On the Nature of Interstellar Grains

Abstract

Whether through absorption, scattering, or emission, the effects of interstellar dust are manifest at nearly all wavelengths of astronomical interest. This dissertation synthesizes observations of the diverse optical phenomenology of interstellar dust to investigate the material composition of interstellar grains and to better understand their electromagnetic properties.

Chapter 1 of this dissertation serves as both a theoretical and observational introduction. First, the computation of absorption, scattering, and extinction cross sections for particles of a specified material and shape is discussed. Chapter 1 then details the translation of these cross sections into the consequent observable dust properties for a population of interstellar grains. Finally, the current state of observations that constrain the properties of interstellar dust, focusing on elemental abundances, extinction, polarized extinction, emission, and polarized emission, is reviewed. These observations establish benchmark behavior against which models of interstellar dust can be compared.

Chapters 2 and 3 focus on the Anomalous Microwave Emission (AME) and in particular whether this emission is produced by spinning polycyclic aromatic hydrocarbons (PAHs). Chapter 2 combines infrared and radio observations of NGC6946 to investigate the relationship between the AME, far-infrared dust emission, and PAH emission. No link between the AME and PAHs is found. Chapter 3 presents a similar analysis for the Galaxy employing a full-sky AME map derived from component separation of the microwave sky. The AME is found to be tightly correlated with the dust radiance but not with the presence of PAHs.

Drawing on the observations presented in Chapter 1, Chapter 4 presents new models of interstellar dust that reproduce simultaneously current observations of dust extinction and emission, both total and polarized. Particular attention is paid to the frequency dependence of the polarized infrared emission, which is found to be compatible with both the presence of ferromagnetic nanoparticles embedded in silicate grains or magnetic dipole emission in large, highly conducting graphitic grains. Chapter 4 concludes by discussing observational predictions of these models and their possible extensions beyond the diffuse interstellar medium.