HI absorption at z~0.7 against the lobe of the powerful radio galaxy PKS 0409-75

Abstract

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 $\sim 1.24M_{\odot }$ yr${}^{-1}$ and an estimated HI column density of $2.16\times 10^{21}$ cm${}^{-2}$, assuming a spin temperature of $T_{\mathrm{spin}}=500$ K and source covering factor of $C_{\mathrm{f}}=0.3$. Using polarisation measurements of PKS 0409-75 from the literature we estimate the magnetic field of the absorbing galaxy to be ~14.5$\hat{I}¼$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.

14 pages, 6 figures, accepted for publication in MNRAS