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

TL;DR

This paper characterizes over 45,000 compact radio sources from the ASKAP-FLASH survey, estimating their redshift distribution and multiwavelength properties to facilitate future studies of cold gas and absorption systems.

Contribution

It provides the first comprehensive photometric redshift analysis and multiwavelength characterization of FLASH radio sources, enabling statistical studies of cold gas at intermediate redshifts.

Findings

Approximately 13% of sources are at z<0.42, 35% within FLASH detectability range, and 52% at z>1.

A catalog of counterparts with photo-z estimates and uncertainties is released.

Insights into AGN content and environments are derived from cross-matching with eROSITA X-ray data.

Abstract

We present the characterisation, including a photometric redshift (photo-$z$) analysis, of the optical counterparts (CTPs) to over 45 000 bright ($S_{856\rm MHz} \geq$ 30 mJy) compact radio sources, identified across all ASKAP First Large Absorption Survey in HI (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 $z<0.42$ (foreground), 35% within the detectability range of FLASH (`in-band'), and 52% at $z>1$ (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 HI 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.