Constraining Spinning Dust Parameters With the WMAP Five-Year Data

Abstract

We characterize spinning dust emission in the warm ionized medium (WIM) by comparing templates of Galactic dust and Hα with the five-year maps from the Wilkinson Microwave Anisotropy Probe (WMAP). The Hα-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 and Lazarian 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 three-year WMAP data which have been corrected in the five-year data release. We show that the bump is not the result of errors in the Hα 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 Hα-correlated emission at high confidence: ~27σ for the nearly full-sky fit, even after marginalizing over the cosmic microwave background cross-correlation bias. We also extend the previous analysis by searching the parameter space of the Draine and 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 polycyclic aromatic hydrocarbons in the WIM are depleted by a factor of ~3.