Spinning dust emission from ultrasmall silicates: emissivity and polarization spectrum

Abstract

Anomalous microwave emission (AME) is an important Galactic foreground of Cosmic Microwave Background (CMB) radiation. It is believed that the AME arises from rotational emission by spinning polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). In this paper, we assume that a population of ultrasmall silicate grains may exist in the ISM, and quantify rotational emissivity from these tiny particles and its polarization spectrum. We found that spinning silicate nanoparticles can produce strong rotational emission when those small grains follow a log-normal size distribution. The polarization fraction of spinning dust emission from tiny silicates increases with decreasing the dipole moment per atom ($\beta$) and can reach $P\sim 20\%$ for $\beta\sim 0.1$D at grain temperature of 60 K. We identify a parameter space $(\beta,Y_{Si})$ for silicate nanoparticles in which its rotational emission can adequately reproduce both the observed AME and the polarization of the AME, without violating the observational constraints by the ultraviolet extinction and polarization of starlight. Our results reveal that rotational emission from spinning silicate may be an important source of the AME.