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

TL;DR

The GDIGS survey uses the Green Bank Telescope to map ionized gas in the Galactic midplane via radio recombination lines, providing sensitive data that reveal the distribution and properties of ionized plasma across the inner Galaxy.

Contribution

This paper presents the first data release of GDIGS, detailing survey parameters, data processing, and the sensitivity achieved in detecting ionized gas through radio recombination lines.

Findings

Sensitive detection of RRLs across the inner Galaxy.

Data cubes with spectral noise ~10 mK enabling plasma studies.

Survey covers key regions W47 and W49.

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 $\langle n_e \rangle \gtrsim 30$cm$^{-3}$ for a 1pc path length or $\langle n_e \rangle \gtrsim 1$cm$^{-3}$ for a 1kpc path length.