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. nominal survey zone is $32.3^{\circ}> l >-5^{\circ}$, $|b|<0.5^{\circ}$, but coverage extends above and below plane select fields, additionally includes areas around W47 ($l \simeq 37.5^{\circ}$) W49 43^{\circ}$). GDIGS simultaneously observes 22 Hn$\alpha$ (15 usable), 25 Hn$\beta$ (18 8 Hn$\gamma$ RRLs (all as well multiple molecular transitions (including of H$_2^{13}$CO, H$_2$CO, CH$_3$OH). Here, we describe parameters characterize RRL data, focusing primarily on data. We produce sensitive data cubes averaging usable RRLs, after first smoothing to a common spectral resolution 0.5km/s spatial 2.65' for Hn$\alpha$, 2.62' Hn$\beta$, 2.09' Hn$\gamma$. average noise per spaxel \hna\ $\sim\!10$mK ($\sim\!5$mJy/beam). This sensitivity allows detect from plasma throughout inner Galaxy. are emission measures $EM \gtrsim 1100$cm$^{-6}$pc, which corresponds mean electron density $\langle n_e \rangle 30$cm$^{-3}$ 1pc path length or 1$cm$^{-3}$ 1kpc length.