Faint objects are also devoid of neutral hydrogen gas
Previously, Sydney based CAASTRO Research Fellow Dr Stephen Curran discussed far-away radio galaxies and quasars that seem not to contain any cool neutral hydrogen gas – the fuel for star formation – (2008 publication in MNRAS) and presented a physically plausible model to explain these observations (2012 publication in ApJ). He and his colleagues concluded that the gas in these distant galaxies was completely ionised due to strong ultraviolet radiation, such that there was no detection of cool hydrogen above a critical ultra-violet luminosity. However, a selection bias towards optically bright objects in these studies could mean that there remains the possibility of a population of fainter objects where hydrogen is not ionised.
In a new publication (“A survey for the missing hydrogen in high redshift radio sources”, accepted for publication in Monthly Notices of the Royal Astronomical Society), Dr Curran, together with CAASTRO co-author Professor Elaine Sadler, Dr Matthew Whiting (CSIRO), and Dr Carl Bignell (National Radio Astronomy Observatory, USA), set out to look closely at optically faint, radio-loud objects in which, according to the above logic, cool neutral hydrogen gas should be present and detectable.
Using data from the Green Bank Telescope in the USA and the Westerbork Synthesis Radio Telescope in the Netherlands, the research team analysed 11 radio sources of which eight yielded analysable data. Despite expecting at least three detections of the neutral hydrogen signal, there were none – which might be due to extended radio emission of the selected sources that reduces coverage of the absorbing gas and therefore makes the survey less sensitive. Discussing further reasons for the non-detections, the authors consider the fact that galaxies in the distant, early Universe are smaller and their hydrogen more likely to be fully ionised. To study even fainter objects, a more sensitive radio telescope like the Square Kilometre Array (SKA) will be required.
S. J. Curran, M. T. Whiting, E. M. Sadler, C. Bignell in MNRAS 2012