Odd Chemical Found Common in Our Galaxy
ESA's Herschel infrared observatory has an unprecedented view on the cold universe, bridging the gap between what can be observed from the ground and earlier infrared space missions. Infrared radiation can penetrate the gas and dust clouds that hide objects from optical telescopes, looking deep into star-forming regions, galactic centers and planetary systems. Also cooler objects, such as tiny stars and molecular clouds, even galaxies enshrouded in dust that are barely emitting optical light, can be visible in the infrared.
Credit: ESA/C. Carreau

A peculiar chemical compound has been found to be ubiquitous in interstellar gas clouds throughout our Milky Way galaxy and may give scientists a better way to track hydrogen across the universe.

The compound, called hydrogen fluoride, was detected by the European-built Herschel infrared space observatory. It is a mix of the elements hydrogen and fluorine and could serve as a signpost for astronomers trying to track hydrogen caches in the cosmos, researchers said.

"Hydrogen fluoride may be a more robust tracer of hidden molecular hydrogen than any other molecule used at present," said study team member Edwin Bergin of the University of Michigan in Ann Arbor, Mich.

This is because hydrogen fluoride molecules can be found in interstellar gas clouds of all sizes, and not just the largest ones. Hydrogen fluoride can sometimes be found on Earth too as a pale yellow gas attached to ash belched by erupting volcanoes, according to the United States Geological Survey.

Hot hydrogen fluoride was first detected in space by ESA's Infrared Space Observatory in 1997.

But the Herschel observatory's latest finding is the first detection of cold hydrogen fluoride and shows that almost all fluorine in clouds of hydrogen molecules is transformed into the hydrogen fluoride molecular compound, researchers said.

This confirms a prediction made by the observing team five years ago, and also presents astronomers with a new way to trace hydrogen in the Milky Way.

Hydrogen, the most abundant element in the universe, makes up three-quarters of the universe's regular matter. But at low temperatures, like those found in star-forming clouds, hydrogen tends to pair up into molecules that do not readily emit radiation, meaning the molecules cannot be detected by astronomers' telescopes.

Traditionally, scientists have looked for hydrogen by first searching for carbon monoxide, which does emit radiation at extremely low temperatures.

However, this method of detection is imperfect because carbon monoxide does not form directly from hydrogen - intermediate steps are required to create other molecules first. Fluorine's highly reactive nature makes it attack the surrounding hydrogen and create the hydrogen fluoride compound in a single step, researchers said.

Herschel's hydrogen fluoride discovery in the Milky Way is one of several findings by the space telescope announced by the European Space Agency last week.