The Warm Ionized Medium (WIM) is a low density, diffuse ionized component of the Milky Way. The WIM is the last major component of the interstellar medium to be studied at high spatial and spectral resolution, and therefore many of its fundamental properties are not clear. Radiation from massive, OB-type stars, which live in the inner galaxy, is thought to escape discrete HII regions to ionize the WIM. However, the inner Galaxy has not been well studied due to extinction from dust at optical wavelengths. GDIGS is a fully-sampled Radio Recombination Line (RRL) survey of the inner Galactic Plane at C-band (4-8 GHz). RRL emission is not affected by extinction from dust, and GDIGS has sufficient spatial resolution to distinguish between HII regions and the WIM emission. Here we discuss the status of GDIGS and some preliminary results from the spectral analysis of the RRLs.
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A 5 GHz Continuum Map of the Inner Galactic Plane with the Green Bank Telescope
Despite the fact that Warm Ionized Medium (WIM) is a major component of the Interstellar Medium our knowledge about it and its relationship with H II regions is very limited. Understanding the WIM better will give us insight into the formation of galaxies and evolution of high-mass star formation regions. Previous surveys of the WIM and H II regions had low spectral and spatial resolutions or looked at Hα, which suffers from extinction. In this project we attempt to get additional value from the GBT Diffuse Ionized Gas Survey (GDIGS), by making continuum maps using the existing data. The goal is to assess whether the GDIGS data can be used to measure the radio recombination line to continuum emission ratio for the diffuse ionized gas.
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- Award ID(s):
- 1714688
- NSF-PAR ID:
- 10342808
- Date Published:
- Journal Name:
- American Astronomical Society Meeting Abstracts #235
- Volume:
- 52
- Issue:
- 1
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4–8 GHz. The nominal survey zone is 32.3° > l > -5°, |b| < 0.5°. Here, we analyze GDIGS Hnα ionized gas emission toward discrete sources with sizes comparable to the 2.065' GDIGS Hnα beam. We use GDIGS data to identify the correct velocity of 39 H II regions that have multiple RRL velocity components. We identify and characterize RRL emission from 88 H II regions that previously lacked measured ionized gas velocities. We additionally identify and characterize RRL emission from eight locations that appear to be previously-unidentified H II regions and 41 locations of RRL emission that do not appear to be H II regions based on their lack of mid-infrared emission. We identify 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane and we compare the objects’ RRL data to 13CO, H I and mid-infrared data. These sources do not have the expected 24 μm emission characteristic of H II regions. Based on this comparison we do not think these objects are H II regions, but we are unable to classify them as a known type of object.more » « less
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Abstract The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4 to 8 GHz. The nominal survey zone is 32.°3 >
ℓ > −5°, ∣b ∣ < 0.°5. Here, we analyze GDIGS Hnα ionized gas emission toward discrete sources. Using GDIGS data, we identify the velocity of 35 Hii regions that have multiple detected RRL velocity components. We identify and characterize RRL emission from 88 Hii regions that previously lacked measured ionized gas velocities. We also identify and characterize RRL emission from eight locations that appear to be previously unidentified Hii regions and 30 locations of RRL emission that do not appear to be Hii regions based on their lack of mid-infrared emission. This latter group may be a compact component of the Galactic Diffuse Ionized Gas. There are an additional 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane. We compare these objects’ RRL data to13CO, Hi, and mid-infrared data, and find that these sources do not have the expected 24μ m emission characteristic of Hii regions. Based on this comparison we do not think these objects are Hii regions, but we are unable to classify them as a known type of object. -
We present an overview of the Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) and the GBT Diffuse Ionized Gas Survey at Low Frequencies (GDIGS-Low). Both GDIGS surveys trace ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission. GDIGS observes RRLs in the 4-8 GHz range and GDIGS-Low maps RRL emission at 800 MHz and 340 MHz. The nominal survey zone for both surveys is 32.3° > ℓ > -5°, |b| < 0.5°, with extensions above and below that latitude limit in select fields as well as coverage of the areas around W47 (ℓ≃37.5°), W49 (ℓ≃43°), and Cygnus X (ℓ≃80°). The goal of these surveys is to better understand the planar Diffuse Ionized Gas (DIG), including its physical properties, its dynamical state and distribution, its relationship with HII regions, and the means by which it is ionized. We discuss an analysis of the DIG around the HII region complex W43 (Luisi et. al. 2020) and a study of discrete sources of emission in the GDIGS survey area (Linville et. al. 2023). We also discuss how we will use GDIGS data to determine the ionic 4He+/ H+ abundance ratio (y+) in the DIG and how we will combine RRL observations from GDIGS and GDIGS-Low to calculate the electron density of the DIG.more » « less
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null (Ed.)Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N II ] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N II ] 122μm data, and spectrally resolved 12 CO. Methods. We observed eight lines of sight (LOS) in the 20° < l < 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N II ] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N II ] and RRL linewidths. Results. The regions with [N II ] 205 μm emission are characterized by electron densities, n ( e ) ~ 10−35 cm −3 , temperatures range from 3400 to 8500 K, and nitrogen column densities N(N + ) ~ 7 × 10 16 to 3 × 10 17 cm −2 . The ionized hydrogen column densities range from 6 × 10 20 to 1.7 × 10 21 cm −2 and the fractional nitrogen ion abundance x (N + ) ~ 1.1 × 10 −4 to 3.0 × 10 −4 , implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N II ] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C II ] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N + resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium.more » « less
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