Detection of Hi21 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 HI 21cm-line absorption in the Warm Neutral Medium of the Galaxy toward theextragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/swith a full width at half maximum of 53+/-6 km/s, is associated with thePerseus 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 densityis 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600K. In addition, we have also detected HI 21 cm-line absorption from highvelocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of-117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperaturecomponents in the high velocity clouds with spin temperatures of greater than1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table