Polycyclic Aromatic Hydrocarbons, the Anomalous Microwave Emission, and Their Connection to the Cold Neutral Medium

Abstract

Using new large area maps of the cold neutral medium (CNM) fraction, f CNM , we investigate the relationship between the CNM, the abundance of polycyclic aromatic hydrocarbons (PAHs), and the anomalous microwave emission (AME). We first present our f CNM map based on full-sky HI4PI data, using a convolutional neural network to covert the spectroscopic Hi data to f CNM . We demonstrate that f CNM is strongly correlated with the fraction of dust in PAHs as estimated from mid-and farinfrared dust emission. In contrast, we find no correlation between f CNM and the amount of AME per dust emission, nor between f CNM and the AME peak frequency. These results suggest PAHs preferentially reside in cold, relatively dense gas, perhaps owing to enhanced destruction in more diffuse media. The lack of correlation between f CNM and AME peak frequency is in tension with expectations from theoretical models positing different spectral energy distributions of AME in the cold versus warm neutral medium. We suggest that different PAH abundances and emission physics in different interstellar environments may explain the weaker than expected correlation between 12 µm PAH emission and AME even if PAHs are the AME carriers.

QUIJOTE Scientific Results -- VII. Galactic AME sources in the QUIJOTE-MFI Northern Hemisphere Wide-Survey

“…Our study is focused on high column density regions pervaded by molecular clouds, i.e., including cold neutral medium (CNM) phases, mainly located along the Galactic plane. Using a completely different method, Hensley et al (2021) investigated the relationship between the CNM, the AME and the abundance of PAHs over large areas associated with diffuse ISM regions (𝑁 HI < 4 × 10 20 cm −2 ) at high Galactic latitudes (| 𝑏 |> 30 • ).…”