Luminous Compact Galaxies at Intermediate Redshifts: Progenitors of Bulges of Massive Spirals?

VLT spectra of 14 luminous compact galaxies (LCGs) reveal strong metallic absorption line sytems as well as narrow and intense emission lines. Their gas extinction is found to be large (AV ∼ 1.5 mag) leading to an upward revision of their star formation rate (SFR) to an average value of ∼ 40 M⊙yr−1. Large extinction values are also supported by the large rate of detection in one field observed by ISO. Gas metal abundances in LCGs have about half the solar value. LCG absorption spectra can be synthesized with a mix of a few Gyr old and relatively metal-rich (generally solar to over-solar values) stellar population and a younger stellar population (< 5×108 years) having a metal abundance similar to that of the gas. We argue that LCGs are the progenitors of present-day spiral bulges. LCGs have masses and light concentrations similar to those of present-day bulges. They could have been formed entirely during a period of a few Gyr prior to the epoch of their observations if the star formation has been sustained at the observed rate. As in present-day galactic bulges, LCG stars show a wide range of abundances. Thus, observing LCGs allows us to directly witness an important stage in the formation of a massive galaxy, the building of the bulge prior to that of the disk.