2013MNRAS.430..157A

We report conclusive verification of the detection of associated Hi 21cm absorption in the early-type host galaxy of the compact radio source PMNJ2054-4242. We estimate an effective spectral line velocity width of 418 ± 20 km/s and observed peak optical depth of 2.5±0.2percent, making this one of the broadest and weakest 21cm absorption lines yet detected. For Tspin/f > 100K the atomic neutral hydrogen column density is N_H I \gtrsim 2 ×10^21/cm2. The observed spectral line profile is redshifted by 187±46 km/s, with respect to the optical spectroscopic measurement, perhaps indicating that the Hi gas is infalling towards the central active galactic nucleus. Our initial tentative detection would likely have been dismissed by visual inspection, and hence its verification here is an excellent test of our spectral line detection technique, currently under development in anticipation of future next-generation 21cm absorption-line surveys.