Tracing the neutral gas environments of young radio AGN with ASKAP
  At present neutral atomic hydrogen (HI) gas in galaxies at redshifts above $z
\sim 0.3$ (the extent of 21-cm emission surveys in individual galaxies) and
below $z \sim 1.7$ (where the Lyman-$\alpha$ line is not observable with
ground-based telescopes) has remained largely unexplored. The advent of
precursor telescopes to the Square Kilometre Array will allow us to conduct the
first systematic radio-selected 21-cm absorption surveys for HI over these
redshifts. While HI absorption is a tracer of the reservoir of cold neutral gas
in galaxies available for star formation, it can also be used to reveal the
extreme kinematics associated with jet-driven neutral outflows in radio-loud
active galactic nuclei. Using the six-antenna Boolardy Engineering Test Array
of the Australian Square Kilometre Array Pathfinder, we have demonstrated that
in a single frequency tuning we can detect HI absorption over a broad range of
redshifts between $z = 0.4$ and $1.0$. As part of our early science and
commissioning program, we are now carrying out a search for absorption towards
a sample of the brightest GPS and CSS sources in the southern sky. These
intrinsically compact sources present us with an opportunity to study the
circumunuclear region of recently re-started radio galaxies, in some cases
showing direct evidence of mechanical feedback through jet-driven outflows.
With the sensitivity of the full ASKAP array we will be able to study the
kinematics of atomic gas in a few thousand radio galaxies, testing models of
radio jet feedback well beyond the nearby Universe
