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11th of November 2021
HI absorption detection in front of a lobe of the bright radio galaxy PKS0409-75 at z~0.7
by Elizabeth Mahony
In preparation for the First Large Absorption line survey in HI absorption (FLASH) to be carried out with ASKAP, Mahony et al. recently published a pilot study searching for 21-cm HI absorption at intermediate redshifts using ASKAP-BETA, the commissioning array of ASKAP. This study revealed two detections of HI absorption; one in the compact steep spectrum source PKS 0023−26 at z=0.322 and another towards the bright FRII radio galaxy PKS 0409−75 (shown in the figure). Intriguingly, the HI absorption was detected at z=0.674, significantly blueshifted from the systemic velocity of the host galaxy (labelled A) by∼3300km/s. Follow-up optical imaging and spectroscopy with GMOS on Gemini-South confirmed that the HI absorption is associated with a foreground galaxy in front of the south-eastern radio lobe (labelled B). Using the optical and radio data combined we estimate this foreground galaxy to have a stellar mass of 3.2−6.8×10^11 solar masses and an HI column density of 2.16×10^21 cm−2. Combining the HI absorption with archival polarisation measurements of PKS 0409−75 from Morganti et al. (1999), we can estimate the magnetic field in the foreground galaxy to be∼14.5μG, giving the first estimate of the magnetic field of a galaxy at z∼0.7.

Although only a small sample was searched for HI absorption in this work, it provides a pilot study of what could be detected by future large absorption line surveys such as FLASH. Most previous searches for HI absorption have focused on compact radio sources with known redshifts to increase the likelihood of detection. As such, the absorption detected towards PKS0409-75 would have been missed by this selection. By searching for absorption lines towards all bright radio sources in the ASKAP field of view, FLASH will detect more sources that would not have been included in traditional searches, as well as providing the first estimate of the typical detection rate towards extended radio sources.

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