The Millennium Galaxy Catalogue: the severe attenuation of bulge flux by dusty spiral discs

Using the Millennium Galaxy Catalogue we quantify the dependency of the disc and bulge luminosity functions on galaxy inclination. Using a contemporary dust model we show that our results are consistent with galaxy discs being optically thick in their central regions (τ f/B = 3.8±0.7). As a consequence the measured B-band fluxes of bulges can be severely attenuated by 50% to 95% depending on disc inclination. We argue that a galaxy's optical appearance can be radically transformed by simply removing the dust, e.g. during cluster infall, with mid-type galaxies becoming earlier, redder, and more luminous. Finally we derive the mean photon escape fraction from the integrated galaxy population over the 0.1 m range, and use this to show that the energy of starlight absorbed by dust (in our model) is in close agreement with the total far-IR emission.