The Millennium Galaxy Catalogue: the B-band attenuation of bulge and disc light and the implied cosmic dust and stellar mass densities

Based on our sample of 10 095 galaxies with bulge-disc decompositions we derive the empirical B-MGC-band internal attenuation-inclination relation for galaxy discs and their associated central bulges. Our results agree well with the independently derived dust models of Tuffs et al., leading to a direct constraint on the mean opacity of spiral discs of tau(f)(B) = 3.8 +/- 0.7 (central face-on B-MGC-band opacity). Depending on inclination, the B-MGC-band attenuation correction varies from 0.2 to 1.1 mag for discs and from 0.8 to 2.6 mag for bulges. We find that, overall, 37 per cent of all B-MGC-band photons produced in discs in the nearby Universe are absorbed by dust, a figure that rises to 71 per cent for bulge photons. The severity of internal dust extinction is such that one must incorporate internal dust corrections in all optical studies of large galaxy samples. This is particularly pertinent for optical Hubble Space Telescope comparative evolutionary studies as the dust properties will also be evolving. We use the new results to revise our recent estimates of the spheroid and disc luminosity functions. The implied stellar mass densities at redshift zero are somewhat higher than our earlier estimates: rho(discs) = (3.8 +/- 0.6) -> (4.4 +/- 0.6) x 10(8) h M-circle dot Mpc(-3) and rho(bulges) = (1.6 +/- 0.4) -> (2.2 +/- 0.4) x 10(8) h M-circle dot Mpc(-3). From our best-fitting dust models we derive a redshift zero cosmic dust density of rho(dust) approximate to (5.3 +/- 1.7) x 10(5) h M-circle dot Mpc(-3). This implies that (0.0083 +/- 0.0027)h per cent of the baryons in the Universe are in the form of dust and (11.9 +/- 1.7) h per cent (Salpeter-'lite' initial mass function) are in the form of stars (similar to 58 per cent reside in galaxy discs, similar to 10 per cent in red elliptical galaxies, similar to 29 per cent in classical galaxy bulges and the remainder in low-luminosity blue spheroid systems/components).