by James Allison (University of Sydney)

Abstract. The Square Kilometre Array (SKA) and its pathfinder telescopes will provide a unique perspective on the cold gas that fuels star formation and black hole growth in galaxies throughout cosmic history. By detecting the HI 21-cm line in absorption towards distant radio-loud active galactic nuclei (AGN), we will use the SKA to construct a census of neutral gas in intervening galaxies out to look back times of 10 billion years and beyond. I will discuss a recent paper in which I proposed a method for measuring the fraction of cold-phase atomic hydrogen using a statistical approach based on the number of expected detections with the SKA and its pathfinders. I will also present recent science results with the Australian SKA Pathfinder (ASKAP), which have successfully demonstrated the feasibility of carrying out such a survey.

The Figure above shows the number density of intervening Damped Lyman-alpha Absorbers (DLAs; N_{HI} > 2 x 10^{20} cm^{-2}) as a function of redshift, from optical (z > 1.7), UV (z < 1.7) and 21-cm line emission (z << 0.1) surveys. Recent results from a targeted survey of 50 radio-loud quasars with ASKAP are also shown (Sadler et al. in prep.). Adapted from Rao et al. (2017).