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Accepted manuscript

The First Large Absorption Survey in H i (FLASH): II. Pilot Survey data release and first results

Published online by Cambridge University Press:  09 June 2025

Hyein Yoon Open the ORCID record for Hyein Yoon [Opens in a new window] ,
Elaine M. Sadler ,
Elizabeth K. Mahony ,
J.N.H.S. Aditya ,
James R. Allison Open the ORCID record for James R. Allison [Opens in a new window] ,
Marcin Glowacki ,
Emily F. Kerrison ,
Vanessa A. Moss Open the ORCID record for Vanessa A. Moss [Opens in a new window] ,
Renzhi Su  and
Simon Weng
...Show all authors
Hyein Yoon*
Affiliation:
Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) Institute for Data Innovation in Science, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea Astronomy Program, Department of Physics and Astronomy, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea
Elaine M. Sadler
Affiliation:
Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Elizabeth K. Mahony
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
J.N.H.S. Aditya
Affiliation:
Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D)
James R. Allison
Affiliation:
First Light Fusion Ltd., Unit 9/10 Oxford Pioneer Park, Mead Road, Yarnton, Kidlington OX5 1QU, UK
Marcin Glowacki
Affiliation:
Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, United Kingdom International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, Cape Town, South Africa
Emily F. Kerrison
Affiliation:
Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Vanessa A. Moss
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Renzhi Su
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311100, China Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, 80 Nandan Road, Shanghai 200030, China University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
Simon Weng
Affiliation:
Sydney Institute for Astronomy, School of Physics A28, University of Sydney, NSW 2006, Australia ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching bei München, 85748, Germany
Matthew Whiting
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
O. Ivy Wong
Affiliation:
ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) ATNF, CSIRO, Space and Astronomy, PO Box 1130, Bentley, WA 6102, Australia International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
Joseph R. Callingham
Affiliation:
ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, The Netherlands Anton Pannenkoek Institute for Astronomy, University of Amsterdam, Science Park 904, Amsterdam, The Netherlands
Stephen J. Curran
Affiliation:
School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand
Jeremy Darling
Affiliation:
Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA
Alastair C. Edge
Affiliation:
Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham, DH1 3LE, UK
Sara L. Ellison
Affiliation:
Department of Physics & Astronomy, University of Victoria, Finnerty Road, Victoria, British Columbia, V8P 1A1, Canada)
Kimberly L. Emig
Affiliation:
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
Lilian Garratt-Smithson
Affiliation:
ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
Gordon German
Affiliation:
Australian SKA Regional Centre (AusSRC), CSIRO, Space and Astronomy, PO Box 1130, Bentley, WA 6102, Australia
Kathryn Grasha
Affiliation:
ARC Centre of Excellence for All-Sky Astrophysics in 3 Dimensions (ASTRO 3D) Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
Bärbel S. Koribalski
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW 2751, Australia
Raffaella Morganti
Affiliation:
ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, The Netherlands Kapteyn Astronomical Institute, University of Groningen, Postbus 800, NL-9700 AV Groningen, The Netherlands
Tom Oosterloo
Affiliation:
ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, NL-7991 PD Dwingeloo, The Netherlands Kapteyn Astronomical Institute, University of Groningen, Postbus 800, NL-9700 AV Groningen, The Netherlands
Céline Péroux
Affiliation:
European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching bei München, 85748, Germany Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, F-13388, Marseille, France
Max Pettini
Affiliation:
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Kevin A. Pimbblet
Affiliation:
Centre of Excellence for Data Science, AI, and Modelling (DAIM), University of Hull, Cottingham Road, Kingston-upon-Hull, UK. HU6 7RX
Zheng Zheng
Affiliation:
National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang, District, Beijing, 100101, China
Martin Zwaan
Affiliation:
European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching bei München, 85748, Germany
Lewis Ball
Affiliation:
SKA Observatory, Jodrell Bank, Lower Withington, Cheshire, SK11 9FT, UK
Douglas C.-J. Bock
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
David Brodrick
Affiliation:
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
John D. Bunton
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
F. R. Cooray
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Philip G. Edwards
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Douglas B. Hayman
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Aidan W. Hotan
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 1130, Bentley, WA 6102, Australia
K. Lee-Waddell
Affiliation:
International Centre for Radio Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia ATNF, CSIRO, Space and Astronomy, PO Box 1130, Bentley, WA 6102, Australia Australian SKA Regional Centre (AusSRC), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
N. M. McClure-Griffiths
Affiliation:
Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
A. Ng
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Chris J. Phillips
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Wasim Raja
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Maxim A. Voronkov
Affiliation:
ATNF, CSIRO, Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
Tobias Westmeier
Affiliation:
International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
*
Author for correspondence: Hyein Yoon, Email: hiyoon.astro@gmail.com.
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Abstract

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The First Large Absorption Survey in H i (FLASH) is a large-area radio survey for neutral hydrogen in and around galaxies in the intermediate redshift range 0.4 < z < 1.0, using the 21-cm H i absorption line as a probe of cold neutral gas. The survey uses the ASKAP radio telescope and will cover 24,000 deg2 of sky over the next five years. FLASH breaks new ground in two ways – it is the first large H i absorption survey to be carried out without any optical preselection of targets, and we use an automated Bayesian line-finding tool to search through large datasets and assign a statistical significance to potential line detections. Two Pilot Surveys, covering around 3000 deg2 of sky, were carried out in 2019-22 to test and verify the strategy for the full FLASH survey. The processed data products from these Pilot Surveys (spectral-line cubes, continuum images, and catalogues) are public and available online. In this paper, we describe the FLASH spectral-line and continuum data products and discuss the quality of the H i spectra and the completeness of our automated line search. Finally, we present a set of 30 new H i absorption lines that were robustly detected in the Pilot Surveys, almost doubling the number of known H i absorption systems at 0.4 < z < 1. The detected lines span a wide range in H i optical depth, including three lines with a peak optical depth τ > 1, and appear to be a mixture of intervening and associated systems. Interestingly, around two-thirds of the lines found in this untargeted sample are detected against sources with a peaked-spectrum radio continuum, which are only a minor (5-20%) fraction of the overall radio-source population. The detection rate for H i absorption lines in the Pilot Surveys (0.3 to 0.5 lines per 40 deg2 ASKAP field) is a factor of two below the expected value. One possible reason for this is the presence of a range of spectral-line artefacts in the Pilot Survey data that have now been mitigated and are not expected to recur in the full FLASH survey. A future paper in this series will discuss the host galaxies of the H i absorption systems identified here.

Keywords

galaxies: activegalaxies: ISMmethods: observationalradio lines: galaxiesradio continuum: generalsurveys
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Accepted manuscript , pp. 1 - 44
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2025. Published by Cambridge University Press on behalf of Astronomical Society of Australia