THE THERMAL PRESSURE OF THE HOT INTERSTELLAR-MEDIUM DERIVED FROM CLOUD SHADOWS IN THE EXTREME-ULTRAVIOLET

We have used the Deep Survey telescope of the Extreme Ultraviolet Expl orer to investigate shadows in the diffuse EUV/soft X-ray background t hat are cast by clouds in the interstellar medium. We confirm the exis tence of a shadow previously reported and provide evidence for two new shadows. We used IRAS data to identify the clouds producing these sha dows and to determine their optical depth to EUV radiation. The EUV-ab sorbing clouds are optically thick in the EUV, and all EUV emission de tected in the direction of these shadows must be produced from materia l in front of the clouds. We obtained new optical data to determine th e distance to these clouds. We use a new differential cloud technique to obtain the pressure of the interstellar medium. These results do no t depend on any zero-level calibration of the data. Our results provid e evidence that the pressure of the hot interstellar gas is the same i n three different directions in the local interstellar medium and is a t least 8 times higher than that derived for the Local Cloud surroundi ng our Sun. This provides new evidence for large thermal pressure imba lances in the local ISM and directly contradicts the basic assumption of thermal pressure equilibrium used in almost all present models of t he interstellar medium.