Phys.org—Hydrogen is the most common element in the Universe, making up 75% 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 nuclear fusion starts – the fusing of atoms together which releases 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, and hence older, 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.
Dr Stephen Curran, from the University of Sydney’s School of Physics and CAASTRO – the ARC Centre for All-sky Astrophysics – and Dr Matthew Whiting, from CSIRO Astronomy and Space Science, have addressed this problem by devising a model that shows how the supermassive black hole, lurking within the centre of each active galaxy, is able to ionise 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 ionised, it cannot be detected through radio waves at 21-centimetres – the way cold star-forming gas is normally found.
via Galaxies without stars: The problem of the missing hydrogen in the early Universe.