Constraining Spinning Dust Parameters with the WMAP Five-Year Data

TL;DR

This study analyzes WMAP five-year data to characterize spinning dust emission in the warm ionized medium, confirming the presence of a broad emission bump around 40 GHz and constraining dust grain properties.

Contribution

It provides the first detailed comparison of spinning dust models with WMAP five-year data, refining the peak frequency and amplitude estimates, and constraining dust grain depletion factors.

Findings

The microwave emission bump peaks at ~40 GHz, lower than previously claimed.

The bump is consistent with spinning dust models, not free-free emission.

Small PAHs are depleted by a factor of ~3 in the warm ionized medium.

Abstract

We characterize spinning dust emission in the warm ionized medium by comparing templates of Galactic dust and Halpha with the 5-year maps from the Wilkinson Microwave Anisotropy Probe. The Halpha-correlated microwave emission deviates from the thermal bremsstrahlung (free-free) spectrum expected for ionized gas, exhibiting an additional broad bump peaked at ~40 GHz which provides ~20% of the peak intensity. We confirm that the bump is consistent with a modified Draine & Lazarian (1998) spinning dust model, though the peak frequency of the emission is somewhat lower than the 50 GHz previously claimed. This frequency shift results from systematic errors in the large-scale modes of the 3-year WMAP data which have been corrected in the 5-year data release. We show that the bump is not the result of errors in the Halpha template by analyzing regions of high free-free intensity, where the WMAP K-band map may be used as the free-free template. We rule out a pure free-free spectrum for the Halpha-correlated emission at high confidence: ~27sigma for the nearly full-sky fit, even after marginalizing over the CMB cross-correlation bias. We also extend the previous analysis by searching the parameter space of the Draine & Lazarian model but letting the amplitude float. The best fit for reasonable values of the characteristic electric dipole moment and density requires an amplitude factor of ~0.3. This suggests that small PAHs in the warm ionized medium are depleted by a factor of ~3.