Cosmic hydrogen gas has not evolved over past four billion years
16 May 2016
Understanding the evolution of the atomic gas (HI) content of galaxies remains one of the key challenges in the study of galaxy evolution. The sensitivity of the current generation of radio telescopes is insufficient to detect HI from individual galaxies at cosmologically significant redshifts in reasonable integration times. In fact, the detection of HI from individual galaxies at high redshifts was one of the original motivations and remains one of the key science drivers for the proposed Square Kilometre Array (SKA). It is also one of the key programs for several of the upcoming SKA pathfinder telescopes such as ASKAP and MeerKAT.
Although it is challenging to detect HI in individual galaxies at redshifts z ≥ 0.2 with the current generation of radio telescopes, it is possible to make measurements of the average HI content of a sample of galaxies. If the positions and redshifts of all of these galaxies are known, we can stack their spectra to determine their average HI content. This is called the HI spectral stacking technique, and it is what CAASTRO researcher Dr Jonghwan Rhee and colleagues used in their recent publication.
Observing with the Giant Metrewave Radio Telescope in Pune, India, they applied the HI spectral stacking technique to data from the COSMOS field – a field for that a wealth of multi-wavelength data is available. The aim was to determine the cosmic HI mass density ΩHI at a redshift z ~ 0.37. The researchers found ΩHI = (0.42 ± 0.16) × 10-3 at z ~ 0.37, which is the highest-redshift measurement of ΩHI ever made using HI spectral stacking. Their result is consistent with those measured from large blind 21-cm surveys at z = 0, as well as measurements from other HI stacking experiments at lower redshifts. In conjunction with earlier measurements, their result also indicates that there has been no significant evolution of HI gas abundance over the last 4 Gyr. The ΩHI measured here from HI 21-cm emission measurements at z ≤ 0.4 is approximately half that measured from Damped Lyman-α Absorption (DLA) systems at z ≥ 2 though. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of ΩHI in the redshift range intermediate between z ∼ 0.4 and the range probed by DLA observations.