The GBT Diffuse Ionized Gas Survey: Tracing the Diffuse Ionized Gas around the Giant Hii Region W43

Abstract

The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) is a fully sampled radio recombination line (RRL) survey of the inner Galaxy at C-band (4-8 GHz). We average together ∼15 Hnα RRLs within the receiver bandpass to improve the spectral signal-to-noise ratio. The average beam size for the RRL observations at these frequencies is ∼2'. We grid these data to have spatial and velocity spacings of 30″ and 0.5 $\mathrm{km}\,{{\rm{s}}}^{-1}$ , respectively. Here we discuss the first RRL data from GDIGS: a 6 deg² area surrounding the Galactic H II region complex W43. We attempt to create a map devoid of emission from discrete H II regions and detect RRL emission from the diffuse ionized gas (DIG) across nearly the entire mapped area. We estimate the intensity of the DIG emission by a simple empirical model, taking only the H II region locations, angular sizes, and RRL intensities into account. The DIG emission is predominantly found at two distinct velocities: ∼40 and ∼100 $\mathrm{km}\,{{\rm{s}}}^{-1}$ . While the 100 $\mathrm{km}\,{{\rm{s}}}^{-1}$ component is associated with W43 at a distance of ∼6 kpc, the origin of the 40 $\mathrm{km}\,{{\rm{s}}}^{-1}$ component is less clear. Since the distribution of the 40 $\mathrm{km}\,{{\rm{s}}}^{-1}$ emission cannot be adequately explained by ionizing sources at the same velocity, we hypothesize that the plasma at the two velocity components is interacting, placing the 40 $\mathrm{km}\,{{\rm{s}}}^{-1}$ DIG at a similar distance as the 100 $\mathrm{km}\,{{\rm{s}}}^{-1}$ emission. We find a correlation between dust temperature and integrated RRL intensity, suggesting that the same radiation field that heats the dust also maintains the ionization of the DIG.