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Detection of HI 21cm Absorption by the Warm Neutral Medium

C. L. Carilli·K. S. Dwarakanath·W. M. Goss
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Abstract

We have detected HI 21cm line absorption by the Warm Neutral Medium (WNM) using the Westerbork Synthesis Radio Telescope. The absorption was detected toward Cygnus A at LSR velocities of -40 and -70 km s^{-1}. These two velocity ranges were previously identified as being relatively free of cold absorbing clouds. The measured optical depth for the WNM along the line of sight to Cygnus A is 8.9+/-1.9 x10^{-4} at -70 km s^{-1}, and 8.5+/-2.0 x10^{-4} at -40 km s^{-1}, with corresponding spin temperatures of 6000+/-1700 K and 4800+/-1600 K, respectively. The volume filling factor for the WNM appears to be fairly high (f = 0.4).

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C. L. Carilli, K. S. Dwarakanath, W. M. Goss. 1998-05-13. Detection of HI 21cm Absorption by the Warm Neutral Medium. https://doi.org/10.1086/311483

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