Where is the Missing Galactic Hydrogen?

All-sky surveys of neutral hydrogen at 21 cm give us an opportunity to study the interstellar medium and galactic structure in new ways. Here, we use the Leiden/Argentine/Bonn data to investigate the differences in the neutral hydrogen structure between the northern and southern hemispheres at high galactic latitudes. Examining longitude-velocity planes of the latitude-longitude-velocity data cube at high negative latitudes shows the expected distribution of low-velocity galactic gas as we look through the galactic plane toward the relative emptiness of intergalactic space. Comparison with the equivalent images at high positive latitudes shows the well-known anomalous-velocity features, dramatically illustrating a north-south asymmetry in the high-latitude hydrogen distribution. Another thing to notice are the gaps in the northern hemisphere low-velocity gas. Where is this missing hydrogen? It could be shifted to either a different velocity or a neighboring position. We can check both these options by integrating along the line of sight at the specific longitudes where the gaps occur and integrating over the entire longitude range of the disturbance. If we use the equivalent southern hemisphere data as the “standard,” we find that column densities in the south are about three time higher than these northern directions, not supporting either conjecture. A third possibility is that the missing hydrogen is ionized. Assuming a scale height of about 1 kpc, the resulting electron density is in good agreement with results from pulsar dispersion measures.