The Shape and Color of the Galactic Disk

A model of the old Galactic disk has been derived from the survey of the Diffuse Infrared Background Experiment (DIRBE) of the Cosmic Background Explorer at wavelengths of 1.25, 2.2, 3.5, and 4.9 μm. The model includes an exponential radial dependence and a sech² vertical dependence of the Galactic luminosity density and allows for a radial truncation of the disk and a global warping characterized by five parameters. A dust layer is also modeled to account for emission by dust as well as extinction. The fit of model to DIRBE surface brightness data is good. According to the model, if we are 8.5 kpc from the Galactic center, the Sun is 15.6 pc above the midplane of the disk, which has radial and vertical scale lengths of 2.64 and 0.333 kpc, respectively, and is warped in a way consistent with the H I layer. The disk is truncated at a radius of 12 kpc, but this may be only a lower limit if the vertical scale length increases outside the solar circle. There are no indications of a thick disk.