The Interstellar Medium of M31. I. A Survey of Neutral Hydrogen Emission

Initial results, in the form of images at fixed velocity and an integral of neutral hydrogen emission, are presented for the northeast half of M31. The resolution is about 10" spatially and 5 km s^-1^ in velocity. A uniform rms sensitivity per velocity channel of 6.6 K was obtained over an area of about 30' x 75' by observing at 10 closely spaced pointings in the B-, C-, and D-configurations of the Very Large Array. Some dramatic results include the widespread detection of high brightness temperature (140-180 K) HI structures, which are apparently resolved spatially (~ 100 pc) but only marginally in velocity. Significant optical depths are likely to be present along many lines of sight.