Statistical tool beats radio telescopes in measuring distant gas
26 April 2016
Intensity mapping is a novel technique that uses the Hydrogen emission at radio wavelengths of galaxies as a proxy for galaxy distribution on large scales. The statistical properties of this distribution help us understand the cosmological principles of our Universe. Hydrogen gas is the most abundant element in the Universe, and it is the main ingredient to fuel star-formation processes in galaxies and is thus driving galaxy evolution. The individual detection of cold Hydrogen gas in distant galaxies is not yet feasible with radio telescopes. However, using intensity mapping, we observe the combined – or binned – emission of many galaxies through low-resolution maps and are able to observe the Cosmic Web over very large distance ranges.
The galaxy distribution is quantified by measuring the cosmological power spectrum. These power spectra contain information on the clustering strength of galaxies as a function of distance or scale. In recent analysis, the intensity mapping data was jointly analysed with optical data of the same regions to measure the cross-power spectrum, to eliminate foregrounds and instrumental noise contaminations in the maps.
As part of the CAASTRO intensity mapping project, CAASTRO post-doctoral fellow Dr Laura Wolz and colleagues at the University of Melbourne have theoretically investigated the signature of the cross-power spectrum of intensity maps with optical galaxy data. They modelled the intensity mapping signal using numerical simulations of the evolution of galaxies with cosmic time. In addition, they synthesised a mock galaxy catalogue of optical telescope observations based on the same simulation. In their study, the researchers show that the cross-power spectrum of intensity maps and optical galaxies has a different shape depending on how the optical galaxies were selected: blue galaxies, which are highly star-forming, show a much higher clustering signal on small scales than red galaxies, which are relatively passive. This proves that these signals can be linked back to the Hydrogen content of the optical galaxies as it correlates with the galaxy colour: blue galaxies are rich in cold gas while red galaxies are relatively gas poor. Intensity mapping in combination with optical galaxy measurements may therefore indirectly detect the Hydrogen gas content of the optical galaxies that were selected according to their colours. This theoretical study is the first proof of concept of this new Hydrogen detection technique, and the approach opens up new possibilities in understanding the gas content, and thus the star-formation activity, of very distant galaxies which are inaccessible to direct observations with our current generation of radio telescopes.