Based on our sample of 10095 galaxies with bulge-disc decompositions we derive the empirical B _ { \mbox { \tiny \sc 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 _ { B } ^ { f } = 3.8 \pm 0.7 ( central face-on B _ { \mbox { \tiny \sc MGC } } -band opacity ) .
Depending on inclination , the B _ { \mbox { \tiny \sc MGC } } -band attenuation correction varies from 0.2 – 1.1 mag for discs and from 0.8 – 2.6 mag for bulges .
We find that , overall , 37 per cent of all B _ { \mbox { \tiny \sc 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 HST 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 \pm 0.6 ) \rightarrow ( 4.4 \pm 0.6 ) \times 10 ^ { 8 } h M _ { \odot } Mpc ^ { -3 } and \rho _ { bulges } = ( 1.6 \pm 0.4 ) \rightarrow ( 2.2 \pm 0.4 ) \times 10 ^ { 8 } h M _ { \odot } Mpc ^ { -3 } .
From our best fitting dust models we derive a redshift zero cosmic dust density of \rho _ { dust } \approx ( 5.3 \pm 1.7 ) \times 10 ^ { 5 } h M _ { \odot } Mpc ^ { -3 } .
This implies that ( 0.0083 \pm 0.0027 ) h per cent of the baryons in the Universe are in the form of dust and ( 11.9 \pm 1.7 ) h per cent ( Salpeter- ‘ lite ’ IMF ) are in the form of stars ( \sim 58 per cent reside in galaxy discs , \sim 10 per cent in red elliptical galaxies , \sim 29 per cent in classical galaxy bulges and the remainder in low luminosity blue spheroid systems/components ) .