Optical spectral signatures of dusty starburst galaxies

We analyse the optical spectral properties of the complete sample of Very Luminous Infrared Galaxies presented by Wu et al. (1998a,b) and we find a high fraction (∼ 50 %) of spectra showing both a strong Hδ line in absorption and relatively modest [Oii] emission (e(a) spectra). The e(a) signature has been proposed as an efficient method to identify dusty starburst galaxies and we study the star formation activity and the nature of these galaxies, as well as the effects of dust on their observed properties. We examine their emission line characteristics, in particular their [Oii]/Hα ratio, and we find this to be greatly affected by reddening. A search for AGN spectral signatures reveals that the e(a)’s are typically HII/LINER galaxies. We compare the star formation rates derived from the FIR luminosities with the estimates based on the Hα line and find that the values obtained from the optical emission lines are a factor of 10-70 (Hα) and 20-140 ([Oii]) lower than the FIR estimates (50-300 M⊙ yr ). We then study the morphological properties of the e(a) galaxies, looking for a near companion or signs of a merger/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 different environments both at low and high redshift. Finally, we discuss the role of dust in determining the e(a) spectral properties and we propose a scenario of selective obscuration in which the extinction decreases with the stellar age. Subject headings: galaxies: clusters: general — galaxies: evolution — infrared: galaxies