Polarization Observations of the Anomalous Microwave Emission in the Perseus Molecular Complex with the COSMOSOMAS Experiment

The anomalous microwave emission detected in the Perseus molecular complex byWatson \ea has been observed at 11 GHz through dual orthogonal polarizationswith the COSMOSOMAS experiment. Stokes U and Q maps were obtained at aresolution of \sim 0.9deg. for a 30deg. X 30deg. region including the Perseusmolecular complex. A faint polarized emission has been measured; we find Q=-0.2% \pm1.0%, while U=-3.4^{+1.8}_{-1.4}% both at the 95% confidence level with asystematic uncertainty estimated to be lower than 1% determined from tests ofthe instrumental performance using unpolarized sources in our map as nullhypothesis. The resulting total polarization level is \Pi = 3.4^{+1.5}_{-1.9}%.These are the first constraints on the polarization properties of an anomalousmicrowave emission source. The low level of polarization seems to indicate thatthe particles responsible for this emission in the Perseus molecular complexare not significantly aligned in a common direction over the whole region, as aconsequence of either a high structural symmetry in the emitting particle or alow-intensity magnetic field. Our weak detection is fully consistent withpredictions from electric dipole emission and resonance relaxation at thisfrequency.