The First Large Absorption Survey in HI (FLASH) is a large-area radio survey for neutral hydrogen in and around galaxies in the redshift range 0.4 < z < 1.0, using the 21-cm HI absorption line as a probe of cold neutral gas. This ASKAP survey will cover 24,000 square degrees of sky over the next five years. FLASH breaks new ground in two ways – it is the first large HI absorption survey to be carried out without any optical preselection of targets, and it uses an automated Bayesian line-finding tool to search through large datasets and assign a statistical significance to potential line detections. The science goals of the survey are to explore the neutral gas content of galaxies in an unbiased way at a cosmic epoch where almost no HI data are currently available, and to investigate the role of neutral gas in AGN fuelling and feedback in galaxies at intermediate redshift.

Yoon et al. describe the FLASH spectral-line and continuum data products from Pilot Survey observations and discuss the quality of the HI spectra and the completeness of the automated line search. Thirty new H I absorption lines that were robustly detected, spanning a wide range in HI optical depth, are presented which are a mixture of intervening and associated systems. The processed data products from these Pilot Surveys (spectral-line cubes, continuum images, and catalogues) are now available online. The figure above is an example of a spectral-line plot used in data validation. The grey band is set at five times the rms spectral-line noise, so a line that extends beyond this band will have peak signal-to-noise ratio of greater than 5. The source shown here (NVSS J015516-251423) has an HI absorption line at a frequency of 823.4 MHz, corresponding to a redshift of 0.725.