The hydrogen gas in high redshift galaxies is fully ionised

In their 2008 publication, Stephen Curran and colleagues discussed their study of far-away galaxies that are devoid of cool hydrogen gas – the fuel for star formation. Since observing distant galaxies also means looking far back in time (in this case, 11.5 billion years, i.e. just 2 billion years after the Big Bang), a dearth of hydrogen is unexpected as these galaxies should be in an active star-forming state. Discussing this surprising result, they concluded that the gas could be superheated and ionised and that follow-up observations with a much more sensitive next generation radio telescope, such as the Square Kilometre Array (SKA), will be required.

Meanwhile, as construction of the SKA precursor instruments in Western Australia and South Africa progress, Curran – a CAASTRO Research Fellow at the University of Sydney – and his collaborator Matthew Whiting (CSIRO Astronomy & Space Science) have analysed their data carefully to find a physically plausible model to explain their observations. In their new publication, “Complete Ionisation of the Neutral Gas: why there are so few Detections of 21-cm Hydrogen in High Redshift Radio Galaxies and Quasars” (The Astrophysical Journal 759-117, November 2012), they present their ’21-cm effect’ that is a cut-off in the detection rate of cool hydrogen gas at a single critical luminosity in the ultraviolet wavelength range. Having determined the ionising photon rate to be just under 3 x 1056 photons per second, the UV luminosity proves sufficient to ionise all gas in a galaxy, and the model also accounts for a balance in the particle dynamics as photon density decreases exponentially with distance from the galaxy nucleus, the ionising source. Their model therefore re-creates the observed cut-off in detections, naturally explains how a large-scale distribution of atomic gas in large spiral galaxies, such as our own Milky Way, can be completely ionised, and rules out that previous observations were corrupted by effects of viewing angle or, indeed, telescope sensitivity.

So while we are waiting for the SKA to come online, Stephen has shown that diligent number-crunching and thorough analysis can still teach us a lot about the distant Universe!


Publication details:

S.J. Curran & M.T. Whiting in ApJ 759-117

Complete Ionisation of the Neutral Gas: why there are so few Detections of 21-cm Hydrogen in High Redshift Radio Galaxies and Quasars

Image Credit: NASA/JPL-Caltech