times 4"$ field of view. Overlaid contours show the extended dust emission as the main panels. Where relevant, arrows mark regions where shocks are prevalent, based on [FeII] enhancement.
• Figure 2: Similar to Fig. \ref{['dust_in_NLR_AGN']} but for NGC 7172, NGC 2992, NGC 3227, and NGC 5135.
• Figure 3: Left: Spectral energy distributions taken over five filters (F560W, F1000W, F1500W, F1800W, F2100W) normalised by the F1500W filter. Different colours represent different galaxies. Each curve shows the median SED of all ROIs for that galaxy. For each galaxy, the dashed lines represent the median SED of star forming regions. Where relevant, we plot the median for regions covering dust in the NLR, represented as solid lines. The latter have been shifted up in the Y-axis for clarity. The shaded area represents the 16-84 percentile. Right: Galaxy median SEDs for dust in the NLR (solid) and for star-forming clumps (dashed).
• Figure 4: Monte Carlo blackbody fits for the dusty NLRs of ESO 428-G14, NGC 4388 (top row), NGC 3081, and NGC 5728 (bottom row). Each panel shows an SED extracted with an aperture diameter of $0\farcs7$ at $\sim$150 pc from the nucleus, where shocks dominate (see Fig. \ref{['dust_in_NLR_AGN']}). Apertures are marked as orange circles in the upper left of each panel. Red stars are the photometric points. The solid lines represent the median of the fits and the shaded bands show the 16–84th percentiles. For each galaxy, we show the median temperature $T_{\rm med}$, and the 16–84th values ($T_{\rm low}$, $T_{\rm high}$).
• Figure 5: JWST/MIRI "Dust in the Wind" sample. (Left:) Extended dust emission revealed by the JWST/MIRI F1000W image, shown after PSF subtraction and decontamination. (Middle:) Radio continuum emission from VLA observations. (Right:) Comparison between the dust morphology (image) and the radio emission (shown as white contours). The zoomed-in $4" \times 4"$ region displays the [SiVI] coronal line emission from SINFONI.