The Nature of Blue Early-Type Galaxies in the Goods Fields

Abstract

We present a study of the nature of the blue early-type galaxies (BEGs) in the GOODS north and south fields using the GOODS HST/ACS archival data. Using visual inspection, we have selected 58 BEGs and 113 normal red early-type galaxies (REGs) in the sample of 1,949 galaxies with spectroscopic redshifts. We find that the BEGs are generally bluer, fainter, and less-massive than the REGs, although a few BEGs are exceptionally bright and massive. The number fraction of the BEGs to total early-type galaxies is almost constant ($\sim0.3$) at $z \le 1.1$. In addition, we find that the size of the BEGs in given redshift bin decrease as redshift decreases. The BEGs look similar to the REGs in the images and surface brightness profiles. However, at least 27 BEGs show traces of tidal disturbances in their fine structures: elongated cores, off-centered cores, asymmetric internal color distributions, tidally distorted outer structures, collisional rings, or very nearby companions. Twenty-one BEGs are detected in the X-ray bands and eleven of them are as luminous as $L_{0.5-10 {\rm keV}} \ge 10^{43.5} {\rm erg~s}^{-1}$, indicating the existence of AGNs in their centers. These results show that at least a half of the BEGs may be descendants of mergers/interacting-galaxies and that at least a quarter of the BEGs may be AGN-host galaxies. The BEGs may evolve into REGs, and the size evolution of the BEGs is consistent with the galactic \emph{downsizing} scenario.