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 $32.3^{\circ }>l>-5^{\circ }$, $|b|<0.5^{\circ }$, but coverage extends above and below the plane in select fields, and additionally includes the areas around W47 ($l\simeq 37.5^{\circ }$) and W49 ($l\simeq 43^{\circ }$). 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 of H${}_2^{13}$CO, H${}_2$CO, and CH${}_3$OH). 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 0.5km/s 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 \hna\ data cubes is $\sim 10$mK ($\sim 5$mJy/beam). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hn$\alpha$ data are sensitive to emission measures $EM\gtrsim 1100$cm${}^{-6}$pc, which corresponds to a mean electron density $⟨n_e⟩\gtrsim 30$cm${}^{-3}$ for a 1pc path length or $⟨n_e⟩\gtrsim 1$cm${}^{-3}$ for a 1kpc path length.

Accepted for publication by ApJS. Data may be downloaded here: http://astro.phys.wvu.edu/gdigs/