The GBT Diffuse Ionized Gas Survey (GDIGS) is a large Green Bank Telescope survey that mapped radio recombination line emission from diffuse ionized gas in the Galactic plane.
Diffuse ionized gas ("DIG") known as the is a major component of the interstellar medium, making up ∼20% of the total Milky Way gas mass and providing an important source of pressure at the midplane. Given that we have known about it for over 50 years, it is surprising that there remain major unanswered questions regarding the origin, distribution, and characteristics of the DIG.
Most DIG studies have been conducted by observing Hα emission. Although Hα is very bright compared to other ionized gas tracers, it suffers from extinction. This drastically limits the distance to which the DIG in the inner Galaxy can be studied. Radio observations give us an opportunity to investigate the Galactic mid-plane DIG distribution throughout the Galactic disk.
Due to its large collecting area and its ability to observe a large number of radio recombination line transitions simultaneously, the GBT is an ideal instrument to map emission from the DIG. By using the C-band receiver in combination with the VEGAS backend, we create fully-sampled, high-sensitivity maps of the DIG at a higher spatial and spectral resolution than previous radio recombination line surveys in a reasonable amount of telescope time. The GBT beam is 2.65' and the data are gridded with 30" pixels and 0.5 km s-1 wide channels. The GDIGS spectral rms is ∼10 mJy per 0.5 km-1 channel.
The GDIGS survey range spans the inner Galaxy from 32.3° > ℓ > -5°. and |b| < 0.5°, as well as some explorations above and below the midplane near high-mass star-forming regions. There is additional coverage around high-mass star forming complexes in the Galaxy (M16, M17, W40, W47, W49, W51, Orion, S235, Cygnus, and NGC7538).
With these data we will 1) determine the dynamical state and distribution of the DIG, 2) study the ionization state of the DIG, 3) explore the relationship between the DIG and HII regions, and 4) analyze the impact of leaking ionizing radiation from HII regions on dust emission.
The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) is a fully sampled radio recombination line (RRL) survey of the inner Galaxy at C-band (48 GHz). We average together ~15 Hnα RRLs within the receiver bandpass to improve the spectral signal-to-noise ratio. The average beam size for the RRL observations at these frequencies is ~2'. We grid these data to have spatial and velocity spacings of 30" and 0.5 km s-1, respectively. Here we discuss the first RRL data from GDIGS: a 6 square degree area surrounding the Galactic HII region complex W43. We attempt to create a map devoid of emission from discrete HII regions and detect RRL emission from the diffuse ionized gas (DIG) across nearly the entire mapped area. We estimate the intensity of the DIG emission by a simple empirical model, taking only the HII region locations, angular sizes, and RRL intensities into account. The DIG emission is predominantly found at two distinct velocities: ~40 and ~100 km s-1. While the 100 km s-1 component is associated with W43 at a distance of ~6 kpc, the origin of the 40 km s-1 component is less clear. Since the distribution of the 40 km s-1 emission cannot be adequately explained by ionizing sources at the same velocity, we hypothesize that the plasma at the two velocity components is interacting, placing the 40 km s-1 DIG at a similar distance as the 100 km s-1 emission. We find a correlation between dust temperature and integrated RRL intensity, suggesting that the same radiation field that heats the dust also maintains the ionization of the DIG.
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 (~37.5°) and W49 (~43°). GDIGS simultaneously observes 22 Hnα (15 usable), 25 Hnβ (18 usable), and 8 Hnγ RRLs (all usable), as well as multiple molecular line transitions (including those of H213CO, H2CO, and CH3OH). Here, we describe the GDIGS survey parameters and characterize the RRL data, focusing primarily on the Hnα 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 Hnα, 2.62' for Hnβ, and 2.09' for Hnγ. The average spectral noise per spaxel in the Hnα data cubes is <10 mK (<5 mJy beam-1). This sensitivity allows GDIGS to detect RRLs from plasma throughout the inner Galaxy. The GDIGS Hnα data are sensitive to emission measures EM ≲1100 cm-6 pc, which corresponds to a mean electron density ⟨ne⟩ ≳ 30 cm-3 for a 1 pc path length or ⟨ne⟩ ≳ 1 cm-3 for a 1 kpc path length.
