The distribution of neutral gas in the Milky Way

Over the past decade, the uncertainty on the H2 mass of the inner Galaxy has been reduced from a factor ∼4 to less than a factor 2, comparable to the uncertainty which still exists for H i. Gas masses beyond the solar circle remain more uncertain, owing mainly to uncertainties in the rotation curve and CO‐to‐H2 mass conversion factor there. H i and H2 masses are roughly equal within the solar circle, but H i dominates beyond. GMCs are superior to H i clouds for tracing Galactic structure, but kinematic distance errors and severe velocity crowding near the terminal velocity in the inner Galaxy remain formidable obstacles to sorting out the structure of that region.Large‐scale near‐infrared surveys of the Galactic plane, particularly those recently obtained by the COBE satellite, hold great promise for determining both distances and masses for large GMCs in the inner Galaxy. An earlier survey by the Infrared Telescope (IRT) has already revealed a remarkably tight anticorrelation between CO and near‐infrared...