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

TL;DR

This study maps the cold neutral medium and explores its relationship with PAHs and anomalous microwave emission, finding strong correlation with PAHs but no link to AME, challenging existing models.

Contribution

Introduces a new full-sky $f_{CNM}$ map using neural networks and investigates its connection to PAHs and AME, revealing unexpected results.

Findings

$f_{CNM}$ correlates with PAH dust fraction

No correlation between $f_{CNM}$ and AME intensity

No link between $f_{CNM}$ and AME peak frequency

Abstract

Using new large area maps of the cold neutral medium (CNM) fraction, $f_{\rm CNM}$, we investigate the relationship between the CNM, the abundance of polycyclic aromatic hydrocarbons (PAHs), and the anomalous microwave emission (AME). We first present our $f_{\rm CNM}$ map based on full-sky HI4PI data, using a convolutional neural network to covert the spectroscopic HI data to $f_{\rm CNM}$. We demonstrate that $f_{\rm CNM}$ is strongly correlated with the fraction of dust in PAHs as estimated from mid- and far-infrared dust emission. In contrast, we find no correlation between $f_{\rm CNM}$ and the amount of AME per dust emission, nor between $f_{\rm CNM}$ 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 $f_{\rm CNM}$ 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$\mu$m PAH emission and AME even if PAHs are the AME carriers.