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Universe has more hydrogen than we thought

‘Undark’ matter hidden in plain view

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A re-analysis of radio telescope observations from three countries has yielded a surprising result: nearby galaxies harbour one-third more hydrogen than had previously been estimated.

While nothing like enough matter to solve physics’ “dark matter” problem, the work by CSIRO astronomer Dr Robert Braun (chief scientist at the agency’s Astronomy and Space Science division in Sydney) also helps explain why the rate of star formation has slowed down. While there’s more hydrogen than astronomers had thought, its distribution makes star formation more difficult.

Andromeda – the galaxy headed for a catastrophic collision with our own in about four billion years – provided the clue for Dr Braun. SpaceRef says he was piqued by “gaps” in the 21-cm radio signal that suggested absorption of the radiation neutral hydrogen gas (tagged HI in astronomy) would normally have emitted.

This, he says, suggests self-absorption of the radiation – glowing gases emit a 21-cm signal, but it’s absorbed by cooler gas in front of it (from our point of view). Researchers have tended to ignore self-absorption in formulating their estimates of hydrogen mass, Dr Braun said.

By taking self-absorption into account, Dr Braun says, his measurements reveal small but massive clouds of HI in all of the galaxies he analyzed – M31, M33 and the Large Magellanic Cloud – sufficient to add between 30 percent and 36 percent to the galactic mass estimates.

The research also shows there’s less HI available in galactic haloes than there was 12 billion years ago – which helps explain why the rate of star formation is so much slower in the modern universe; the haloes act as the reservoirs that re-fuel star formation.

Dr Braun notes that “Although there’s more atomic hydrogen than we thought, it’s not big enough to solve the Dark Matter problem. If what we are missing had the weight of a large kangaroo, what we have found would have the weight of a small echidna.”

As new instruments like the Square Kilometer Array are brought into service, Dr Braun says similar measurements can be made of more distant galaxies. The 50-light-year resolution that Australia’s Parkes Radio Telescope provided for the Large Magellanic Cloud isn’t sufficient for distant galaxies, he said.

However, the current study will feed into a sky survey to be undertaken by Australia’s Square Kilometer Array Pathfinder telescope in Western Australia. The FLASH (first Large Absorption Survey in HI) survey will use background radio continuum sources to identify and characterize neutral hydrogen in the foreground, and increase the accuracy of our measurement of HI absorption lines. ®

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