The Thermal Pressure of the Hot Interstellar Medium Derived from Cloud Shadows in the Extreme Ultraviolet

We have used the Deep Survey telescope of EUVE to investigate shadows in thediffuse EUV/Soft X-Ray background cast by clouds in the interstellar medium. Weconfirm the existence of a shadow previously reported, and provide evidence fortwo new shadows. We used IRAS data to identify the clouds producing theseshadows and to determine their optical depth to EUV radiation. TheEUV-absorbing clouds are optically thick in the EUV, and all EUV emissiondetected in the direction of these shadows must be produced from material infront of the clouds. We obtained new optical data to determine the distance tothese clouds. We use a new differential cloud technique to obtain the pressureof the interstellar medium. These results do not depend on any zero levelcalibration of the data. Our results provide evidence that the pressure of thehot interstellar gas is the same in three different directions in the localinterstellar medium, and is at least 8 times higher than derived for the localcloud surrounding our Sun. This provides new evidence for large thermalpressure imbalances in the local ISM, and directly contradicts the basicassumption of thermal pressure equilibrium used in almost all present models ofthe interstellar medium.Comment: 13 pages, 13 figure