Detection of \hi 21 Centimeter Absorption by the Warm Neutral Medium
C. L. Carilli, K. S. Dwarakanath, W. M. Goss;
ApJ, 1998, 502, L79
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 ).
KEYWORDS: ism: structure, ism: atoms, galaxy: fundamental parameters, radio lines: ism
PERSOKEY:h_i, 21 cm, ABSORPTION, general ism, ,
CODE: carilli98