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

Abstract

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.

Figures (6)

• Figure 1: The PAH spectrum as modeled in Hensley2023 is shown in blue. Grey rectangles indicate the wavelength extents of DIRBE bands 1 -- 4 as marked. The PAH 3.3 $\mu$m emission feature due to the C-H stretching mode in band 3 is also marked. We note that bands 1 and 2 are dominated by the stellar continuum, which we indicate with the orange line.
• Figure 2: Upper left: The 2-component Commander AME model evaluated at 30 GHz. Upper right: Planck 857 GHz dust map. Bottom: The Planck radiance map with an $N_{\rm side}$ of 512. All maps are smoothed to a 0.5$^{\circ}$ resolution and are shown in a logarithmic color scale.
• Figure 3: Top: 3.3 $\mu$m PAH emission feature derived from the COSMOGLOBE DIRBE data products and smoothed to a 0.5$^{\circ}$ resolution. The map is shown in a linear scale with bright stellar sources and PAH emission coinciding with DIRBE signal-to-noise $<$ 5 masked out, as indicated by the gray points in the image. Bottom: the signal-to-noise ratio of COSMOGLOBE DIRBE band 3 used to mask the PAH map, smoothed to a 0.5$^{\circ}$ resolution and shown in a logarithmic scale. The mask applied to the PAH emission has also been applied to the displayed signal-to-noise map.
• Figure 4: Galactic Plane correlations between the dust, PAH, and AME. Left: correlation between the AME and the dust emission in log-log space. Right: correlation between the AME and the PAH 3.3 $\mu$m emission feature in log-log space. The Spearman-r rank correlation coefficient is shown in the upper left of both panels.
• Figure 5: Correlations of the PAH emission and the AME in 5$^{\circ}$ wedges of the sky as described in the text. Each histogram is presented in the same way as the right-hand panel of Figure \ref{['fig:corr_full']}. The Spearman-r rank correlation coefficient is also shown in each panel.