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

TL;DR

This paper reports the detection of a broad, weak 21 cm hydrogen absorption line in an early-type galaxy, demonstrating a spectral-line detection technique for future large-scale surveys.

Contribution

It provides a verified detection of a rare broad, weak 21 cm absorption line and introduces a spectral-line detection method for upcoming surveys.

Findings

Detected one of the broadest, weakest 21 cm absorption lines.

Estimated hydrogen column density > 2 x 10^{21} cm^{-2}.

Observed redshifted profile suggests gas infall towards the galaxy nucleus.

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.