11.5.2. Observations of External Galaxies
For a rotating galaxy, hydrogen radiating Galactic and Extra-Galactic Radio Astronomy at different velocities will arise from regions of the galaxy (such as those in Figure 11.2) delimited by the isovelocity contours. If observations are made with an interferometer whose baseline has an orientation close to the position angle of the major axis of the galaxy, and whose length, D, is not so great that the hydrogen distribution is appreciably resolved (i.e., a > / D, where a is the diameter of the galaxy), then the interferometer phase in each velocity channel indicates the distance along the major axis of hydrogen radiating in that velocity interval. The position profile is then a plot of the centroid positions of the regions in Figure 11.2 weighted by the hydrogen distribution (and smeared by random velocities, the observing beam, and the velocity filter). The amplitude in each velocity channel is roughly the product of the hydrogen density multiplied by the area of these regions. Observations of a number of galaxies have been made in this manner at the Owens Valley Radio Observatory (Rogstad, Rougoor, and Whiteoak, 1967). The amplitude-velocity and phase-velocity plots are a function of both the hydrogen distribution and the rotation curve, and the position profile obtained is subject to various interpretations.