First Observations Of Cold Ionized Hydrogen Hint At Its Star-Stopping Capabilities

This composite image shows the Milky Way, the Engineering Development Array and the plot indicating the presence of ionized hydrogen. Engineering Development Array image courtesy of ICRAR. Milky Way image courtesy of Sandino Pusta.

A hydrogen atom can lose its electron and become ionized when it gets heated to temperatures of over 10,000°C (18,032°F). Now, researchers have discovered a cloud of ionized hydrogen towards the core of the Milky Way with an unexpectedly low temperature: -230°C (-382°F).

This is the first time that such cold ionized hydrogen has been unequivocally observed. As reported in the Monthly Notices of the Royal Astronomical Society, the team conducted a series of observations and found signals corresponding to this cold hydrogen plasma deep inside gas clouds.

The ionized hydrogen is likely to form after atoms are hit by high-energy particles accelerated by supernovae and black holes. Light from massive stars alone can’t cut it. These particles are likely to penetrate through the clouds, which can have galactic consequences. The team suspects that this bombardment is halting star formation.

"The possible existence of cold ionized gas had been hinted at in previous work, but this is the first time we clearly see it," lead author Dr Raymond Oonk, from the Netherlands Institute for Radio Astronomy, said in a statement.

"This discovery shows that the energy needed to ionize hydrogen atoms can penetrate deep into cold clouds. Such cold clouds are believed to be the fuel from which new stars are born. However, in our Galaxy we know that the stellar birth rate is very low, much lower than naively expected. Perhaps the energy observed here acts as a stabilizer for cold clouds, thereby preventing them from collapsing on to themselves and forming new stars."

The observations were conducted with the Engineering Development Array (EDA), a precursor to what will be the next state-of-the-art radio observatory, the Square Kilometer Array. The EDA focuses on low-frequency emissions from the sky. This approach is key to studying radio recombination lines, the emission of ionized atoms that have managed to snatch back the electrons they have lost.

The Square Kilometer Array will be built in both South Africa and Australia and comprise multiple antennae in different research stations. The first construction contracts are expected to be awarded in late 2020. Once completed, researchers expect it to be 50 times more sensitive than any other radio instrument.

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