The GBT Diffuse Ionized Gas Survey (GDIGS): Survey Overview and First Data Release

Abstract

The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by measuring 4–8 GHz radio recombination line (RRL) emission. The nominal survey zone is 32.°3 > ℓ > − 5°, ∣ b ∣ < 0.°5, but coverage extends above and below the plane in select fields and additionally includes the areas around W47 ( ℓ ≃ 37.°5) and W49 ( ℓ ≃ 43°). GDIGS simultaneously observes 22 Hn α (15 usable), 25 Hn β (18 usable), and 8 Hn γ RRLs (all usable), as well as multiple molecular line transitions (including those of , H 2 CO, and CH 3 OH). Here, we describe the GDIGS survey parameters and characterize the RRL data, focusing primarily on the Hn α data. We produce sensitive data cubes by averaging the usable RRLs, after first smoothing to a common spectral resolution of 0.5 km s −1 and a spatial resolution of 2.′65 for Hn α , 2.′62 for Hn β , and 2.′09 for Hn γ . The average spectral noise per spaxel in the Hn α data cubes is ∼10 mK (∼5 mJy beam −1 ). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hn α data are sensitive to emission measures EM ≳ 1100 cm −6 pc, which corresponds to a mean electron density 〈 n e 〉 ≳ 30 cm −3 for a 1 pc path length or 〈 n e 〉 ≳ 1 cm −3 for a 1 kpc path length.