Astronomical surveys mapping regions of the Galaxy have been collected and studied for decades. These surveys allow researchers to compare previous data, further characterize objects or images of the sky, and learn more through statistical analysis. For the National Science Foundation’s Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS), researchers took advantage of the power of the GBT, located in Pocahontas County, West Virginia, to better understand the impact of massive stars in the Milky Way.
The international team, led by West Virginia University associate professor Loren Anderson, took a new approach to their survey by focusing on the diffuse ionized gas, or DIG, in the Galactic plane. Ionized gas, also known as plasma, is created by high energy photons that remove electrons from atoms. The DIG, sometimes also called the warm ionized medium, is a low-density plasma that is a major component of the Galactic interstellar medium. Despite decades of mapping, there remain numerous unanswered questions surrounding its origin, distribution, and characteristics. GDIGS gives the clearest view yet of the large-scale distribution of ionized gas in the Milky Way.
Massive stars create an enormous number of high energy photons, which leak out into the Galaxy, creating ionized gas when they interact with material in the Milky Way. The creation of ionized gas can then inhibit future star formation since ionized gas is hotter than the cool material required to make stars.
The Galactic Plane
The Galactic plane can be thought of as an equator, an imaginary plane slicing the Milky Way in half. Nearly all star formation, and an even higher percentage of massive star formation, takes place in the Galactic plane. By creating a detailed map of the Galactic plane, researchers can make an unbiased view of star formation, and study the impact of star formation on the Galaxy.
Resolution through Radio Recombination Lines
Radio recombination lines, or RRLs, are useful tools for understanding the physical conditions of the interstellar medium. RRLs are produced in an ionized medium after an electron and an ion recombine. “Observations of RRLs give us an opportunity to investigate the Galactic plane DIG distribution throughout the Galactic disk at high spatial and spectral resolution,” Anderson explains. Since RRLs are in the radio regime, they are not absorbed by dust in the Galactic plane, and therefore researchers can trace ionized gas throughout the Galaxy.
Green Bank Telescope (GBT)
The GBT is the largest fully steerable radio telescope in the world, with a dish large enough to hold two football fields. Due to its massive collecting area and its ability to observe a substantial number of RRLs simultaneously, the GBT is an ideal instrument to map emission from the warm ionized medium. GDIGS uses the GBT C-band receiver, which can span a wide frequency range. Along with the C-band receiver, the VEGAS spectrometer was used which can simultaneously observe 64 spectral lines.
“The Galactic plane has been mapped in every available way, but this is the first such spectroscopic survey of ionized gas; GDIGS will allow us to create a complete picture.”Loren Anderson
Anderson and team published their survey results with the intention of providing a more complete map of Milky Way ionized gas to the broader astronomical community. Their research characterized the GDIGS RRL data and described enhanced data products that they then provide freely to the public.
Anderson is an associate professor of physics and astronomy at West Virginia University and is a researcher in the Center for Gravitational Waves and Cosmology. His research focuses on the impact of high-mass stars on the Milky Way. Among other projects, Dr. Anderson created the most complete catalog of Galactic HII regions, the zones of ionized gas surrounding massive stars, using WISE telescope data. He is following up on objects in this catalog with radio observations using the GBT, the VLA, and the ATCA.
The Green Bank Observatory is a facility of the National Science Foundation and is operated by Associated Universities, Inc.
Holly Legleiter, Public Relations Coordinator, Center for Gravitational Waves and Cosmology, West Virginia University, ude.u1702085194vw.li1702085194am@re1702085194tielg1702085194elh1702085194
Jill Malusky, Public Relations, Green Bank Observatory, ude.o1702085194arn@y1702085194ksula1702085194mj1702085194