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

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Abstract
VLT spectra of 14 luminous compact galaxies (LCGs) reveal strong metallic absorption-line systems 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 the Infrared Space Observatory (ISO). Gas metal abundances in LCGs have about half the solar value. LCG absorption spectra can be synthesized with a mix of a few gigayears old and relatively metal-rich (generally solar to oversolar values) stellar population and a younger stellar population (less than 5 × 108 yr) 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 gigayears 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 witness directly 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. An infall scenario naturally explains the gas metal abundance, which is generally lower than that of the older stellar component. At least for the strongest star-forming LCGs, there is clear imaging evidence for the presence of companions. 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 = 1011 L☉, 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.
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