Detection of H [CSC]i[/CSC] 21 Centimeter Absorption by the Warm Neutral Medium

We have detected H I 21 cm 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) × 10 −4 at -70 km s −1 and (8.5 ± 2.0) × 10 −4 at -40 km s −1 , with corresponding spin temperatures of 6000 ± 1700 and 4800 ± 1600 K, respectively. The volume filling factor for the WNM appears to be fairly high ( f ≈ 0.4).