Modelling the spectral energy distribution of galaxies^*

III. Attenuation of stellar light in spiral galaxies

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany e-mail: Richard.Tuffs;Cristina.Popescu@mpi-hd.mpg.de
² Research Associate, The Astronomical Institute of the Romanian Academy, Str. Cuţitul de Argint 5, Bucharest, Romania
³ University of Crete, Physics Department, PO Box 2208, 71003 Heraklion, Crete, Greece
⁴ Foundation for Research and Technology-Hellas, 71110 Heraklion, Crete, Greece
⁵ Research School of Astronomy & Astrophysics, The Australian National University, Cotter Road, Weston Creek ACT 2611, Australia

Corresponding author: R. J. Tuffs, Richard.Tuffs@mpi-hd.mpg.de

Received: 14 November 2003
Accepted: 24 January 2004

Abstract

We present new calculations of the attenuation of stellar light from spiral galaxies using geometries for stars and dust which can reproduce the entire spectral energy distribution from the ultraviolet (UV) to the Far-infrared (FIR)/submillimeter (submm) and can also account for the surface brightness distribution in both the optical/Near-infrared (NIR) and FIR/submm. The calculations are based on the model of Popescu et al. ([CITE]), which incorporates a dustless stellar bulge, a disk of old stars with associated diffuse dust, a thin disk of young stars with associated diffuse dust, and a clumpy dust component associated with star-forming regions in the thin disk. The attenuations, which incorporate the effects of multiple anisotropic scattering, are derived separately for each stellar component, and presented in the form of easily accessible polynomial fits as a function of inclination, for a grid in optical depth and wavelength. The wavelength range considered is between 912 Å and 2.2 m, sampled such that attenuation can be conveniently calculated both for the standard optical bands and for the bands covered by GALEX. The attenuation characteristics of the individual stellar components show marked differences between each other. A general formula is given for the calculation of composite attenuation, valid for any combination of the bulge-to-disk ratio and amount of clumpiness. As an example, we show how the optical depth derived from the variation of attenuation with inclination depends on the bulge-to-disk ratio. Finally, a recipe is given for a self-consistent determination of the optical depth from the  line ratio.