A search for HI absorption in distant star-forming galaxies with ASKAP-FLASH -- I. Selection and analysis of the radio sample

TL;DR

This study compiles UV-selected galaxy catalogues near radio sources to facilitate future 21 cm HI absorption searches, revealing that close-impact galaxies are often AGN, while more distant ones are mainly star-forming, with implications for galaxy evolution.

Contribution

The paper introduces two galaxy catalogues with UV and radio data, enabling targeted HI absorption studies at intermediate redshifts, and analyzes galaxy properties related to their radio associations.

Findings

Galaxies within 5 arcsec of radio sources are likely AGN.

Objects between 5 and 20 arcsec are predominantly star-forming.

Radio-associated galaxies tend to be redder and resemble high-excitation radio galaxies.

Abstract

We present and discuss two catalogues of UV-selected (NUV $< 22.8$ mag) galaxies that lie within a 200 deg$^2$ area of sky covered by the ASKAP FLASH survey and have an impact parameter of less than 20 arcsec to a FLASH radio continuum source. These catalogues are designed to enable a future search for 21 cm HI absorption in and around star-forming galaxies at redshift $0.4<z<1$. We outline the production of this UV-bright dataset, which has optical spectroscopy from the WiggleZ and SDSS surveys and a median redshift of $\sim0.6$. Analysis of the optical spectra, using multiple diagnostic diagrams, shows that galaxies with an impact parameter of less than 5 arcsec are likely to be physically associated with the radio source and are five times more likely to be an AGN than objects without a radio match. Conversely, objects with impact factors between 5 and 20 arcsec are largely (>80 percent) star-forming and resemble the overall WiggleZ population. The ($g - i$) colour evolution with redshift is consistent with a history of active star-formation, but the radio-associated objects are typically redder and have colours similar to high-excitation radio galaxies. The redshift distribution of the two catalogues matches the overall distribution for WiggleZ galaxies, despite their otherwise rare radio properties. These catalogues can be expanded in future as new radio data become available, and a forthcoming paper will present the HI absorption results.