Ionization of extended low-density warm ionized medium, HII regions and helium radio recombination line from the inner Galaxy

Raju Baddi*
National Center for Radio Astrophysics, Pune 411007, India

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Hydrogen and helium radio recombination line observations using Westerbork Synthesis Radio Telescope along 28 di erent directions from the inner Galaxy have been presented. These observations at 1.4GHz have resulted in detection of 57 hydrogen RL components and some of these (13) have an associated helium RL. The results of correlation of helium RL detection with HII regions and distance to the line originating ionized gas cloud in the form of a weigh scheme for 11 positions from this sample have already been published in a previous paper. Here results obtained from an additional set of 17 new positions have been combined with the previous set. In total 26 cases of hydrogen RL components have either a detection or a non-detection of associated helium RL (13:13) with a detection confidence factor greater than 1.0. These 26 cases further belong to a selected subset of 40 hydrogen RL from the 57. The remaining 14 from the 40 have detection confidence factor less than 1.0 and hence require more data or a better spectrum to ascertain their implication. The 26 hydrogen RL segregate as expected under the HII region correlation weigh scheme with a di erence of a factor of 10 between the expected weight for a detection from a non-detection. All the He RL non-detection cases are found to have a correlation weight less than the 85% of the He RL detection cases. From this weigh scheme it is proposed that ionization of ELDWIM is directly related to near by HII regions. The ionization spectrum being hot or cold depends on the presence of near by HII regions which supply the required ionization photons. This is reflected in the occurence of an associated helium RL with the hydrogen RL, as the first ionization potential of helium (24.6 eV) is nearly twice that of hydrogen (13.6 eV). On the other hand ELDWIM can also form the outer envelopes of the near by HII regions or perhaps exists in both the forms.

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Keywords : HII regions; Radio recommbination lines; ISM