Analyzing PAHs as a Tracer of Anomalous Microwave Emission Near the Galactic Plane Using the COSMOGLOBE DIRBE Reduction

The physical mechanism producing Anomalous Microwave Emission (AME) has been an unresolved puzzle for close to 30 years. One candidate mechanism is rotational emission from polycyclic aromatic hydrocarbons (PAHs) which can have the necessary electric dipole moment and size distribution to account for the AME in representative interstellar environments. However, previous investigations have found that AME is better correlated with the far-infrared dust emission rather than the PAH emission. In this work we analyze the correlations between the AME and the PAH and far-infrared dust emission using the 3.3 $μ$m PAH emission feature as observed by band 3 of the Diffuse Infrared Background Experiment (DIRBE). This analysis builds on previous work conducted in individual molecular clouds and extends it into fainter, more diffuse structures. In addition, we utilize the COSMOGLOBE DIRBE reduction for this work, building on previous studies that used the original DIRBE data set. We find that the AME is better correlated with far-infrared dust emission ($ρ\sim$0.9) than the PAH emission ($ρ\sim$0.7) in the central $|b|\leq$ 10\degree\ region of the sky. This could indicate either that non-PAH dust grains or an alternative physical emission mechanism is primarily responsible for the AME in the Galactic Plane, or that the excitation conditions for mid-infrared emission and for AME from PAHs differ substantially.

💡 Research Summary

This paper investigates the long‑standing mystery of Anomalous Microwave Emission (AME) by examining its correlation with polycyclic aromatic hydrocarbon (PAH) emission and far‑infrared (FIR) dust emission across the entire Galactic plane (|b| ≤ 10°). Using the COSMOGLOBE re‑processing of the COBE/DIRBE data, the authors construct a full‑sky map of the 3.3 µm PAH feature, which is confined to DIRBE band 3 (central wavelength 3.5 µm). The methodology involves masking bright stellar sources (pixels exceeding 0.5 MJy sr⁻¹ above a local background), modeling the residual starlight with the Faint Source Model (FSM), and performing a linear least‑squares (LLS) fit of two basis functions—one for PAH emission and one for FSM—across DIRBE bands 1–4. The fit yields amplitude maps for PAH (a₁) and starlight (a₂). A signal‑to‑noise cut of 5 in band 3 is applied, resulting in a 0.5°‑resolution PAH map that retains only reliable detections.

For the AME component, the authors adopt the Planck 2016 Commander two‑component model evaluated at 30 GHz, smoothed to the same 0.5° resolution. FIR dust emission is traced by the Planck PR2 857 GHz map, while total dust radiance is taken from the Planck PR2 foreground products. All maps are placed on a HEALPix grid with Nside = 512 (≈0.1° pixels) and then smoothed to the common resolution.

Correlation analysis is performed in log‑log space using two‑dimensional histograms and Spearman rank coefficients (ρ). Across the full |b| ≤ 10° region, the AME–FIR correlation is strong (ρ ≈ 0.906), whereas the AME–PAH correlation is weaker (ρ ≈ 0.740). The authors further split the data into two latitude wedges (0° ≤ |b| < 5° and 5° ≤ |b| < 10°). In the inner wedge, ρ(AME,FIR) = 0.93 and ρ(AME,PAH) = 0.77; in the outer wedge, ρ(AME,FIR) = 0.87 and ρ(AME,PAH) = 0.36. The correlation coefficients are largely insensitive to variations in the S/N threshold, indicating that the observed differences are not driven by noise or fitting artifacts.

The paper discusses two primary interpretations of these results. First, non‑PAH ultra‑small grains, such as nano‑silicates, could dominate the AME, consistent with theoretical work suggesting that any small grain possessing an electric dipole moment can emit rotational radiation. Second, PAHs may indeed be the AME carriers, but the physical conditions governing their mid‑infrared (3.3 µm) emission and their rotational emission differ substantially—variations in electron density, gas temperature, and grain charging can decouple the two observables, reducing the apparent correlation. An alternative hypothesis—that AME arises from thermal vibrational emission rather than spinning dust—is also mentioned.

The study’s significance lies in extending correlation analyses from isolated molecular clouds to the entire Galactic plane, providing the first large‑scale quantitative comparison of PAH and FIR tracers of AME. The finding that FIR dust emission remains a better predictor of AME than PAH emission imposes a strong constraint on models of the anomalous microwave component. The authors recommend future work that combines higher‑resolution PAH spectroscopy (e.g., JWST, SPICA) with next‑generation microwave surveys (e.g., C‑BASS, QUIJOTE) and refined nano‑grain rotational models to disentangle the relative contributions of PAH and non‑PAH carriers.