Unravelling the morphologies of luminous compact galaxies using the HST/ACS GOODS survey

Context. Luminous Compact Galaxies (LCGs) (M-B <= - 20, R-1/2 <= 4.5 kpc and EW0(OII) >= 15 angstrom) constitute one of the most rapidly evolving galaxy populations over the last similar to 8 Gyr history of the universe. Due to their inherently compact sizes, any detailed quantitative analysis of their morphologies has proved to be difficult in the past. Hence, the morphologies and thereby the local counterparts of these enigmatic sources have been hotly debated. Aims. Our aim is to use the high angular resolution, deep, multiband HST/ACS imaging data, from the HST/ACS GOODS survey, to study the quantitative morphology of a complete sample of LCGs in the redshift range 0.5 <= z <= 1.2. Methods. We have derived structural parameters for a representative sample of 39 LCGs selected from the GOODS-S HST/ACS field, using full 2-dimensional surface brightness profile fitting of the galaxy images in each of the four filters available. B-435W - z(850LP) color maps are constructed for the sample to aid in the morphological classification. We then use the rest frame B band bulge flux fraction (B/T) to determine the morphological class of galaxies which are well fit by a bulge+disk two dimensional structure. Mergers were essentially identified visually by the presence of multiple maxima of comparable intensity in the rest frame B band images, aided by the color maps to distinguish them from HII regions. We also make use of the Spitzer 24 mu m source catalog of sources in the CDFS to derive the dust enshrouded star formation rates (SFR) for some of the sample LCGs Results. We derive the following morphological mix for our sample of intermediate redshift LCGs: Mergers: similar to 36%, Disk dominated: similar to 22%, S0: similar to 20%, Early types: similar to 7%, Irr/tadpole: similar to 15%. We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 <= Log(10)(M/M-circle dot) <= 10.96, with a median mass of Log(10)(M/M-circle dot) = 10.32. We also derive SFR values ranging from a few to similar to 65 M-circle dot/ year as expected for this class of objects. We find that LCGs account for similar to 26% of the M-B <= - 20 galaxy population in the redshift range 0.5 <= z <= 1.2. We estimate a factor similar to 11 fall in the comoving number density of blue LCGs from redshifts 0.5 < z <= 1.2 to the current epoch, even though this number is subject to large uncertainties given the small sample size at zero redshift available from the literature. Conclusions. The strong redshift evolution exhibited by LCGs, and the fact that a significant fraction of LCGs are in merging systems, seem to indicate that LCGs might be an important phase in the hierarchical evolution of galaxies. We envisage that some of the LCGs that are classified as merging systems, might go on to rebuild their disks and evolve into disk galaxies in the local universe.