Australian Astronomers in Search for Missing Hydrogen in Early Universe

A study by Dr Stephen Curran of the University of Sydney and Dr Matthew Whiting of the CSIRO Astronomy and Space Science has addressed the mystery of why astrophysicists cannot detect hydrogen in the most distant regions of the Universe.

This image shows a black hole squashing star formation (NASA / JPL / Caltech)

Hydrogen is the most common element in the Universe, making up 75 percent of all normal matter and the content of stars. Although stars themselves are hot, they can only form out of the coldest gas when a massive cloud of hydrogen can collapse under its own gravity until the point when nuclear fusion starts – the fusing of atoms together releasing the huge amounts of energy we see as starlight.

Astronomers have been puzzled as to why they could not detect this cold star-forming gas in the most distant regions of the Universe. At such vast look-back times, astronomers expected the gas to be much more abundant as it has yet to be consumed by star formation.

Now, the team has addressed this problem by devising a model that shows how the supermassive black hole, lurking within the center of each active galaxy, is able to ionize all of the surrounding gas even in the very largest galaxies. When hydrogen gas is in this state, where the electron is ripped out of the atom, the gas is too agitated to allow the cloud to collapse and form stars. Also, when ionized, it cannot be detected through radio waves at 21 cm – the way cold star-forming gas is normally found.

“Previously, we had not known just how much of the gas was ionized by the black hole accretions disks – we had thought that perhaps it was just enough to take the abundance of cool gas to below the detection threshold of current radio telescopes. So we’d thought that it was maybe a telescope sensitivity issue,” explained Dr Curran, who co-authored the study published in the Astrophysical Journal (arXiv.org version).

The results show that the extreme ultra-violet radiation given off by the material being sucked in – at near light-speeds – to the black hole, is sufficient to ionize all of the gas in even the very largest galaxies.

“In order to probe further back in time, we choose the most distant radio sources. What appears as faint light from these, to us on earth, is actually extreme ultra-violet, dimmed and stretched to visible light on its several billion year journey to us,” Dr Curran said. “Unfortunately, these are the only objects we know of at the very limits of the cosmos and within these the radiation from the central black hole is so intense as to heat all the gas to the point where it cannot form stars.”

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Bibliographic information: Curran SJ and Whiting MT. 2012. Complete Ionization of the Neutral Gas: Why There are So Few Detections of 21 cm Hydrogen in High-redshift Radio Galaxies and Quasars. Astrophysical Journal, vol. 759, no. 2; doi: 10.1088/0004-637X/759/2/117