Internal Absorption and the Luminosity of Disk Galaxies

We investigate the correlation of the optical depth of dust in galactic disks with galaxy luminosity. We examine normal late-type (spiral and irregular) galaxies with measured far-ultraviolet (UV, λ ∼ 2000 Å) fluxes and compile the corresponding fluxes in the far-infrared (FIR, λ ∼ 40-120 μm) as measured by IRA S. The UV-to-FIR flux ratio is found to decrease rapidly with increasing FIR and FIR + UV luminosities. Since both the UV and FIR radiation originate 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 produced by young stars escaping from galaxy disks. Thus, the strong correlations above imply that the dust opacity increases with the luminosity of the young stellar population. We also find that the ratio of the UV-to-FIR flux decreases with increasing galaxy blue luminosity (a tracer of the intermediate-age stellar population) and with galaxy rotation speed (an indicator of galaxy mass). We supplement the UV sample of galaxies with an optically selected sample and find that the blue-to-FIR flux ratio declines with both FIR luminosity and galaxy rotation speed. We also examine a sample of galaxies for which the Hβ/Hα flux ratios can be obtained and find that the Hβ/Hα ratio, which also measures the extinction, decreases with the increasing FIR luminosity. We model the absorption and emission of radiation by dust to normal galactic disks with a simple model of a uniform plane-parallel slab in which the dust that radiates in the IRAS band is heated exclusively by UV light from relatively nearby hot stars. We then find that the relation between the UV-to-FIR flux ratio and the observed luminosities can be explained by the face-on extinction optical depth τ varying with the intrinsic luminosity as a power law in the intrinsic UV luminosity: τ = τ1(L/L1)β. The same scaling law may also account for the various correlations found between the blue-to-FIR flux ratio and luminosities of late-type galaxies, although the possible dependence of the intrinsic ratio of UV to the blue luminosity leads to uncertainties here. The Hβ/Hα ratio is less affected by the problem, and the observations are consistent with the above scaling law. Comparisons of our simple model to the observations show that, expressed in the blue band, the total extinction optical depth is TB,* * = 0.8±0.3 at the fiducial observed blue luminosity of a Schechter L* galaxy and β = 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 both galaxy metallicity (dust cross section per unit mass of interstellar gas) and galaxy surface mass density with increasing luminosity.