2012ApJ...752..134W

We present a study on spectral energy distributions, morphologies, and star formation for an IRAC-selected extremely red object sample in the GOODS Chandra Deep Field-South. This work was enabled by new HST/WFC3 near-IR imaging from the CANDELS survey as well as the deepest available X-ray data from Chandra 4 Ms observations. This sample consists of 133 objects with the 3.6 µm limiting magnitude of [3.6] = 21.5 and is approximately complete for galaxies with M_*> 10¹¹ M_☉ at 1.5 ≤ z ≤ 2.5. We classify this sample into two types, quiescent and star-forming galaxies (SFGs), in the observed infrared color-color ([3.6]-[24] versus K - [3.6]) diagram. The further morphological study of this sample shows a consistent result with the observed color classification. The classified quiescent galaxies are bulge dominated and SFGs in the sample have disk or irregular morphologies. Our observed infrared color classification is also consistent with the rest-frame color (U - V versus V - J) classification. We also found that quiescent and SFGs are well separated in the nonparametric morphology parameter (Gini versus M₂₀) diagram measuring their concentration and clumpiness: quiescent galaxies have a Gini coefficient higher than 0.58 and SFGs have a Gini coefficient lower than 0.58. We argue that the star formation quenching process must lead to or be accompanied by the increasing galaxy concentration. One prominent morphological feature of this sample is that disks are commonly seen in this massive galaxy sample at 1.5 ≤ z ≤ 2.5: 30% of quiescent galaxies and 70% of SFGs with M_*> 10¹¹ M_☉ have disks in their rest-frame optical morphologies. The prevalence of these extended, relatively undisturbed disks challenges the merging scenario as the main mode of massive galaxy formation.