THE REDDENING LAW OUTSIDE THE LOCAL GROUP GALAXIES - THE CASE OF NGC-7552 AND NGC-5236

The dust reddening law from the UV to the near-IR for the extended reg ions of galaxies is here derived from the spectral distributions of th e starburst spiral galaxies NGC 7552 and NGC 5236. The centers of thes e galaxies have similar absorption and emission line spectra, differin g only in the strength of their interstellar lines and in the continuu m distribution, with NGC 7552 appearing more reddened than NGC 5236. T he disk of NGC 7552 is more inclined, and there is evidence that its c enter is observed through additional foreground dust and gas clouds, a s compared to the center of NGC 5236. While the galaxies can be expect ed to have similar dust content, they are known to have different dust path lengths to our line of sight. Therefore, differences in the shap e of the spectra can be attributed mainly to the effects of dust, allo wing us to probe for the first time the properties of the reddening la w outside the local group of galaxies. We derive the reddening law bas ed on the optical depth of the emission lines of Halpha and Hbeta and also based on the continuum distribution. We find that the optical dep th from the emission line regions are about twice the optical depth of the continuum regions. Thus, dereddening a starburst galaxy by scalin g the Milky Way reddening laws to optical depths obtained from the Hal pha/Hbeta line ratio overcompensates for the effect of dust.