GALACTIC STRUCTURE FROM THE SPACELAB INFRARED TELESCOPE .2. LUMINOSITY MODELS OF THE MILKY-WAY

The three-dimensional luminosity distribution of the Milky Way has been determined from a 2.4-mu-m map of the northern Galactic plane. The radial surface brightness profile of the disk has an exponential scale length of 3.0 kpc (assuming R0 = 8 kpc). The vertical distribution of light follows the law exp (-\z\/h(z)) more closely than a canonical sech2 profile. In the solar neighborhood, the scale height h(z) is approximately 247 pc (assuming R0 = 8 kpc), in agreement with star counts perpendicular to the Galactic plane. The scale height is not constant with radius but decreases to approximately 165 pc at R = 5 kpc. To first order, the bulge can be represented by an oblate spheroid with an axis ratio b/a = 0.61 and again, an exponential luminosity profile. The 2.2-mu-m/12-mu-m flux ratio for the bulge is typical of other dust-free spheroidal systems. Luminosity fluctuations along the galactic plane are found to be caused chiefly by variations in the line-of-sight extinction. These fluctuations can be reproduced quantitatively by a detailed model for the dust distribution and allow an approximate calibration of the dust/gas ratio in regions of high (10 mag at V) optical depth.