H I absorption at z 0.7 against the lobe of the powerful radio galaxy PKS 0409-75

We present results from a search for the H I 21-cm line in absorption towards 16 bright radio sources with the six-antenna commissioning array of the Australian Square Kilometre Array Pathfinder. Our targets were selected from the 2-Jy sample, a flux-limited survey of the southern radio sky with extensive multiwavelength follow-up. Two sources were detected in H I absorption including a new detection towards the bright Fanaroff-Riley Type II radio galaxy PKS 0409-75 at a redshift of $z$ = 0.674. The H I absorption line is blueshifted by ~3300 km s-1 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 H I absorption is associated with a galaxy in front of the southern radio lobe with a stellar mass of 3.2-6.8 × 1011 M⊙, a star formation rate of ~1.24 M⊙ yr-1, and an estimated H I column density of 2.16 × 1021 cm-2, assuming a spin temperature of Tspin = 500 K and source covering factor of Cf = 0.3. Using polarization 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 H I absorption towards all bright radio sources as opposed to smaller targeted samples