| Abstract
| - Using HST/WFPC2 imaging in F606W (or F450W) and F814W filters, we obtained the color maps in observed frame for 36 distant (0.4 < z < 1.2) luminous infrared galaxies (LIRGs, $L_{\rm IR}(8{-}1000~\mu$m) ≥$10^{11}~L_odot$), with average star formation rates of ~100 $M_odot$ yr -1. Stars and compact sources are taken as references to align images after correction of geometric distortion. This leads to an alignment accuracy of 0.15 pixel, which is a prerequisite for studying the detailed color properties of galaxies with complex morphologies. A new method is developed to quantify the reliability of each pixel in the color map without any bias against very red or blue color regions.
Based on analyses of two-dimensional structure and spatially resolved color distribution, we carried out morphological classification for LIRGs. About 36% of the LIRGs were classified as disk galaxies and 22% as irregulars. Only 6 (17%) systems are obvious ongoing major mergers. An upper limit of 58% was found for the fraction of mergers in LIRGs with all the possible merging/interacting systems included. Strikingly, the fraction of compact sources is as high as 25%, similar to that found in optically selected samples. From their K band luminosities, LIRGs are relatively massive systems, with an average stellar mass of about 1.1 $\times$ 10 $^{11}~M_odot$. They are related to the formation of massive and large disks, from their morphologies and also from the fact that they represent a significant fraction of distant disks selected by their sizes. If sustained at such large rates, their star formation can double their stellar masses in less than 1 Gyr. The compact LIRGs show blue cores, which could be associated with the formation of the central region of these galaxies. We find that all LIRGs are distributed along a sequence which relate their central color to their concentration index. This sequence links compact objects with blue central color to extended ones with relatively red central color, which are closer to the local disks. We suggest that there are many massive disks which have been forming a large fraction of their stellar mass since z = 1. For most of them, their central parts (bulge?) were formed prior to the formation of their disks.
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