An anomalous Wilkinson Microwave Anisotropy Probe signal in the ecliptic plane

We report the detection of a high Galactic latitude, large-scale, 7s signal in Wilkinson Microwave Anisotropy Probe (WMAP) 5-year data and spatially correlated with the ecliptic plane. Two possible candidates are studied, namely unresolved sources and Zodiacal light emission. We determine the strength of the Zodiacal light emission at WMAP frequencies and estimate the contribution from unresolved extragalactic sources. Neither the standard Zodiacal light emission nor the unresolved sources alone seem to be able to explain the observed signal. Other possible interpretations such as Galactic foregrounds and diffuse Sunyaev-Zel'dovich effect also seem unlikely. We check if our findings could affect the low-l anomalies which have been reported in the WMAP data. Neither Zodiacal light emission nor unresolved point-source residuals seem to affect significantly the quadrupole and octupole measurements. However, a signal with a quasi-blackbody spectrum and with a spatial distribution similar to the Zodiacal light emission could explain both the anomalous signal and the low-l anomalies. Future data (Planck) will be needed in order to explain the origin of this signal.