The distribution of the ISM in the Milky Way

A three-dimensional large-scale model

¹ University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece
² National Observatory of Athens, I. Metaxa & Vas. Pavlou Str., Palaia Penteli, 15236 Athens, Greece
³ Astronomical Laboratory, Department of Physics, University of Patras, 26110 Patras, Greece

Received: 1 December 2005
Accepted: 4 July 2006

Abstract

We use the COBE/DIRBE (1.2, 2.2, 60, 100, 140, and 240 μm) maps and the COBE/FIRAS spectra (for the wavelength range 100-1000 μm) to constrain a model for the spatial distribution of the dust, the stars, and the gas in the Milky Way. By assuming exponential axisymmetric distributions for the dust and the stars and by performing the corresponding radiative transfer calculations we closely (given the simple geometry of the model) reproduce the FIR and NIR maps of the Milky Way. Similar distributions for the atomic and molecular hydrogen in the disk are used (with an inner cut-off radius for the atomic hydrogen) to fit the gas data. The star formation rate as a function of the Galactic radius is derived from the FIR emission and is well in agreement with existing estimates from various star formation tracers. The gas surface density is plotted against the star formation rate density and an “intrinsic” Galactic Schmidt law is derived with excellent agreement with the “external” Schmidt law found for spiral galaxies. The Milky Way is found to consume ~ and ~ of its gas in the outer and inner regions respectively (for a period of 0.1 Gyr) to make stars. The dust-induced  color excess observed in various directions and distances (up to ~6.5 kpc) with well-studied Cepheid stars is compared with the model predictions showing a good agreement. The simple assumption of exponential distributions of stars and dust in the Galaxy is found to be quite instructive and adequate in modeling all the available data sets from 0.45 μm (B-band) to 1000 μm.