Structure and Dynamics of Bulges in the Milky way and External Galaxies

The properties of bulges in external galaxies are reviewed with emphasis on quantitative comparisons with the bulge in the Galaxy. The luminosity profile of the Milky Way bulge is derived from 2.4-micron data obtained with the Spacelab 2 Infrared Telescope. Small bulges like that of the Milky Way are better represented by exponential laws than by a De Vaucouleurs law. The Milky Way bulge profile is quantitatively similar to those of M31 and NGC 4565. An oblate isotropic-rotator dynamical model is constructed for the Milky Way bulge and compared with existing data. The ratio of mass to V-band light is approximately 3. A comparison of the dynamical mass density in the direction of Baade's Window with the mass in stars inferred from direct star counts shows that only of order 30 percent of the mass is unseen. Circular rotation curves in bulges derived from observations of gas-emission lines are often incompatible with the mass distribution inferred from the stellar kinematics; there is some evidence that bulges have a triaxial shape and that the gas motions deviate significantly from circular rotation.