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The average optical depth of disc galaxies

The average optical depth of disc galaxies is calculated assuming that their infrared output originates from the dust reprocessing of stellar light. Essentially, the luminosity functions at both optical (B-band) and FIR (IRAS) wavelengths are integrated to give a local luminosity density in the optical and FIR. This method was first proposed by Saunders et al., who found a B-band optical depth of tau_B=0.26. In this paper we reconsider the method and employ more realistic models. We conclude that on average a B-band photon originates from a region with an optical depth of tau_B~0.9, a value that indicates that galaxies have high rather than low optical depths. Adjusting the FIR luminosity for flux at wavelengths longer than those detected by IRAS could increase this value further (to tau_B>1). Interpreting this, a galaxy with the dust distributed in a similar manner to the stars would have a central face-on optical depth of tau_B>2.5, and this `typical' galaxy would become optically thin at ~1 scalelength from the centre.