Kinematical structure of the local interstellar medium: The Galactic center hemisphere

A survey of interstellar Na I D-1 and D-2 absorption features found in the spectra of 31 early-type stars in the first and fourth Galactic quadrants reveals a new picture of the large-scale kinematics of interstellar gas in the vicinity of the Sun. While absorption features found in the spectra of stars in the fourth Galactic quadrant show the gas expanding from stars in the Scorpio-Centaurus OB association, stars in the first Galactic quadrant, in the range of distances covered by this survey (74-312 pc from the Sun), show interstellar gas falling toward the Galactic plane with a bulk motion whose component parallel to that plane is directed toward the Scorpio-Centaurus association. This picture differs completely from generally accepted models in which the nearby interstellar gas flows away from the Scorpio-Centaurus association toward the Galactic anticenter. Evidence for the presence of an expanding shell-like feature that is reaching the solar system is presented.