Optical Spectral Signatures of Dusty Starburst Galaxies

We analyse the optical spectral properties of the complete sample of VeryLuminous Infrared Galaxies presented by Wu et al. (1998a,b) and we find a highfraction (~50 %) of spectra showing both a strong H_delta line in absorptionand relatively modest [OII] emission (e(a) spectra). The e(a) signature hasbeen proposed as an efficient method to identify dusty starburst galaxies andwe study the star formation activity and the nature of these galaxies, as wellas the effects of dust on their observed properties. We examine their emissionline characteristics, in particular their [OII]/H_alpha ratio, and we find thisto be greatly affected by reddening. A search for AGN spectral signaturesreveals that the e(a)'s are typically HII/LINER galaxies. We compare the starformation rates derived from the FIR luminosities with the estimates based onthe H_alpha line and find that the values obtained from the optical emissionlines are a factor of 10-70 (H_alpha) and 20-140 ([OII]) lower than the FIRestimates (50-300 M_sun yr^-1). We then study the morphological properties ofthe e(a) galaxies, looking for a near companion or signs of amerger/interaction. In order to explore the evolution of the e(a) population,we present an overview of the available observations of e(a)'s in differentenvironments both at low and high redshift. Finally, we discuss the role ofdust in determining the e(a) spectral properties and we propose a scenario ofselective obscuration in which the extinction decreases with the stellar age.Comment: 26 pages, Latex, including 7 postscript figures, accepted for publication in the Astrophysical Journa