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

Abstract

We report conclusive verification of the detection of associated HI 21 cm 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^{-1} and observed peak optical depth of 2.5 +/- 0.2 per cent, making this one of the broadest and weakest 21 cm absorption lines yet detected. For T_{spin}/f > 100 K the atomic neutral hydrogen column density is N_{HI} > 2 x 10^{21} cm^{-2}. The observed spectral line profile is redshifted by 187 +/- 46 km s^{-1}, 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 21 cm absorption-line surveys.