We have used the Australian Square Kilometre Array Pathfinder ( ASKAP ) radio telescope to search for intervening 21 cm neutral hydrogen ( HI ) absorption along the line of sight to 53 bright radio continuum sources .
Our observations are sensitive to HI column densities typical of Damped Lyman Alpha absorbers ( DLAs ) in cool gas with an HI spin temperature below about 300–500 K. The six-dish Boolardy Engineering Test Array ( BETA ) and twelve-antenna Early Science array ( ASKAP-12 ) covered a frequency range corresponding to redshift 0.4 < z < 1.0 and 0.37 < z < 0.77 respectively for the HI line .
Fifty of the 53 radio sources observed have reliable optical redshifts , giving a total redshift path \Delta z = 21.37 .
This was a spectroscopically untargeted survey , with no prior assumptions about the location of the lines in redshift space .
Four intervening HI lines were detected , two of them new .
In each case , the estimated HI column density lies above the DLA limit for HI spin temperatures above 50-80 K , and we estimate a DLA number density at redshift z \sim 0.6 of n ( z ) = 0.19 \substack { +0.15 \ -0.09 } .
This value lies somewhat above the general trend of n ( z ) with redshift seen in optical DLA studies .
Although the current sample is small , it represents an important proof of concept for the much larger 21 cm First Large Absorption Survey in HI ( FLASH ) project to be carried out with the full 36-antenna ASKAP telescope , probing a total redshift path \Delta z \sim 50 , 000 .