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 ($S_{856\rm MHz} \geq$ 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.

Figures (11)

• Figure 1: FLASH survey footprint in equatorial coordinates. Black boxes indicate the fields observed up to April 2025, with those included in the pilot surveys marked by star symbols. The eROSITA-DE reference of the western Galactic hemisphere and the coverage of tenth release of the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys South (DR10-South) are overplotted for context.
• Figure 2: Photo-$z$ versus spec-$z$ for six distinct spectroscopic subclasses of radio galaxies (HERG, LERG, AeB, SF, Unusual, and NA) following the classification scheme of Ching2017
• Figure 3: Colour–colour diagram showing $g-r$ vs. $z-W1$ for sources with S/N $\geq 3$ in $g, r, z$, and $W1$. Points are colour-coded by Gaussian kernel density. Overplotted are empirical template tracks of various galaxy types, with square markers denoting specific steps in redshift (0.2, 0.4, 0.6, 1, 1.6, 2, 3) and circles marking the starting redshift. The black dashed line represents the Salvato2022 Galactic/extragalactic selection boundary in colour space.
• Figure 4: Spectroscopic versus photometric redshifts for high-$z$ CTPs in the FLASH sample. Points are colour-coded by and plotted in increasing order of their LS10-$r$ band AB magnitude. The dashed lines indicate the $\frac{|\Delta z|}{1+z} \geq 0.15$ boundaries used to identify outliers.
• Figure 5: Top: FLASH compact source PICZL photo-$z$ distributions coloured by different LS10 morphological classes. Middle: Same distribution as above but coloured by the PDF degeneracy class: none — single strong peak; light — weak secondary peak(s); mild — noticeable but subdominant secondary peaks; strong — multiple peaks of comparable strength. Bottom: Same distribution as above but coloured by the S/N of all LS10 ($g,r,i,z$) bands.