Dust in the wind: Crystalline silicates, corundum and periclase in PG 2112+059

TL;DR

This study analyzes the dust composition in the quasar PG 2112+059, revealing a complex mix of amorphous silicates, corundum, periclase, and crystalline silicates, indicating diverse dust formation conditions in quasar environments.

Contribution

First detection of corundum and periclase in quasar dust, providing evidence for a clumpy grain formation region and detailed mineralogical composition analysis.

Findings

Presence of Mg-rich amorphous silicates with olivine stoichiometry

Detection of corundum (Al2O3) and periclase (MgO) in quasar dust

Identification of crystalline forsterite at 11.2 microns

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_2O_3) 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% of the identified dust mass, while corundum takes up 38 wt.%. Depending on the choice of continuum, a range of mass fractions is observed for periclase ranging from 2.7% in the most conservative case to 9% in a less constrained continuum. In addition, we identify a feature at 11.2 micron as the crystalline silicate forsterite, with only a minor contribution from polycyclic aromatic hydrocarbons. The 5% crystalline silicate fraction requires high temperatures such as those found in the immediate quasar environment in order to counteract rapid destruction from cosmic rays.