Dust Destruction by Drift-Induced Sputtering in Active Galactic Nuclei

Abstract

Recent mid-infrared high spatial resolution observations have revealed that active galactic nuclei (AGNs) may host a polar dust region with the size of several pc, and such dust may be carried by radiation from the central engine. The polar dust emission often exhibits very weak or absence of the silicate 10-$\mu$m emission feature. A possible explanation is that the polar dust is dominated by micron-sized large grains because these grains do not show the silicate feature, while it remains unclear how large grains are preferentially supplied to the polar region. We here propose a new scenario describing the prevalence of large grains at the polar region. We show that grains are accelerated to the hypersonic drift velocity by the radiation pressure from AGN, and the hypersonic drift results in dust destruction via kinetic sputtering. Sputtering destroys small grains faster than the large ones, and thus larger grains will be preferentially blown over longer distance. Although the hypersonic drift, or kinetic sputtering, tends to be suppressed for very small grains due to the Coulomb drag, they might also be disrupted by Coulomb explosion. Removal of small grains and/or survival of large grains may explain the lack of a silicate 10-$\mu$m emission feature in polar dust emission.

17 pages, 14 Figures; Accepted for publication in ApJ

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• Infrared Spectral Energy Distribution and Variability of Active Galactic Nuclei: Clues to the Structure of Circumnuclear Material
• Dust attenuation evolution in $z\sim 2$-$12$ JWST galaxies
• Infrared emission of $z\sim 6$ galaxies: AGN imprints
• Hypercubes of AGN Tori (HYPERCAT) -- I. Models and Image Morphology
• The dust attenuation law in $z\sim 6$ quasars
• Dust beyond the torus: Revealing the mid-infrared heart of local Seyfert ESO 428-G14 with JWST/MIRI
• The role of grain size in AGN torus dust models
• The VMC Survey -- XLIX. Discovery of a population of quasars dominated by nuclear dust emission behind the Magellanic Clouds
• Modelling Grain Rotational Disruption by Radiative Torques and Extinction of Active Galactic Nuclei
• Fading Light, Fierce Winds: JWST Snapshot of a Sub-Eddington Quasar at Cosmic Dawn
• H1821+643: The most X-ray and infrared luminous AGN in the Swift/BAT survey in the process of rapid stellar and supermassive black hole mass assembly