The mineralogy of newly formed dust in active galactic nuclei

TL;DR

This study analyzes dust composition in AGN tori, revealing a predominance of alumina and amorphous silicates with significant crystalline fractions, which differ from Galactic dust and vary with luminosity.

Contribution

It provides the first detailed quantification of dust mineralogy in AGN tori, highlighting higher crystallinity levels than in the Galaxy and their dependence on source properties.

Findings

Dust mainly consists of alumina and amorphous silicates.

Crystallinity averages 8%, higher than in the Galaxy.

Crystallinity correlates with oxide fraction and luminosity.

Abstract

The tori around active galactic nuclei (AGN) are potential formation sites for large amounts of dust, and they may help resolve the so-called dust budget crisis at high redshift. We investigate the dust composition in 53 of the 87 Palomar Green (PG) quasars showing the 9.7 micron silicate feature in emission. By simultaneously fitting the mid-infrared spectroscopic features and the underlying continuum, we estimate the mass fraction in various amorphous and crystalline dust species. We find that the dust consists predominantly of alumina and amorphous silicates, with a small fraction in crystalline form. The mean crystallinity is 8 +/- 6%, with more than half of the crystallinities greater than 5%, well above the upper limit determined for the Galaxy. Higher values of crystallinity are found for higher oxide fractions and for more luminous sources.