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Ancient Hydrogen Signals --a Key to the Evolution of the Universe


“Hydrogen is the building block of the Universe, it’s what stars form from and what keeps a galaxy ‘alive’,” said Jacinta Delhaize a researcher with theInternational Centre for Radio Astronomy Research (ICRAR). In research published in the Monthly Notices of the Royal Astronomical Society, Delhaize has studied distant galaxies en masse to determine one of their important properties – how much hydrogen they contain – by ‘stacking’ their signals. “Galaxies in the past formed stars at a much faster rate than galaxies now. We think that past galaxies had more hydrogen, and that might be why their star formation rate is higher."

“Distant, younger, galaxies look very different to nearby galaxies, which means that they’ve changed, or evolved, over time,” said Delhaize. “The challenge is to try and figure out what physical properties within the galaxy have changed, and how and why this has happened.”

Delhaize and her supervisors set out to observe how much hydrogen was in far away galaxies, but the faint radio signals of this distant hydrogen gas are almost impossible to detect directly. This is where the new stacking technique comes in.

To gather enough data for her research, Delhaize combined weak signals from thousands of individual galaxies, stacking them to produce a strong averaged signal that is easier to study.

“What we are trying to achieve with stacking is sort of like detecting a faint whisper in a room full of people shouting,” said Delhaize. “When you combine together thousands of whispers, you get a shout that you can hear above a noisy room, just like combining the radio light from thousands of galaxies to detect them above the background.”

The research used CSIRO’s Parkes radio telescope to survey a large section of the sky for 87 hours, collecting signals from hydrogen over an unmatched volume of space and up to two billion years back in time.

“The Parkes telescope views a big section of the sky at once, so it was quick to survey the large field we chose for our study,” said ICRAR Deputy Director and Jacinta’s supervisor, Professor Lister Staveley-Smith.

Delhaize said observing such a large volume of space meant that she could accurately calculate the average amount of hydrogen in galaxies at a certain distance from Earth, corresponding to a particular period in the Universe’s history. This provides information that can be used in simulations of the Universe’s evolution and clues to how galaxies formed and changed over time.

Next generation telescopes like the international Square Kilometre Array (SKA) and CSIRO’s Australian SKA Pathfinder (ASKAP) will be able to observe even larger volumes of the Universe with higher resolution.

“That makes them fast, accurate and perfect for studying the distant Universe. We can use the stacking technique to get every last piece of valuable information out of their observations,” said Delhaize. “Bring on ASKAP and the SKA!”

The combined graphic below shows a high-resolution image (inset) from the Green Bank Telescope of recently discovered hydrogen clouds between the Andromeda Galaxy M31 (upper right) and Triangulum Galaxy M33 (bottom left). 




The Daily Galaxy via The Monthly Notices of the Royal Astronomical Society are published by Oxford University Press.

Image credit: Bill Saxton, NRAO/AUI/NSF


What makes hydrogen gas clouds surrounding galaxies hot? The conversion of atomic hydrogen to molecular hydrogen.
D Marmet Canadian scientist proposed that molecular hydrogen suffuses space.

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