  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 Mo/yr. 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. 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. 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. The gas needed to feed the observed star formation is likely
to be falling in from the outskirts of the galaxy, being tidally pulled out
from interacting companion galaxies. Some LCGs also show evidence for the
beginning of a disk formation. If the above scenario holds for most LCGs, we
estimate that at least 20% of present-day spiral galaxies have formed the bulk
of their stars at relatively recent epochs, during the last 8-9 Gyr, at
redshifts less than ~ 1. Since they are heavily extincted, we predict their IR
luminosities to be relatively large, around Lir = 10^11 Lsun, i.e. near or
slightly below the luminosities of the galaxies detected by ISO in the same
redshift range. Taking into account the integrated IR luminosity of the LCG
galaxy population can lead to a significant upward revision of the cosmic SFR
density in the redshift range from 0.5 to 1.
