Spinning Dust Radiation: A Review of the Theory

This paper reviews the current status of theoretical modeling of electric dipole radiation from spinning dust grains. The fundamentally simple problem of dust grain rotation appeals to a rich set of concepts of classical and quantum physics, owing to the diversity of processes involved. Rotational excitation and damping rates through various mechanisms are discussed, as well as methods of computing the grain angular momentum distribution function. Assumptions on grain properties are reviewed. The robustness of theoretical predictions now seems mostly limited by the uncertainties regarding the grains themselves, namely, their abundance, dipole moments, and size and shape distribution. 1. Introduction Rotational radiation from small grains in the interstellar medium (ISM) has been suggested as a source of radio emission several decades ago already. The basic idea was first introduced by Erickson (1957) [1] and then revisited by Hoyle and Wickramasinghe (1970) [2] and Ferrara and Dettmar (1994) [3]. Rouan et al. (1992) [4] were the first to provide a thorough description of the physics of rotation of polycyclic aromatic hydrocarbons (PAHs), although not including all gas processes. Shortly after the discovery of the anomalous dust-correlated microwave emission (AME) in the galaxy by Leitch et al. (1997) [5], Draine and Lazarian (1998, hereafter DL98) [6, 7] suggested that spinning dust radiation might be responsible for the AME and provided an in-depth theoretical description of the process. Understanding the spinning dust spectrum in as much detail as possible is important. First, the AME constitutes a foreground emission to cosmic microwave background (CMB) radiation. Second, it provides a window into the properties of small grains, which play crucial roles for the physics and chemistry of the ISM. Motivated by these considerations and the accumulating observational evidence for diffuse and localized AME, several groups have since then revisited and refined the DL98 model [8–12]. New physical processes were accounted for, which can significantly affect the predicted spectrum. A publicly available code to evaluate spinning dust emissivities (SPDUST) is now available, including most (but not all thus far) processes recently investigated (SPDUST is available at http://www.sns.ias.edu/~yacine/spdust/spdust.html.). The purpose of this paper is to provide an overview of the physics involved in modeling spinning dust spectra. We attempt to provide a comprehensive description of the problem at the formal level, and let the interested reader learn about the