Star Formation and Selective Dust Extinction in Luminous Starburst Galaxies

We investigate the star formation and dust extinction properties of veryluminous infrared galaxies whose spectra display a strong Hdelta line inabsorption and a moderate [OII] emission (e[a] spectrum). This spectralcombination has been suggested to be a useful method to identify dustystarburst galaxies at any redshift on the basis of optical data alone. Wecompare the average e(a) optical spectrum with synthetic spectra that includeboth the stellar and the nebular contribution, allowing dust extinction toaffect differentially the stellar populations of different ages. We find thatreproducing the e(a) spectrum requires the youngest stellar generations to besignificantly more extinguished by dust than older stellar populations, andimplies a strong ongoing star formation activity at a level higher than inquiescent spirals. A model fitting the optical spectrum does not necessarilyproduce the observed FIR luminosity and this can be explained by the existenceof stellar populations which are practically obscured at optical wavelengths.Models in which dust and stars are uniformly mixed yield a reddening of theemerging emission lines which is too low compared to observations: additionalforeground reddening is required.Comment: 17 pages, 4 Postscript figures, ApJ in pres