| 11th of November 2021 |
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| HI absorption detection in front of a lobe of the bright radio galaxy PKS0409-75 at z~0.7 |
| by Elizabeth Mahony |
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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. |