| The star formation quenching and morphological transformation of galaxies are hot issues in the studies on the formation and evolution of high-redshift massive galaxies.Stellar mass,environmental density,and active galactic nucleus(AGN)feedback are primary factors to influence the evolution of galaxies.When and how stellar mass,environment and AGN feedback begin to dominate galaxy evolution is not settled.In this paper,using the data from the multi-band surveys in 3D-HST/CANDELS,we study the effect of stellar mass,environment,and AGN fraction on star formation quenching and morphological transformation by constructing a sample of massive galaxies(M*? 1010M?)at 0.5?z?2.5,and analyze the role of AGN feedback in galaxy evolution by our statistics about the physical properties and morphological structures of AGN and non-AGNs in the massive transitional galaxies(i.e.,green valley galaxies)at the redshift of 1.5<z<2.5.Based on their star formation activities and morphological types,we split the massive galaxies at 0.5<z<2.5 into four subsamples:quiescent early-type galaxies(qEs),quiescent late-type galaxies(qLs),star-forming early-type galaxies(sEs),star-forming late-type galaxies(sLs),to study the role of stellar mass and environment on the processes of star formation quenching and morphological transformation.According to the distributions of mass and environment for these four subsamples,we find that a mass dependence is presented in quenching process and morphological transformation.With decreasing redshifts,the mass dependence of quenching is weakened,while the mass dependence of morphological transformation becomes stronger.It is also found that a environment dependence is only presented in morphological transformation.With cosmic time,this environment dependence grows more significantly.In addition,among the four populations,the AGN fraction in qLs peaks at 2<z?2.5,and rapidly declines with cosmic time.The sEs are found to have higher AGN fractions of 20-30%at 0.5 ?z<2.The redshift evolution of AGN fractions in qLs and sEs suggests that the AGN feedback could have played important roles in the formation of qLs and sEs.Green valley(GV)galaxies are regarded as transitional galaxies with intermediate star formation activities and structures between blue cloud and red sequence galaxies,possessing the highest AGN fraction at z?2.Based on the deep X-ray observations in GOODS-S and GOODS-N fields,we choose massive green valley galaxies at 1.5?z?2.5 to compare the physical properties,morphological measurements and environmental conditions for the AGN host galaxies and non-AGN galaxies.By eliminating the central AGN component on spectral energy distribution(SED)and structure measurements of galaxies,we find that the specific star formation rate(sSFR)and interstellar color excess,E(B-V),of AGN host galaxies hold different distributions for the non-AGN galaxies at z>2,while such difference is not remarkable at z<2.The lower sSFR and E(B-V)values in the AGN host galaxies at z>2 show that AGN feedback might be possible to suppress the activity of star formation.It is also found that similar structural distributions of Sersic index(n),Gini coefficient and M20 between the samples of AGN host galaxies and non-AGN galaxies at 1.5 ?z ?2.5.However,their distributions of effective radius(re)and compactness parameter(?1.5)are different.The AGN host galaxies are found to have smaller re and higher?1.5.Finally,we also find a similar distribution of environmental overdensity between the AGN hosts and non-AGN galaxies at 1.5<z<2.0,which might indicate that the environmental condition at z<2 didn't exert any influence in triggering central AGN.For the GV galaxies at 2.0<z<2.5,the AGN and nonAGN samples come to exhibit different distributions of overdensity.The AGN host galaxies prefer to reside in denser environment,which implies that galaxy merger might have played roles in trigering central AGNs at higher redshifts. |
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