Discovery of HI gas in a young radio galaxy at $z = 0.44$ using the
  Australian Square Kilometre Array Pathfinder
  We report the discovery of a new 21-cm HI absorption system using
commissioning data from the Boolardy Engineering Test Array of the Australian
Square Kilometre Array Pathfinder (ASKAP). Using the 711.5 - 1015.5 MHz band of
ASKAP we were able to conduct a blind search for the 21-cm line in a continuous
redshift range between $z = 0.4$ and 1.0, which has, until now, remained
largely unexplored. The absorption line is detected at $z = 0.44$ towards the
GHz-peaked spectrum radio source PKS B1740$-$517 and demonstrates ASKAP's
excellent capability for performing a future wide-field survey for HI
absorption at these redshifts. Optical spectroscopy and imaging using the
Gemini-South telescope indicates that the HI gas is intrinsic to the host
galaxy of the radio source. The narrow OIII emission lines show clear
double-peaked structure, indicating either large-scale outflow or rotation of
the ionized gas. Archival data from the \emph{XMM-Newton} satellite exhibit an
absorbed X-ray spectrum that is consistent with a high column density obscuring
medium around the active galactic nucleus. The HI absorption profile is
complex, with four distinct components ranging in width from 5 to 300 km
s$^{-1}$ and fractional depths from 0.2 to 20 per cent. In addition to systemic
HI gas, in a circumnuclear disc or ring structure aligned with the radio jet,
we find evidence for a possible broad outflow of neutral gas moving at a radial
velocity of $v \sim 300$ km s$^{-1}$. We infer that the expanding young radio
source ($t_{\rm age} \approx 2500$ yr) is cocooned within a dense medium and
may be driving circumnuclear neutral gas in an outflow of $\sim$ 1
$\mathrm{M}_{\odot}$ yr$^{-1}$.
