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

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 degrees.3 > l > -5 degrees, vertical bar b vertical bar < 0 degrees.5, but coverage extends above and below the plane in select fields and additionally includes the areas around W47 (l similar or equal to 37 degrees.5) and W49 (l similar or equal to 43 degrees). GDIGS simultaneously observes 22 Hn alpha (15 usable), 25 Hn beta (18 usable), and 8 Hn gamma RRLs (all usable), as well as multiple molecular line transitions (including those of (H2CO)-C-13, H2CO, and CH3OH). Here, we describe the GDIGS survey parameters and characterize the RRL data, focusing primarily on the Hn alpha 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 alpha, 2'.62 for Hn beta, and 2'.09 for Hn gamma. The average spectral noise per spaxel in the Hn alpha data cubes is similar to 10 mK (similar to 5 mJy beam(-1)). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hn alpha data are sensitive to emission measures EM greater than or similar to 1100 cm(-6) pc, which corresponds to a mean electron density < n(e)> greater than or similar to 30 cm(-3) for a 1 pc path length or < n(e)> greater than or similar to 1 cm(-3) for a 1 kpc path length.