INTERNAL ABSORPTION AND THE LUMINOSITY OF DISK GALAXIES

We investigate the correlation of the optical depth of dust in galacti c disks with galaxy luminosity. We examine normal late-type (spiral an d irregular) galaxies with measured far-ultraviolet (UV, lambda simila r to 2000 Angstrom) fluxes and compile the corresponding fluxes in the far-infrared (FIR, lambda similar to 40-120 mu m) as measured by IRAS . The UV-to-FIR flux ratio is found to decrease rapidly with increasin g FIR and FIR+UV luminosities. Since both the UV and FIR radiation ori ginate mostly from the young stellar population in late-type galaxies, the UV-to-FIR flux ratio is a measure of the fraction of the light pr oduced by young stars escaping from galaxy disks. Thus, the strong cor relations above imply that the dust opacity increases with the luminos ity of the young stellar population. We also find that the ratio of th e UV-to-FIR flux decreases with increasing galaxy blue luminosity (a t racer of the intermediate-age stellar population) and with galaxy rota tion speed (an indicator of galaxy mass). We supplement the UV sample of galaxies with an optically selected sample and find that the blue-t o-FIR flux ratio declines with both FIR luminosity and galaxy rotation speed. We also examine a sample of galaxies for which the H beta/H al pha flux ratios can be obtained and find that the H beta/H alpha ratio , which also measures the extinction, decreases with the increasing FI R luminosity. We model the absorption and emission of radiation by dus t to normal galactic disks with a simple model of a uniform plane-para llel slab in which the dust that radiates in the IRAS band is heated e xclusively by UV light from relatively nearby hot stars. We then find that the relation between the UV-to-FIR flux ratio and the observed lu minosities can be explained by the face-on extinction optical depth ta u varying with the intrinsic luminosity as a power law in the intrinsi c UV luminosity: tau = tau(1)(L/L(1))(beta). The same scaling law may also account for the vaious correlations found between the blue-to-FIR flux ratio and luminosities of late-type galaxies, although the possi ble dependence of the intrinsic ratio of UV to the blue luminosity lea ds to uncertainties here. The H beta/H alpha ratio is less affected by the problem, and the observations are consistent with the above scali ng law. Comparisons of our simple model to the observations show that, expressed in the blue band, the total extinction optical depth is tau (B,) = 0.8 +/- 0.3 at the fiducial observed blue luminosity of a Sche chter L() galaxy and beta = 0.5 +/- 0.2. Thus, our models imply that most galaxies are optically thin to dust in the blue. The increase in optical depth with luminosity can be attributed to the increase in bot h galaxy metallicity (dust cross section per unit mass of interstellar gas) and galaxy surface mass density with increasing luminosity.