The first large absorption survey in H I (FLASH): II. Pilot survey data release and first results

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 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