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We report conclusive verification of the detection of associated HI 21 cmabsorption in the early-type host galaxy of the compact radio sourcePMNJ2054-4242. We estimate an effective spectral line velocity width of 418 +/-20 km s^{-1} and observed peak optical depth of 2.5 +/- 0.2 per cent, makingthis one of the broadest and weakest 21 cm absorption lines yet detected. ForT_{spin}/f > 100 K the atomic neutral hydrogen column density is N_{HI} > 2 x10^{21} cm^{-2}. The observed spectral line profile is redshifted by 187 +/- 46km s^{-1}, with respect to the optical spectroscopic measurement, perhapsindicating that the HI gas is infalling towards the central active galacticnucleus. Our initial tentative detection would likely have been dismissed byvisual inspection, and hence its verification here is an excellent test of ourspectral line detection technique, currently under development in anticipationof future next-generation 21 cm absorption-line surveys.

Broad, weak 21 cm absorption in an early type galaxy: spectral-line finding and parametrization for future surveys