One sightline, many systems: a FLASH discovery of H <scp>i</scp> towards scintillating quasar PKS 0405-385

Abstract We report the discovery of an intervening 21 cm absorption line at z = 0.882 towards the z = 1.284 quasar PKS 0405–385, identified in the First Large Absorption Survey in H i (FLASH). This quasar once displayed the most rapid known intraday variability at radio frequencies, from which it earned the title of ‘the smallest radio quasar’. Although its size was revised upwards soon after based on updated scattering theory, PKS 0405–385 remains an important probe of Galactic plasma, and now also of intervening gas discovered through H i absorption. We present new long-slit spectroscopy spanning both PKS 0405–385 and the candidate host of the intervening H i gas. We identify Mg ii and Fe ii absorption lines in this spectrum consistent with the redshift of the intervening H i , as well as two additional, independent metal-line systems at z = 0.907 and z = 0.966, but we cannot accurately pinpoint the host(s) of this intervening gas in current data. We revisit the radio variability of PKS 0405–385 in light of advances in scintillation theory, as well as extended monitoring with the Australia Telescope Compact Array and the Australian SKA Pathfinder, and find a revised linear size ≥ 0.3 pc, but no new evidence of repeating intraday variability.

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Publication Details

Journal
Publications of the Astronomical Society of Australia
Published
2026-04-17
DOI
https://doi.org/10.1017/pasa.2026.10190
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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article

One sightline, many systems: a FLASH discovery of H i towards scintillating quasar PKS 0405-385

R. D. Ekers, V. A. Moss, P. G. Edwards, M. Glowacki et al.
Publications of the Astronomical Society of Australia
Galaxies: Formation, Evolution, Phenomena
article

One sightline, many systems: a FLASH discovery of H i towards scintillating quasar PKS 0405-385

R. D. Ekers, V. A. Moss, P. G. Edwards, M. Glowacki, S. J. Curran, M. T. Whiting, E. K. Mahony, Joshua Pritchard, E. M. Sadler, Emily F. Kerrison, Hyein Yoon, Hayley Bignall, Aditya J. N. H. S., Yijung Kang, J. R. Allison, J. Stevens, A. Tuntsov, R. Su
article en

Abstract

Abstract We report the discovery of an intervening 21 cm absorption line at z = 0.882 towards the z = 1.284 quasar PKS 0405–385, identified in the First Large Absorption Survey in H i (FLASH). This quasar once displayed the most rapid known intraday variability at radio frequencies, from which it earned the title of ‘the smallest radio quasar’. Although its size was revised upwards soon after based on updated scattering theory, PKS 0405–385 remains an important probe of Galactic plasma, and now also of intervening gas discovered through H i absorption. We present new long-slit spectroscopy spanning both PKS 0405–385 and the candidate host of the intervening H i gas. We identify Mg ii and Fe ii absorption lines in this spectrum consistent with the redshift of the intervening H i , as well as two additional, independent metal-line systems at z = 0.907 and z = 0.966, but we cannot accurately pinpoint the host(s) of this intervening gas in current data. We revisit the radio variability of PKS 0405–385 in light of advances in scintillation theory, as well as extended monitoring with the Australia Telescope Compact Array and the Australian SKA Pathfinder, and find a revised linear size ≥ 0.3 pc, but no new evidence of repeating intraday variability.

Publications of the Astronomical Society of Australia
Commonwealth Scientific and Industrial Research Organisation (AU), The University of Sydney (AU), Seoul National University (KR), University of Cape Town (ZA), Shanghai Astronomical Observatory (CN), Korea Astronomy and Space Science Institute (KR), Manly Hospital (AU), Victoria University of Wellington (NZ), Kavli Institute for Particle Astrophysics and Cosmology (US), ARC Centre of Excellence for Gravitational Wave Discovery (AU), Astronomy and Space (AU), ARC Centre of Excellence for All-sky Astrophysics (AU), First Light Fusion (GB), University of Edinburgh (GB)
Openalex Percentile: Top 12%
Galaxies: Formation, Evolution, Phenomena
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