A survey for H I in the distant Universe: the detection of associated 21-cm absorption at z = 1.28

Abstract

We have undertaken a survey for H I 21-cm absorption within the host galaxies of z ∼ 1.2-1.5 radio sources, in the search of the cool neutral gas currently ‘missing’ at z ≳ 1. This deficit is believed to be due to the optical selection of high-redshift objects biasing surveys towards sources of sufficient ultraviolet luminosity to ionize all of the gas in the surrounding galaxy. In order to avoid this bias, we have selected objects above blue magnitudes of B ∼ 20, indicating ultraviolet luminosities below the critical value of LUV ∼ 1023 W Hz−1, above which 21-cm has never been detected. As a secondary requirement to the radio flux and faint optical magnitude, we shortlist targets with radio spectra suggestive of compact sources, in order to maximize the coverage of background emission. From this, we obtain one detection out of 10 sources searched, which at z = 1.278 is the third highest redshift detection of associated 21-cm absorption to date. Accounting for the spectra compromised by radio frequency interference, as well as various other possible pitfalls (reliable optical redshifts and turnover frequencies indicative of compact emission), we estimate a detection rate of ≈30 per cent, close to that expected for LUV ≲ 1023 W Hz−1 sources.