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

TL;DR

This study uses JWST/MIRI imaging to reveal that shocks, rather than just AGN illumination, significantly heat dust in the narrow line regions of nearby AGN, highlighting a previously overlooked mechanism.

Contribution

It demonstrates that shocks are a key factor in dust heating in AGN NLRs, supported by spatially resolved observations and simple modeling, expanding understanding of dust heating processes.

Findings

Dust in NLRs has steeper spectral slopes than star-forming regions.

Dust temperatures range from 150 to 220 K at ~150 pc from the nucleus.

Shocks with velocities of 200-400 km/s can explain the excess MIR emission.

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.