Star formation and selective dust extinction in luminous starburst galaxies

We investigate the star formation and dust extinction properties of very luminous infrared galaxies whose spectra display a strong H delta line in absorption and a moderate [O II] emission [e(a) spectrum]. This spectral combination has been suggested to be a useful method to identify dusty starburst galaxies at any redshift on the basis of optical data alone. We compare the average e(a) optical spectrum with synthetic spectra that include both the stellar and the nebular contribution, allowing dust extinction to affect differentially the stellar populations of different ages. We find that reproducing the e(a) spectrum requires the youngest stellar generations to be significantly more extinguished by dust than older stellar populations and implies a strong ongoing star formation activity at a level higher than in quiescent spirals. A model fitting the optical spectrum does not necessarily produce the observed FIR luminosity, and this can be explained by the existence of stellar populations which are practically obscured at optical wavelengths. Models in which dust and stars are uniformly mixed yield a reddening of the emerging emission lines which is too low compared to observations: additional foreground reddening is required.