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Stars don't form out of nothing, but tracking the gas and dust that do eventually form stars is hard. They float around the galaxy at almost absolute zero, emitting essentially no light, and generally making life difficult for astronomers. But part of how they make life difficult is actually the key to studying them鈥攖hey have absorption lines that detail what kind of material the light is passing through on its way to Earth.

A posted to the arXiv preprint server by Harvey Liszt of America's National Radio Astronomy Observatory and Maryvonne Gerin of the Sorbonne details how tracking those absorption lines via radio astronomy can trace the "dark neutral medium" of interstellar gas throughout the galaxy.

The paper describes the findings of 88 sight lines, which in this context is a straight line from Earth to a very bright object, like a quasar or another galaxy. As the light from these bright objects makes its way toward Earth, some of the light is absorbed by the interstellar medium (ISM), creating a distinct dark spot in the spectra coming from the light source.

These absorption lines are particularly strong in the radio spectrum, so the paper focused on data from two different radio antennas. The Atacama Large Millimeter/Submillimeter Array (ALMA), one of the world's best known radio telescopes, the Institut de radioastronomie millim茅trique at the Sorbonne, and the Arizona Radio Observatory, all contributed data to this paper, with some of the data gathered as long as 30 years ago.

Six different ions were the focus of this paper, with varying levels of success. The formyl cation (HCO⁺) was the most commonly found molecule, being present in 72 of the 86 sight lines it had data collected for. It seemed to be the best predictor of where molecular hydrogen gas, the most abundant molecule in the universe, but one that is really hard to directly detect, might be. It forms when H₂ and some other elements are hit by cosmic rays, so a large amount of HCO⁺ would also be indicative that a large amount of H₂ would reside in the same area.

Hydrogen Cyanide (HCN) was another key molecule in the study. Astronomers previously thought this molecule was only present in large quantities in dense clouds of gas where stars were actively forming. However, the paper shows it is present throughout the ISM, forcing some further refinement of the formational process of this molecule.

The ethynyl radical (C₂H) was another key component in the study. It is the second most abundant after HCO⁺, and, as a very simple hydrocarbon, can show how simple hydrocarbons can morph into more complex ones as they undergo reactions in the ISM. The study also notes that the ratio of C₂H to HCO⁺ changes based on the location conditions in that region of space, such as the dust content, so calculating that ratio for different areas might shed (figurative) light on other processes happening there.

Other molecules were harder to track. The study didn't find any carbon monosulfide (CS) at all. Carbon monoxide (CO) was only ever found on sight lines with HCO⁺, making it redundant, even though it was about 100x brighter than the emission from HCO⁺.

Regular formyl radicals (HCO) are also ubiquitous throughout the galaxy, but, according to the paper, their absorption lines are much harder to detect, making them less useful in estimating the presence of these dark gas clouds. HCO⁺ has much more clearly defined lines, making it easier to use for this purpose.

It turns out tracing all these gases throughout the galaxy is one effective way to track down potential areas of star formation, and to watch as the ISM itself starts to clump together in the beginning of that process. As more powerful telescopes come online and we are able to increase the signal to noise ratio of some of these molecules' signals, they will eventually present a clearer picture of this "dark" part of the universe that is teeming with the next round of star stuff.