Detection of H I 21 centimeter absorption by the warm neutral medium

Abstract

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)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 and 4800+/-1600 K, respectively. The volume filling factor for the WNM appears to be fairly high ( f~0.4 ).