Speaker
Description
Obscured active galactic nuclei (AGN) represent a crucial phase of supermassive black hole growth, in which dense circumnuclear dust hides the central engine and reprocesses its radiation into the infrared. Earlier mid-infrared observations with the Spitzer Space Telescope provided key constraints on dusty torus models but lacked the sensitivity and spectral resolution required to reveal subtle ice absorption features in nearby AGN. Recent JWST/MIRI mid-infrared spectra of the central region of Seyfert 2 galaxy NGC 7582 have revealed prominent ice absorption feature. This is the first time we have detected the icy band at 6 μm in the local galaxies. Such an ice absorption band has not been included in existing AGN or starburst dust radiative transfer models. In particular, the template spectra from current AGN model libraries severely underestimate the mid-infrared flux when compared with the JWST data. To resolve this discrepancy, we propose a new class of frosty AGN models. These models incorporate icy grain mantles into clumpy dust torus configurations. Using Monte Carlo radiative transfer calculations of the spectral energy distribution, we account for both the imprint of embedded ice features and the additional heating contribution from Type-I quasar spectra, thereby extending and updating the AGN model library of Siebenmorgen et al. (2014). Our framework enables us to address several key questions: Under which physical conditions can water freeze onto dust grains in AGN environments? Is the ice reservoir primarily associated with the torus or with circumnuclear starburst regions? And more broadly, why do current AGN models fail to reproduce JWST/MIRI mid-infrared spectra? In this talk, we will present the physical conditions required to produce the 6 μm water-ice band in AGN, discuss the implications for the torus structure, and outline how the inclusion of icy dust fundamentally alters the interpretation of spectral energy distribution (SED) of AGN.