The ASKAP-FLASH survey: A first look at the multiwavelength properties and redshift distribution of compact radio sources

Abstract We present the characterisation, including a photometric redshift (photo- z ) analysis, of the optical counterparts (CTPs) to over 45 000 bright ( $S_{856\,\mathrm{MHz}} \geq$ 30 mJy) compact radio sources, identified across all ASKAP First Large Absorption Survey in H I (FLASH) fields observed up to April 2025. These sources constitute a large, homogeneous population of background continuum sightlines specifically selected to enable statistical studies of cold gas at intermediate redshifts of $0.42 \leq z \leq 1$ . As spectroscopic redshift measurements are not available for the majority of these candidate absorbers, we estimate photo- z s for the CTPs of all FLASH continuum sources cross-matched to the tenth data release of the DESI Legacy Imaging Surveys (LS10). Using these estimates, we establish the redshift distribution and find that approximately 13% of continuum sources lie at $z\lt0.42$ (foreground), 35% within the detectability range of FLASH (‘in-band’), and 52% at $z\gt1$ (background). We examine the subset of FLASH continuum sources with CTPs in the eROSITA X-ray survey, providing additional insight into their AGN content, multiwavelength properties, and environments. Finally, we discuss how this information can be used as a statistical prior to aid in distinguishing between associated and intervening H I absorption systems and estimating the total comoving absorption path length of the survey, establishing a framework for incorporating redshift-based priors in future large radio absorption surveys. We release a catalogue of LS10 counterparts to FLASH continuum sources, providing photo- z estimates, associated uncertainties, and measures of redshift degeneracies.