Dust in the Wind: Crystalline Silicates, Corundum, and Periclase in PG 2112+059

Abstract

We have determined the mineralogical composition of dust in the broad absorption line (BAL) quasar PG 2112+059 using mid-infrared spectroscopy obtained with the Spitzer Space Telescope. From spectral fitting of the solid state features, we find evidence for Mg-rich amorphous silicates with olivine stoichiometry, as well as the first detection of corundum (Al 2O3) and periclase (MgO) in quasars. This mixed composition provides the first direct evidence for a clumpy density structure of the grain-forming region. The silicates in total encompass 56.5% ± 1.4% of the identified dust mass, while corundum takes up 38 ± 3 wt.%. Depending on the choice of continuum, a range of mass fractions is observed for periclase ranging from 2.7% ± 1.7% in the most conservative case to 9% ± 2% in a less-constrained continuum. In addition, we identify a feature at 11.2 μm as the crystalline silicate forsterite, with only a minor contribution from polycyclic aromatic hydrocarbons. The 5% ± 3% crystalline silicate fraction requires high temperatures such as those found in the immediate quasar environment in order to counteract rapid destruction from cosmic rays.