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

Abstract

Luminous Compact Galaxies (LCGs) ( M_B ≤ -20, R_1/2 ≤ 4.5 kpc and EW₀(OII) ≥ 15 Å) constitute one of the most rapidly evolving galaxy populations over the last ~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 µm 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: ~36%, Disk dominated: ~22%, S0: ~20%, Early types: ~7%, Irr/tadpole: ~15%. We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 ≤ Log₁₀(M/M_⊙) ≤ 10.96, with a median mass of Log₁₀(M/M_⊙) = 10.32. We also derive SFR values ranging from a few to ~65 M_⊙ /year as expected for this class of objects. We find that LCGs account for ~26% of the M_B ≤-20 galaxy population in the redshift range . We estimate a factor ~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.