HIGH-RESOLUTION IMAGES OF DIFFUSE NEUTRAL CLOUDS IN THE MILKY WAY. I. OBSERVATIONS, IMAGING, AND BASIC CLOUD PROPERTIES

A set of diffuse interstellar clouds in the inner Galaxy within a few hundred parsecs of the Galactic plane has been observed at an angular resolution of ≈1′. 0 combining data from the NRAO Green Bank Telescope and the Very Large Array. At the distance of the clouds, the linear resolution ranges from ∼1.9 to ∼2.8 pc. These clouds have been selected to be somewhat outside of the Galactic plane, and thus are not confused with unrelated emission, but in other respects they are a Galactic population. They are located near the tangent points in the inner Galaxy, and thus at a quantifiable distance: 2.3 ⩽ R ⩽ 6.0 kpc from the Galactic Center and -1000 ⩽ z ⩽ +610 pc from the Galactic plane. These are the first images of the diffuse neutral H I clouds that may constitute a considerable fraction of the interstellar medium (ISM). Peak H I column densities lie in the range N H ɪ = 0.8–2.9 × 10 20 cm −2 . Cloud diameters vary between about 10 and 100 pc, and their H ɪ mass spans the range from less than a hundred to a few thousands M ☉ . The clouds show no morphological consistency of any kind, except that their shapes are highly irregular. One cloud may lie within the hot wind from the nucleus of the Galaxy, and some clouds show evidence of two distinct thermal phases as would be expected from equilibrium models of the ISM.