DUST EXTINCTION OF THE STELLAR CONTINUA IN STARBURST GALAXIES - THE ULTRAVIOLET AND OPTICAL EXTINCTION LAW

We analyze the IUE UV and the optical spectra of 39 starburst and blue compact galaxies in order to study the average properties of dust ext inction in extended regions of galaxies. The optical spectra have been obtained using an aperture which matches that of IUE, so comparable r egions within each galaxy are sampled. The data from the 39 galaxies a re compared with five models for the geometrical distribution of dust, adopting as extinction laws both the Milky Way and the Large Magellan ic Cloud laws. The commonly used uniform dust screen is included among the models. We find that none of the five models is in satisfactory a greement with the data. In order to understand the discrepancy between the data and the models, we have derived an extinction law directly f rom the data in the UV and optical wavelength range. The resulting cur ve is characterized by an overall slope which is more gray than the Mi lky Way extinction law's slope, and by the absence of the 2175 angstro m dust feature. Remarkably, the difference in optical depth between th e Balmer emission lines Halpha and Hbeta is about a factor of 2 larger than the difference in the optical depth between the continuum underl ying the two Balmer lines. We interpret this discrepancy as a conseque nce of the fact that the hot ionizing stars are associated with dustie r regions than the cold stellar population is. The absence of the 2175 angstrom dust feature can be due either to the effects of the scatter ing and clumpiness of the dust or to a chemical composition different from that of the Milky Way dust grains. Disentangling the two interpre tations is not easy because of the complexity of the spatial distribut ion of the emitting regions. The extinction law of UV and optical spec tral continua of extended regions can be applied to the spectra of med ium- and high-redshift galaxies, where extended regions of a galaxy ar e, by necessity, sampled.