Detection of H i 21 Centimeter Line Absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy

Using the Westerbork Synthesis Radio Telescope, we have detected H I 21 cm line absorption in the warm neutral medium of the Galaxy toward the extragalactic source 3C 147. This absorption, at a local standard of rest (LSR) velocity of -29 ± 4kms -1 with a full width at half-maximum of 53 ± 6kms -1 , is associated with the Perseus arm of the Galaxy. The observed optical depth is (1.9 ± 0.2) × 10 -3 . The estimated spin temperature of the gas is 3600 ± 360K. The volume density is 0.4 cm -3 assuming pressure equilibrium. Toward two other sources, 3C 273 and 3C 295, no wide H I 21 cm line absorption was detected. The highest of the 3 σ lower limits on the spin temperature of the warm neutral medium is 2600 K. In addition, we have also detected H I 21 cm line absorption from high-velocity clouds in the outer arm toward 3C 147 and 3C 380 at LSR velocities of -117.3, -124.5, and -113.7 km s -1 , respectively. We find two distinct temperature components in the high-velocity clouds with spin temperatures of greater than 1000 K and less than 200 K, respectively.