Modelling the spinning dust emission from dense interstellar clouds

Context. Electric dipole emission arising from rapidly rotating polycyclic aromatic hydrocarbons (PAHs) is often invoked to explain the anomalous microwave emission. This assignation is based on i) an observed tight correlation between the mid-IR emission of PAHs and the anomalous microwave emission; and ii) a good agreement between models of spinning dust and the broadband anomalous microwave emission spectrum. So far often detected at large scale in the diffuse interstellar medium, the anomalous microwave emission has recently been studied in detail in well-known dense molecular clouds with the help of Planck data. Aims. While much attention has been given to the physics of spinning dust emission, the impact of varying local physical conditions has not yet been considered in detail. Our aim is to study the emerging spinning dust emission from interstellar clouds with realistic physical conditions and radiative transfer. Methods. We use the DustEM code to describe the extinction and IR emission of all dust populations. The spinning dust emission is obtained with SpDust, which we have coupled to DustEM. We carry out full radiative transfer simulations and carefully estimate the local gas state as a function of position within interstellar clouds. Results. We show that the spinning dust emission is sensitive to the abundances of the major ions (H II, C II) and we propose a simple scheme to estimate these abundances. We also investigate the effect of changing the cosmic-ray rate. In dense media, where radiative transfer is mandatory to estimate the temperature of the grains, we show that the relationship between the spinning and mid-IR emissivities of PAHs is no longer linear and that the spinning dust emission may actually be strong at the centre of clouds where the mid-IR PAH emission is weak. These results provide new ways to trace grain growth from diffuse to dense medium and will be useful for the analysis of anomalous microwave emission at the scale of interstellar clouds.