The dominant source of electromagnetic energy in the Universe today (over ultraviolet, optical and near-infrared wavelengths) is starlight. However, quantifying the amount of starlight produced has proven difficult due to interstellar dust grains whi
ch attenuate some unknown fraction of the light. Combining a recently calibrated galactic dust model with observations of 10,000 nearby galaxies we find that (integrated over all galaxy types and orientations) only (11 +/- 2)% of the 0.1 micron photons escape their host galaxies; this value rises linearly (with log(lambda)) to (87 +/- 3)% at 2.1 micron. We deduce that the energy output from stars in the nearby Universe is (1.6+/-0.2) x 10^{35} W Mpc^{-3} of which (0.9+/-0.1) x 10^{35} W Mpc^{-3} escapes directly into the inter-galactic medium. Some further ramifications of dust attenuation are discussed, and equations that correct individual galaxy flux measurements for its effect are provided.
Based on our sample of 10095 galaxies with bulge-disc decompositions we derive the empirical B-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 +/- 0.7 (central face-on B-band opacity). Depending on inclination, the B-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-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. From our best fitting dust models we derive a redshift zero cosmic dust density of rho_{dust} ~ (5.3 +/- 1.7) x 10^5, h M_{odot} 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 IMF) are in the form of stars (~58 per cent reside in galaxy discs, ~10 per cent in red elliptical galaxies, ~29 per cent in classical galaxy bulges and the remainder in low luminosity blue spheroid systems/components). [Abridged]
