HI absorption at z~0.7 against the lobe of the powerful radio galaxy PKS
  0409-75
  We present results from a search for the HI 21-cm line in absorption towards
16 bright radio sources with the 6-antenna commissioning array of the
Australian Square Kilometre Array Pathfinder (ASKAP). Our targets were selected
from the 2-Jy sample, a flux-limited survey of the southern radio sky with
extensive multi-wavelength follow-up. Two sources were detected in HI
absorption including a new detection towards the bright FRII radio galaxy PKS
0409-75 at a redshift of z=0.674. The HI absorption line is blueshifted by
~3300 km/s compared to the optical redshift of the host galaxy of PKS 0409-75
at z=0.693. Deep optical imaging and spectroscopic follow-up with the GMOS
instrument on the Gemini-South telescope reveal that the HI absorption is
associated with a galaxy in front of the southern radio lobe with a stellar
mass of $3.2 - 6.8 \times 10^{11}M_\odot$, a star-formation rate of $\sim 1.24
M_\odot$ yr$^{-1}$ and an estimated HI column density of $2.16\times10^{21}$
cm$^{-2}$, assuming a spin temperature of $T_{\rm spin}=500$ K and source
covering factor of $C_{\rm f}=0.3$. Using polarisation measurements of PKS
0409-75 from the literature we estimate the magnetic field of the absorbing
galaxy to be ~14.5$\mu$G, consistent with field strengths observed in nearby
spiral galaxies, but larger than expected for an elliptical galaxy. Results
from this pilot study can inform future surveys as new wide-field telescopes
allow us to search for 21-cm HI absorption towards all bright radio sources as
opposed to smaller targeted samples.
