J. R. Allison, S. J. Curran, E. M. Sadler, S. N. Reeves
We report conclusive verification of the detection of associated HI 21cm absorption in the early-type host of the compact radio source PMN J2054-4242. We estimate an equivalent spectral-line width of 415 +/- 20 km/s, and observed peak optical depth of 2.5 +/- 0.2 per cent, making this one of the broadest and weakest 21cm absorption-lines yet discovered. For Tspin/f > 100K the column density is NHI > 2 x 10^{21} cm^{-2}. The observed spectral-line profile is redshifted by v = 179 +/- 46 km/s, with respect to the spectroscopic optical measurement, perhaps indicating that the HI gas is infalling toward the central active galactic nucleus. The broad width of the line suggests that the cold gas is either rotating at very high velocity, or that the infall is accelerating (perhaps as a blended series of line-of-sight gas clouds). 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. We find that other such broad-line dominated detections in the literature are comparatively rare, presenting us with the following question: are these systems intrisically more scarce in nature, or are the existing data and analysis techniques not yet of sufficient quality to detect these low peak signal-to-noise systems?
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http://arxiv.org/abs/1211.3452
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