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 (∼ 0.3) at z ≤ 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 0.5−10keV ≥ 10 43.5 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/interactinggalaxies 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 downsizing scenario.

The nature of the Sloan Digital Sky Survey galaxies in various classes based on morphology, colour and spectral features - I. Optical properties

“…Since the r band is the standard band in the SDSS photometry and the photometric uncertainty in the u band is relatively large compared to that in the g band, we selected the ( g − r ) colour as the index for the galaxy colour segregation. We used the method of Lee et al (2006) to segregate blue galaxies from red galaxies. First, we divided the redshift range of 0 ≤ z ≤ 0.4 into eight bins, and derived a colour histogram of the early‐type galaxies with for each redshift bin.…”

“…We used a polynomial function instead of a linear function for more accurate fitting, although the resulting peak line seems almost linear at z < 0.3. It is noted that the peak line shows a somewhat complex curve when it extends to higher redshift ( z ∼ 1; Lee et al 2006). We regard the early‐type galaxies that are 3σ bluer than the Gaussian peak colour as blue early‐type galaxies, and the other early‐type galaxies as red early‐type galaxies.…”

The nature of the Sloan Digital Sky Survey galaxies in various classes based on morphology, colour and spectral features - III. Environments

“…In many previous studies, however, galaxies have been classified using simple schemes based on just one or two properties, which sometimes miss detailed aspects of galaxy evolution. For example, several unusual classes of galaxies such as blue early‐type galaxies (BEGs) (Ferreras et al 2005; Lee, Lee & Hwang 2006) and passive spiral galaxies (Yamauchi & Goto 2004; Ishigaki, Goto & Matsuhara 2007), or the fundamental parameter difference between finely divided classes in the same morphology class (Lee et al 2007; Choi, Goto & Yoon 2009a) are difficult to investigate using simple classification schemes. Recently, Lee et al (2008, 2010, hereafter Paper I and Paper II, respectively) presented studies on the optical and multiwavelength properties of the Sloan Digital Sky Survey (SDSS; York et al 2000) galaxies divided into fine classes based on their morphology, colour and spectral features.…”

The X‐Ray Evolution of Early‐Type Galaxies in the Extended Chandra Deep Field–South

“…However, the luminosity density of red sequence galaxies has been observed to remain approximately constant over z % 0:1Y0:7 (e.g., Bell et al 2004b;Faber et al 2005), and since these galaxies appear to be fading passively with cosmic time, this suggests that an important fraction (% 1 2 ) of the current z ¼ 0 stellar mass content of the early-type galaxy population has emerged since z ¼ 1. One explanation for these observations is that relatively young, lower mass galaxies continuously populate the red sequence (e.g., van der Wel et al 2004; Thomas et al 2005) through either a downsizing mass assembly scenario (e.g., Cowie et al 1996;Bundy et al 2006;Cimatti et al 2006;De Lucia et al 2006;Lee et al 2006) and /or a scenario where lower mass early-type galaxies have different merging histories than their higher mass counterparts (e.g., Khochfar & Burkert 2003). At z k1, distant red galaxies (DRGs; selected using J À K colors) and extremely red objects ( EROs; R À K or I À K selected ), some of which are likely progenitors to early-type galaxies, appear to have large star formation rates (SFRs) and inferred dynamical masses similar to those observed for massive local ellipticals (e.g., Förster- McCarthy 2004;van Dokkum et al 2004;Papovich et al 2006).…”