M ar 2 00 1 Three Dimensional Structure of the Milky Way Disk : the distribution of stars and dust beyond 0 . 35

We present a three dimensional model for the Milky Way fit to the farinfrared (FIR) and near-infrared (NIR) data from the COBE/DIRBE instrument for galactic latitudes |b| < 30 degrees and to within 20 degrees of the Galactic center. Because of the low optical depth at 240μm, the FIR emission traces the distribution of Galactic dust in the Galaxy. We model the dust distribution as due to three components: a warped exponential disk with scale length 0.28 R⊙ and a flaring scale height, a spiral arm component with four arms as traced by Galactic HII regions, and the local (Orion) arm which produces prominent emission features at galactic longitude l ≃ 80 and −100 degrees. A Cosmic Infrared Background of 1.07MJy sr is recovered, consistent with previous determinations. The dust distribution is then used to calculate absorption in J and K, and the stellar emission in these wavebands is modeled with two components: a warped exponential disk with a scale length of 0.28 R⊙ and a spiral arm component dominated by two arms. This small scale length is consistent with a maximal disk model for our Galaxy, which is inconsistent with the cuspy dark matter halos predicted in CDM models. We find different amplitudes for the warp in the stars and dust, which starts within the Solar Circle. Subject headings: galactic structure, dust, IR, absorption Keck Distinguished Visiting Professor of Astrophysics, IAS