NEUTRAL HYDROGEN IN THE OUTER GALAXY

We have used the Bell Laboratories northern hemisphere H I survey data of Stark and coworkers to study the distribution of H I in the outer Galaxy. These measurements are well suited for investigations of faint Galactic H I emission because the sidelobe contamination of the data is low. We used a flat rotation curve to calculate the H I space density n(H) as a function of Galactic position, expressed in a Galactocentric cylindrical coordinate system (R, theta, z). We present a series of contour maps of n(H)(R, theta, z) for fixed values of R, theta, and z for theta < 220-degrees. The data set was examined in a variety of ways in order to study the various systematic problems which influence the derived results. Isodensity views of the Galaxy from an external location proved to be valuable in evaluating the possible presence of contamination of the data by kinematically mispositioned H I. In comparing our results with those of Burton & te Lintel Hekkert, we find good agreement down to the level of n(H) approximately 10(-2) cm-3. However, the contours based on the Bell Laboratories measurements appear "better behaved" at substantially lower values of n(H) which permits us to probe with more reliability the nature of the Galactic warp and the extension of the gas to large z in the outermost portions of the Galaxy than in previous studies. For 12 kpc < R < 18 kpc the z distribution of the H I is complicated and reveals the existence of a confined component and an extended component. The confined component exists only within a Galactocentric radius R approximately 18-20 kpc, while the extended component can be followed in some directions to R approximately 25 to 30 kpc. In the directions of maximum warp (90-degrees < theta < 110-degrees ), the average distance, <z>, of the gas away from the Galactic plane is found to reach approximately 4 kpc at R approximately 24 kpc, and the flaring of the layer as measured by z(rms) increases to approximately 3 kpc. In these directions the warping outer layer appears to be reliably traced over a region at least 10 kpc thick in z. When viewing from the position of the Sun, this implies that the warped and flaring outer Galaxy extends to Galactic latitudes as large as approximately +25-degrees.