Helium line detections from extended low-density warm-ionized medium at 1.4 GHz

Abstract

Helium line observations toward 11 Galactic positions using the Westerbork Synthesis Radio Telescope (WSRT) have been reported. These observations were made toward nearby positions where hydrogen lines had already been detected at sufficiently high intensity (>=50 mK) at 1.4 GHz. This approach provided a fair chance of detecting helium lines as well, keeping in mind the relative abundance (10%) of helium with respect to hydrogen. Care was also taken to avoid the presence of H II regions along the line of sight so that the line emission originated from the extended low-density warm-ionized medium (ELDWIM) of the Galaxy. The observations resulted in the detection of helium lines toward 5 positions out of 11 with a signal-to-noise ratio > 4σ. An attempt has been made to associate detection/non-detection of helium lines with the presence of surrounding H II regions. A weighting scheme that accounts for nearby (<500 pc) H II regions, their distances, and other factors produces favorable results. It is seen from this weighting scheme that a higher weight favors the detection of helium lines, while a lower weight is associated with non-detection. The idea is to correlate ionization of ELDWIM with the surrounding H II regions.