AST- 9803114 Sparke Most of the mass in galaxies is dark, emitting little or no light; we infer the presence only from its gravitational force. The total mass within any radius can be deduced from the speed of gas in a circular orbit there; but to find the shape of the dark halo, or how much mass is distributed within it, we musst look at orbits in more than one plane, or motion which is not symmetric about the center. Dr. Sparke is studying the relation between the observed motions of stars and gas and the mass distribution within galaxies. She is using polar ring systems for part of this work. In polar ring systems, a ring of gas, dust and stars orbits almost at right angles to the equatorial plane of the underlying galaxy. She combines measurements of motions in the ring gas, made in the 21cm line of neutral hydrogen, with optical nad near-IR images and spectra, to determine the flattening of the unseen halo. She is also conducting a theoretical study of star and gas orbits in double-bar potentials. In many barred galaxies, a small secondary nuclear bar can be seen within the larger main bar; probably the figure of the central bar rotates more rapidly around the galaxy center than the larger bar. Another part of this work is the study of how lopsidedness (i.e., the disk of the galaxy does not coincide with the center of an unseen halo) might persist. She has a model - gravitational n-body - to follow the gas flow of these off-center disks. ***
