Massive Invisible Galactic Structure is Discovered – By Accident

Incredibly sensitive spectral observations from the Green Bank Telescope discover previously unknown huge Galactic structure

1-4: Typical narrow-line emission from the OH molecule from spiral arms. 5: A new, broad, and faint structure from the OH molecule–in and between the spiral arms. Credit: NSF/GBO/P.Vosteen

When it comes to the Universe, there is more than meets the eye. Astronomers using the National Science Foundation’s Green Bank Telescope (GBT) have discovered a massive, previously unknown structure in our Galaxy. This discovery was so unexpected, additional observations were taken using the Green Bank Observatory’s 20-meter Telescope to confirm the data. 

How do astronomers miss a structure like this? This research used the radio spectrum, which is not visible to the naked eye. The GBT is the world’s largest fully steerable radio telescope, and thanks to its high level of sensitivity, it was able to detect this massive structure, made of molecular gas not just present in more recognizable celestial objects, but in the vast spaces between these structures, throughout the disk of our Galaxy. Researchers believe that this gaseous structure extends far into the outskirts of our Galaxy.

The Universe is composed of a mysterious interstellar medium, which scientists are still learning more about. The major component of the molecular gas in the interstellar medium is H2, but the H2 is usually undetectable! To map it out, radio astronomers look for “tracers” in the form of signals from other molecules mixed with the H2 in smaller quantities, and the standard tracer is carbon monoxide (CO). However, depending on how diffuse and cold the H2 and CO gases are, they still might not be seen. In 2005, astronomers (Grenier et al.) found an excess of cosmic rays, of unknown origin, emanating from the disk of our Galaxy. Was it possible that these rays were a clue to large reserves of molecular gas not yet been detected, and was there another method to trace this gas and confirm its existence? 

In 2012, astronomer Ron Allen, a professor with the Physics and Astronomy Department of Johns Hopkins University, unexpectedly found OH emission without corresponding CO emission while working on an unrelated project. As OH is also a gas molecule that occurs in clouds of molecular H2, this finding hinted that there might be an abundant portion of H2 not traced by CO, also referred to as “CO-dark” molecular gas. 

Allen worked with Dave Hogg of National Radio Astronomy Observatory to create a new research program using the GBT to observe OH as an alternative tracer of H2. Philip Engelke, a new Ph.D. student at Johns Hopkins University, joined the project soon after. In 2015, the first results of this research were published, showing that OH indeed traces the “CO-Dark” component of H2 remarkably well. While it required long exposure times, the OH observations began filling in the gaps between previous CO observations, showing molecular gas as a major component in the structure of our Galaxy.

Later in 2015, while reviewing data, Engelke noticed a bump-like feature, which he initially thought corresponded to the Outer Arm of our Milky Way Galaxy. Follow-up observations revealed a large, faint, broad feature in the entire line of sight. Allen and team were intrigued, but suspected that this feature could be an instrumental by-product of the GBT itself, rather than an actual feature in the Galaxy. The researchers came up with a test: observe the feature with a completely independent telescope.

In 2018, 100 hours of independent observations were conducted using the Observatory’s 20-meter Telescope. Johns Hopkins PhD student Michael Busch joined Allen’s team and played a major role in this work. The 20-meter, a much smaller and older instrument, is primarily used in educational projects including the University of North Carolina’s SkyNet.

The 20-meter observed the signal from the massive structure. Engelke commented, “To be even more sure, we tried several different independent signal processing techniques on the GBT and 20-meter results to attempt to remove the feature as if it were from the instrumental background, rather than a real signal, and none of these methods managed to remove it.” In 2019, another 100 hours of GBT observations at additional points along the disk of the Galaxy revealed that the feature follows the extent and shape of other known components of Galactic structure. At this point the researchers were convinced that the massive structure was actually real! 

What impact will this new discovery have on astronomy? The existence of this massive structure has implications for star formation theories, as well as the structure, make-up, and total mass of the interstellar medium. 

Professor Allen passed away in August of 2020, as this research was drafted by his former PhD students for publication in the Astrophysical Journal. Busch shared, “Ron was an incredible mentor, a brilliant astronomer, and a great friend to me… I will miss him dearly.” Adds Engelke, who completed his doctorate in 2019, “We were very lucky to have known him. Ron was truly excited about this discovery, and I know he would have been proud of the result. Michael and I look forward to continued research inspired by this discovery.” 

Learn more about the life and work of Ron Allen.

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The Green Bank Observatory is a facility of the National Science Foundation and is operated by Associated Universities, Inc.

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Contact:

Jill Malusky, Green Bank Observatory Public Relations, jmalusky@nrao.edu