Galactic HI and the interstellar medium in URSA Major

Sensitive neutral hydrogen measurements have been made over a 100 square degree field in Ursa Major. The observations cover the area which has the least neutral hydrogen and probably the least interstellar matter of any direction in the sky. It is likely that this data set contains every line of sight that intersects a total H I column density of less than 6.5 x 10(19) cm(-2). The smallest column density is estimated to be 4.4 +/- 0.5 x 10(19) cm(-2). Observations at 10' angular resolution reveal little structure that is not seen at 21' resolution, and we believe that our estimates of the total H I column density are accurate on yet smaller angular scales. The lowest N-HI area is also extremely deficient in H alpha emission, and shows no evidence for significant amounts of H-2. The largest uncertainty in the interstellar inventory of Ursa Major is in the amount of diffuse ionized gas. An electron column density N-e less than or similar to 4 x 10(19) cm(-2) is not inconsistent with the observations, but this figure is quite uncertain.