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

TL;DR

This survey searched for HI 21-cm absorption in high-redshift galaxies to understand the distribution of neutral gas, successfully detecting it at z=1.278, and highlighting the importance of UV luminosity and source compactness.

Contribution

The paper presents a novel survey method selecting low UV luminosity sources to detect associated 21-cm absorption at high redshift, achieving a significant detection rate.

Findings

Detected 21-cm absorption at z=1.278, the third highest redshift detection.

Estimated detection rate of ~30% for sources with UV luminosity below 1e23 W/Hz.

Highlights the bias of optical selection and the importance of source compactness.

Abstract

We have undertaken a survey for HI 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 ultra-violet luminosity to ionise 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 ultra-violet luminosities below the critical value 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 maximise the coverage of background emission. From this, we obtain one detection out of ten 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%, close to that expected for L_UV < 1e23 W/Hz sources.