Morphology and structure of extremely red objects at z ∼ 1 in the CANDELS-COSMOS field**Supported by the National Natural Science Foundation of China.

Abstract

Using high-resolution HST/Wide Field Camera 3 F125W imaging from the CANDELS-COSMOS field, we report the structural and morphological properties of extremely red objects (EROs) at z ∼ 1. Based on the UVJ color criteria, we separate EROs into two types: old passive galaxies (OGs) and dusty star-forming galaxies (DGs). For a given stellar mass, we find that the mean size of OGs (DGs) is smaller by a factor of ∼ 2 (1.5) than that of present-day early-type (late-type) galaxies at a rest-frame optical wavelength. We derive the average effective radii of OGs and DGs, corresponding to 2.09 ± 1.13 kpc and 3.27 ± 1.14 kpc, respectively. Generally, the DGs are heterogeneous, with mixed features including bulges, disks and irregular structures, with relatively high M20, large size and low G. By contrast, OGs have elliptical-like compact morphologies with lower M20, smaller size and higher G, indicating a more concentrated and symmetric spatial extent of the stellar population distribution in OGs than DGs. These findings imply that OGs and DGs have different evolutionary processes, and that the minor merger scenario is the most likely mechanism for the structural properties of OGs. However, the size evolution of DGs is possibly due to the secular evolution of galaxies.