Detection of HI 21 cm - line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy

Using the Westerbork Synthesis Radio Telescope, we have detected HI 21 cm-line absorption in the Warm Neutral Medium of the Galaxy toward the ex-tragalactic source 3C147. This absorption, at an LSR velocity of –29±4 km s −1 with a full width at half maximum of 53±6 km s −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±360 K. The volume density is 0.4 cm −3 assuming pressure equilibrium. Toward two other sources, 3C273 and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the 3σ lower limits on the spin temperature of the Warm Neutral Medium is 2600 K. In addition, we have also detected HI 21 cm-line absorption from high velocity clouds in the Outer Arm toward 3C147 and 3C380 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.