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

Yoon, Hyein; Sadler, Elaine M; Mahony, Elizabeth K; Aditya, J N H S; Allison, James R; Glowacki, Marcin; Kerrison, Emily F; Moss, Vanessa A; Su, Renzhi; Weng, Simon; Whiting, Matthew; Wong, O Ivy; Callingham, Joseph R; Curran, Stephen J; Darling, Jeremy; Edge, Alastair C; Ellison, Sara L; Emig, Kimberly L; Garratt-Smithson, Lilian; German, Gordon; Pettini, Max; Pimbblet, Kevin A; Zheng, Zheng; Zwaan, Martin; Ball, Lewis; Bock, C.-J; Brodrick, David; Bunton, John D; Cooray, F R; Edwards, Philip G; Hayman, Douglas B; Hotan, Aidan W; Lee-Waddell, K; Mcclure-Griffiths, N M; Ng, A; Phillips, Chris J; Raja, Wasim; Voronkov, Maxim A; Westmeier, Tobias

Abstract

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 deg 2 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 deg 2 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 deg 2 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.

Citation

Yoon, H., Sadler, E. M., Mahony, E. K., Aditya, J. N. H. S., Allison, J. R., Glowacki, M., Kerrison, E. F., Moss, V. A., Su, R., Weng, S., Whiting, M., Wong, O. I., Callingham, J. R., Curran, S. J., Darling, J., Edge, A. C., Ellison, S. L., Emig, K. L., Garratt-Smithson, L., German, G., …Westmeier, T. (2025). The First Large Absorption Survey in H i (FLASH): II. Pilot Survey data release and first results. Publications of the Astronomical Society of Australia, 42, Article e088. https://doi.org/10.1017/pasa.2025.10046

• Journal Article Type: Article
• Acceptance Date: May 5, 2025
• Online Publication Date: Jun 9, 2025
• Publication Date: 2025
• Deposit Date: Jun 6, 2025
• Publicly Available Date: Jul 17, 2025
• Print ISSN: 1448-6083
• Publisher: Cambridge University Press
• Peer Reviewed: Peer Reviewed
• Volume: 42
• Article Number: e088
• DOI: https://doi.org/10.1017/pasa.2025.10046
• Keywords: galaxies: active ; galaxies: ISM ; methods: observational ; radio lines: galaxies ; radio continuum: general ; surveys
• Public URL: https://hull-repository.worktribe.com/output/5235110