Stacking of hydrogen signals to study density over cosmic time
16 December 2013
Neutral atomic hydrogen (HI), a fundamental ingredient in a galaxy, is key to our understanding of galaxy formation and evolution. Large optical surveys with ground- and space-based telescopes have allowed us to assess the physical properties of many millions galaxies, focusing on stellar components and leading to a clear history of star formation in galaxies over cosmic time. In contrast though, gas components – especially HI – beyond the local Universe (z > 0.2) have been poorly constrained because existing radio telescopes are not sensitive enough to directly detect weak HI signals from individual galaxies.
While waiting for SKA precursor telescopes to come online, CAASTRO PhD student Jonghwan Rhee, his ANU supervisor Prof Frank Briggs, and colleagues used an HI spectral stacking technique with 21-cm emission data from the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands to overcome the current limitations. The technique required galaxy positions and redshifts (obtained from CNOC2 optical redshift survey) and performed a cross-correlation between 21-cm observation data and optical spectroscopic data. The researchers extracted HI emission lines from radio spectral data cubes taken from radio observations, based on positions and redshifts that were known from optical photometric and spectroscopic observations. They then co-added the extracted HI spectra of multiple galaxies to measure the average HI signal and converted this into HI mass and finally cosmic HI mass density (ΩHI).
In their recent publication (Monthly Notices of the Royal Astronomical Society), the team reports the most reliable measurements beyond the local universe yet using this stacking technique and the highest signal-to-noise measurement of ΩHI at intermediate redshifts. Their ΩHI at z ~ 0.1 and 0.2 was consistent with results at z = 0 based on large scale 21-cm surveys, as well as other measurements at z < 0.2, and ΩHI at z < 0.5 (based on 21-cm emission line observations) was consistent with no evolution in the neutral hydrogen gas density over the last ~ 4 Gyr.
by J. Rhee